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Bulletin No. 131
PLANT NEMATODES
THE GROWER SHOULD KNOW
Reprinted from the Fourth Proceedings of the Soil Science
Society of Florida, 1942 (Publ. Apr. 1, 1949)
Amended 1956
STATE OF FLORIDA
DEPARTMENT OF AGRICULTURE
NATHAN MAYO, Commissioner
TALLAHASSEE
_May, 1956
PLANT NEMATODES THE GROWER
SHOULD KNOW
DR. G. STEINER1
For some decades nematologists have emphasized the presence of
Enormous numbers of nematodes in our crop lands and have singled out
many species which are plant parasites or plant pathogens and therefore
Pf significance to plant growth and crop production. But only during
recent years has an increasing realization of the economic significance
pf plant nematodes as pests of crops, and as limiting factors in crop
production been developed by growers and research workers. That this
plisregard of nematodes as a factor in soil response has had its serious
consequences in research, particularly in soil science, may be illustrated
y the fact that various extensive fertilizer tests made in Florida have
had to be repeated because of their insidious damage. It is obvious that
jest crops having diseased and parasitized root systems will not respond
in the same manner as would those with normal and healthy roots. The
loil scientist should therefore be aware of the modifying effect nema-
todes may have on the results of his studies with fertilizers, cover crops,
race elements, etc.
For years the opinion was prevalent that there was just one signifi-
rant nemic plant pest, the root-knot nematode, and it was spoken of as
"the nematode." Although root-knot nematodes are recognized the world
pver as the worst and most perplexing agricultural pests they are cer-
tainly not the only significant ones. There are many other important
plant nematodes such as the meadow nematodes (a group of forms
elonging to the genus Pratylenchus Filipjev); the spiral nematodes
(Genus Helicotylenchus Steiner); the kidney-shaped nematode (Genus
flotylenclulus Linford); the lance nematodes (Genus Hoploaimus v.
sDaday); the puncturing nematodes (Genus Dolichodorus Cobb); the
stylet nematodes (Genus Tylenchorhynchus Cobb); the ring nematodes
I(Subfamily Criconematinae Chitwood); the pin nematodes (Genus
Paratylenchus Micoletzky); the seed gall nematodes (Genus Anguina
Scopoli), to which belongs, e.g., the wheat nematode (A. tritici [Stein-
'buch] Filipjev); the bulb and stem nematodes (Genus Ditylenchus Fil-
ipjev), which represent a group of different species including, e.g., the
Rice nematode (Ditylenchus angustus [Butler] Filipjev) and the potato
rot nematode (Ditylenchus destructor Thorne); the sugar-beet nema-
Itode (Heterodera schachtii Schmidt); the golden nematode of potatoes
(H. rostochiensis Wollenweber); the citrus nematode (Genus Tylenchu-
Ilus Cobb); the bud and leaf nematodes (certain species of Aphelen-
choides Fischer) such as the fern nematode (A. olesistus Ritzema Bos),
the cocopalm nematode (A. cocophilus Cobb), the strawberry nema-
1 Recently retired Head Nematologist, Agricultural Research Service, U. S. Dept.
of Agriculture.
DEPARTMENT OF AGRICULTURE
tode. (A. fragariae Ritzema Bos) causing spring dwarf in strawberry!
plants, Bessey's strawberry nematode (A. besseyi Christie) causing sum-
mer dwarf, the chrysanthemum nematode (A. ritzemna-bosi); the needlE
nematodes (Genus Xiphinema Cobb); and certain spear nematodes of
the Genera Dorylaimus Bastian, Pungentus Thorne and others. Therq
are known today several hundred different species of nematodes that
attack plants and at least a dozen must be considered major pests whilq
others will undoubtedly be considered as such after they are better
known. In general, however, their significance as pests is certainly under
estimated. There are several reasons for this. One is the soil-bornt
character of all plant nematodes; a second, their small microscopic size
a third, their hidden mode of life either in the soil or inside the plant
tissues; a fourth, that their study is technically very difficult; a fifth, thaj
they are considered a border subject not properly belonging to the
field of the entomologist or that of the plant pathologist or the parasite
ologist. In addition plant nematology and its related subject soil, fresh-
water, and marine nematology are not given as courses in our college
and universities and are considered only a side line of research by mosf
agricultural institutions.
Then, too, there is the tendency on the part of growers and investi-
gators to judge the health and growth conditions of plants mainly orl
the basis of the appearance of the above-ground parts. This generally
leads to an underestimation of the significance of nematodes in theil
role as plant pathogens and their influence on plant growth. Since the
roots and other underground parts of plants are the principal portionI
attacked by nematodes, a full evaluation of their damage naturally
necessitates the uprooting of a sufficient number of plants for inspection
of the root system. This, of course, often means the loss of many plants.
In the case of shrubs and trees this may involve a considerable loss,
without the gain of more than a chance result. The evaluation of the'
significance of other underground pests and diseases of crops, as in
the case of plant nematodes, is also more difficult than that of those
found above ground.
I
Growers frequently are also inclined to blame reduced yields on a
lack of soil fertility, on deficiencies of certain elements, on drought and!
other seasonal conditions, on winter kill, sunburn, lightning, and other
factors, when actually nematodes may be the cause of these conditions.,
Growers also sometimes tell of the wonderful crops they or their parents
used to grow and that the production of such crops is no longer possible,,
even with heavy applications of fertilizers. We are convinced that many
such instances not infrequently would prove upon examination to be
the result of root destruction by nematodes. In other countries the
term "sick soil" is sometimes used to express failure of a given soil to
continue to produce, notwithstanding fertilizer applications or the use
of other means to induce increased production. Nematodes were repeat-
edly found to cause such conditions, often more specifically termed by
PLANT NEMATODES THE GROWER SHOULD KNOW 5
Growers as "alfalfa sick soil," "clover sick soil," "soil tired of beets," "tired
of potatoes," etc.
I It is agreed that with our monoculture, that is, a single identical
crop planted over wide areas not infrequently year after year or at
least in close succession, pests and diseases of such crops are provided
with favorable conditions for development. They therefore increase
Tremendously, particularly in the absence of any natural checks. But
here again in our attempts to look for and control pests and diseases,
phe above-ground parts of our crops have attracted most of our atten-
tion and consumed most of our efforts. This has led to modern pest
control of these aerial parts by spraying and dusting practices and by
fumigation. A great deal of research work has been and still is being
(lone in this connection. However, the root systems and other under-
ground parts of plants have not gotten their share of attention although
Ioil-borne pests and diseases, particularly our plant nematodes, are
equally favored by our monocultures.
) Fortunately plant nematodes are slow in their spread and are usually
not of such a lethal virulence as many of the fungous, bacterial, and
firous diseases. An outright killing may be effected on seedlings but
this is rather an exception on fully grown plants. Nematode diseases
pause a reduction in growth and vitality, but generally are not com-
pletely lethal. Thus it would appear that, in general, the host-parasite
relationship between plants and nematodes is a highly balanced one,
that is, the parasite does not at once kill the host and so deprive itself
mf a means of existence. This situation has made these pests appear less
impressive and thus has been another reason for the under-estimation
pf their importance.
It is also a fact that many of these pathogenic nematodes, more or
less unconsciously, have been kept under control to some degree by cer-
tain agricultural practices. Plowing, disking, harrowing, and cultivation
pre some of the operations that help and are not to be under-estimated
as a means of reducing noxious nematodes in the soil. This is accom-
plished by exposing the nematodes to the sun, to drying by wind, to
starvation by depriving them of a living host, to mechanical injury, etc.
FCrop rotation also has long been recognized as an extremely helpful
agricultural practice through the tendency to reduce the nematode popu-
tation by starvation providing, of course, that one or more crops used
in the rotations are not a host of the particular nematode or nematodes
(present in a given soil. Thus various agricultural practices, to some ex-
tent, have been effecting a certain amount of control over soil-borne
nematode pests of plants, but also have assisted in delaying a proper
appraisal of their importance.
I This situation began to change with the coming of soil sterilization
by steam and other means including the use of efficacious soil fumigants.
The efficient control of root-parasitic nematodes and other biotic factors
6 DEPARTMENT OF AGRICULTURE
antagonistic to plant growth through these methods usually results i*
remarkable increases in plant growth; the higher yields thus proving
quite conclusively the really detrimental effect of nematodes on croq
production. But here again, past and current conceptions have beer
slow to acknowledge the beneficial effects as being mainly the result q
parasite and disease control.
That the sterilization of soil by steam makes for better growth A
plants, frequently to such an extent that fertilizer applications may be
omitted, has long been known. For an explanation of this phenomenon
it was assumed, however, that such sterilization made more nitrogen
available and that this condition rather than parasite and disease conq
trol, was responsible for the growth and yield increase. With the advent
of efficacious soil fumigants such as carbon bisulphide (CS2), chloropicri I
(C ClaNO2), methyl bromide (CHaBr) and others, including the most
recent DD mixture (1,3-Dichloro-propene 1,2-Dichloropropane), tha
same explanation was still promulgated. Here too, an increase of avail,
able nitrogen was assumed and thought to cause growth "stimulation."
Peculiarly enough the question of where this nitrogen originated, par-
ticularly with fumigants not containing it, did not arise. It was argued
that both steam and chemical sterilization will kill most, if not all
organisms in the soil, and thus make more nitrogen available through
the decomposition of the bodies of these organisms. Such an argument
appears fallacious. If it were correct, then man too would have his
living space affected only through decomposition of his body after deatl!
Life is action and it is the living biological factor in the soil that exerts
the more extended, the more intensive, and the more significant action
on plant growth rather than the dead one. That is to say it is the living
plant-parasitic and plant-pathogenic biotic factors in a given soil whicl
are responsible for reducing the growth and production of crops grow-
ing therein. We are convinced that the striking increases in growth ani
yield that frequently follow sterilization, either by steam, hot water, or
fumigants, are largely the result of parasite and disease control ani
more particularly, nematode control. The results of soil fumigation in
our opinion thus emphasize the very great importance of plant-parasiti
nematodes in crop production.
Thus the growing recognition of the significance of plant and soil
nematodes as a factor in crop production is well founded. It is hoped
that it will lead to helpful developments for the control of these cropq
antagonistic biotic factors through the use of more efficacious fumigants,
and better methods of application. Here the way is open for significant
increases in crop production combined with a possible reduction of
acreage and a lesser need for fertilizers. Let us provide healthy rooq
systems for our crop plants through control of the many nematodes and
other agents now causing root lesions, root decay, root amputations
blind root tips, abnormal matted growth, etc., and surprising yield in-
creases will result. A healthy root system will more effectively utilize
I
PLANT NEMATODES THE GROWER SHOULD KNOW 7
lower fertility level in a soil or take more advantage of one that has been
artificially increased by fertilizer applications, than will one that is af-
fected by disease.
Figure 1.-Roots of a tobacco plant grown in South Carolina and disfigured
through a heavy infection by root-knot nematodes. Note the stunted, blinded and
swollen roots and compare with type of root disfiguration and galling shown in
Figure 2. Such variations in character of root galling and disfiguration may be
related to different root-knot nematode species, but may also be the result of different
reactions by the hosts.1
How are the growers to recognize the presence of these microscopic
nematode pests on their crops, and how are they to know the kinds in-
volved? Are there specific symptoms which indicate that a disease or
abnormal behavior is caused by a nematode, or an association of nema-
todes, or a combination of nematodes and fungi or bacteria, or a combi-
nation of all three of them, or a combination of still some other factor or
group of factors with nematodes? There exists one sure way to determine
this, namely the location and identification of the nematodes themselves.
May we therefore emphasize that it is not possible to recognize and
properly diagnose nematode diseases of plants by symptoms alone; in
1 Most of the photographs used to illustrate the booklet were taken by Mr.
Marcel L. F. Faubert of the office of Plant and Operations; others by Mr. Wilfred
T. Mead of the Agricultural Research Service. The author wishes to express to
both of them his appreciation for the excellent service.
8 DEPARTMENT OF AGRICULTURE
every instance the nematode itself should be located and determined
before a final diagnosis is made. Let us support this statement by an
example. It is widely assumed that the root-knot nematodes may be
correctly recognized and properly diagnosed through the symptoms they
cause on the roots of host plants, namely conspicuous swellings, knots
or galls. (Figs. 1-8) But even in this instance it is fallacious; such
symptoms are only indicative but certainly not proof of an infection by
root-knot nematodes because: (1) there are knot-forming nematodes
found on roots of plants other than the root-knot nematodes (e.g. Dity-
lenchus radicicola [Greeff] Filipjev, or Nacobbus dorsalis Thorne &q
Allen), (2) root-knot nematodes may be present without causing the
formation of knots (this is often seen, e.g., in cotton or corn where theq
nematode breaks through the surface of a root without forming knots)
and roots are then mistakenly judged free, (3) other organisms mayq
form similar knots, e..g% the slime molds (Plasmodiophora species) caus-
ing clubroots in members of the cabbage family, the crown gall organism|
(Bacterium tumefaciens Sm. & Town) and certain viruses; also so-called
e.-
Figure 2.-Roots and pods of a peanut plant (Arachis hypogaea L.) infected
with root-knot nematodes which formed only small galls on the roots but blackish I
lesions on the pods. This attack at the ends of pods causes interruption of growth
and misshapen and often dwarfed pods.
Figure 3.-Seedlings of one of the rubber-producing dandelions (Taraxacum koksaghyz Rod.) exhibiting
root swellings caused by root knot nematodes.
10 DEPARTMENT OF AGRICULTURE
1
Figure 4.-A. Heavily knotted and crippled roots of a Laredo soybean plant
grown in Virginia infected with root-knot nematodes; %2 normal size. B. Heavily
knotted and crippled roots of a young Castilla elastica Cerv.; % normal size.
PLANT NEMATODES THE GROWER SHOULD KNOW
A'
IN
Figure 5.-A. Roots of black locust seedlings (Robinia pseudoacacia L.) dam-
Ped by root-knot nematodes. Note the swollen and blinded root tips; % normal
ize. B. Roots of a daylily (Hemerocallis hybrid) infected with root-knot nematodes.
rote again the blinded root tips; otherwise the knots and galls are only slightly
rticeable on the fleshy roots; 3 normal size.
12 DEPARTMENT OF AGRICULTURE 4
bacterial nodules may be mistaken for root-knot galls, but they usually
are easily distinguished from knots caused by root-knot nematodes by
the fact that they are mostly fixed on a short peduncle and hang on th|
side of a root whereas the swelling caused by root-knot nematodes is
most often an axial one of the. root proper (Fig. 9), (4) there arl
certain plants that have normally knotted or swollen roots (e.g. Parthe
nocissus sp.). Thus it is evident that even this common root-knot nemi
tode disease must be diagnosed by the presence of the organism itself
rather than through the occurrence of knots of one kind or another o
the roots.
Figure 6.-A. Root-knot infected sweet potatoes; the sweet potato to the leftI
exhibits cracking which appears to result in certain instances from a root-knot infec-
tion. B. Beet heavily infected and disfigured by root-knot nematodes. C. Carrot
disfigured and made unmarketable by root-knot nematodes; 1/2 normal size.
.D E
Figure 7.-A. Bliss triumph potato badly infected and disfigured by root-knot
nematodes. Specimens collected at Gainesville, Florida. Note the protruding knobs
formed by the nematodes and compare with B where symptoms are sunken, crater-
like spots. B-E. Bliss triumph potato infected and disfigured by root-knot nema-
todes. Specimens collected at Tampa, Florida. It is thought that the difference in
symptoms observed in the potatoes of A and B-E of this Figure are the result of
species differences in the attacking nematode populations; % normal size.
14 DEPARTMENT OF AGRICULTURE
Furthermore, for a proper assay of the status of a given plant asd
host, it should be ascertained whether the nematode is actually p
during progeny on it. For a demonstration of this point root-knot nejA
todes may serve again. Our observations appear to show that these p
parasitic larvae enter the roots or subterraneous stem and leaf form.
B
iJB
T f
,^ tt
p9-
is <
Figure 8.-A. Root-knot infected bean seedling with galls also on the .leaves (I
arrow). B. Root-knot infected bean seedling with galls also on the stem (at arrows
C & D. Malformed and dying bean seedlings with heavy infection of root-knol
nematodes; 1/2 normal size. Attention may be called to the fact that such hea4
invasions of seedlings, leading to the ultimate death of the plants before the invading
nematodes have time to produce progeny, may lead to such a reduction in thl
number of nematodes that a replanting with beans may be fully successful.
tions of almost any type of plant in spite of the fact that, in most case
they are unable to develop in them and to produce progeny. In 1931
it was found that the root tips of French marigolds (Tagetes hybrids
were heavily invaded by larval root-knot nematodes, while galls witE
full grown females producing egg masses were observed in only smaj
numbers. Upon closer study it was shown that most of the larval nema-
todes that invaded these roots were unable to develop to the adus
stage and died before reaching it. Obviously these resistant marigold,
did not furnish the invading root-knot larvae with the proper food, oI
I
PLANT NEMATODES THE GROWER SHOULD KNOW
the nematode larvae did not have the ability to induce the invaded host
to produce their food. Further studies on so-called resistant plants
showed a similar situation to exist, e.g. in Crotalaria spectabilis Roth,
Solanum grandiflorum R. & P., lantana (Lantana camera L.), "dusty
miller" or silver cineraria (Senecio cineraria DC.), Nicotiana megalo-
siphon Huerck & Muell. Arg. and Nicotiana plumbaginifolia Viv. Cro-
*~
Figure 9.-Lespedeza stipulacea Maxim., Korean clover, showing nodules caused
by root-knot nematodes (solid arrows), and nodules caused by beneficial nitrogen-
fixing bacteria (open arrows). Coll: Virginia, 1941; % normal size. It may be
mentioned that this lespedeza is an excellent indicator plant to detect the presence
of root-knot nematodes in the soil. Where these nematodes occur in a soil growing
this species, the plants always will be stunted and yellow.
16 DEPARTMENT OF AGRICULTURE
talaria spectabilis proved to be one of the most interesting plants
yet studied in its host-parasite relationship with root-knot nematodes.
It was observed that the root tips of growing specimens are invaded
by large numbers of larval root-knot nematodes, but up to the pres-
ent time not one larva has ever been found to have reached the adul
stage. They all appear to perish at a very early stage of fixation in'
the root. This legume is therefore an excellent trap plant and cover
crop combined. On the basis of greenhouse and field experiments i
has been found very effective in cleaning infested land of root-knot
nematodes. In entering the roots the nematode appears to be able to
induce the root tissues to produce the so-called giant or nectarial cells
on which they feed, but for some reason the nematodes will not gro
to the adult stage and produce eggs and progeny. However, even in
this case the plant suffers from an invasion of its roots by these nematodes
larvae; this is shown by the fact that small seedlings of C. spectabilis will
die or at least appear sick for a time after invasion. Eventually theq
plant will recover. Solanum grandiflorum R. & P., a weed growing inq
Brazil1 and used there as a root stock for tomato grafts on root-knot-I
infested lands, exhibits characteristics quite similar to those of Crota-
laria spectabilis in its relationship to the root-knot nematodes. Here,|
too, the roots are invaded and their tips blinded by the larval root-knot
nematodes which again are unable to grow and develop as they become
fixed. The Solanum eventually will also attract from the soil larval speci-
mens of the nematodes but will recover after their invasion and death.
Here again the young seedlings may suffer badly from this invasion and,
in fact, may even be killed. Of a total of 100 seeds planted in root-knot-
infested soil only three seedlings survived while a control planting from I
the same lot of seed in sterilized soil gave full germination and only
healthy growth.
Still another variation in host-parasite relationship is shown by the |
rose geranium (Pelargonium graveolens [Thumb] L'Herit). In this
plant the roots also appear to possess considerable resistance against |
attack. Few galls are formed, many of which are empty, and only occa-
sional ones were observed with egg-laying females. In contrast to this
behavior of the roots, the basal portion of the stem appears to be very
attractive to root-knot nematodes and consequently is invaded by large
numbers, galls being formed and female specimens being able to grow
to maturity and produce eggs (Fig. 10).
We are convinced that a study of the numerous host plants affected
by root-knot nematodes, including the so-called resistant and immune
types, will bring to light additional information of much value for a
better understanding of these numerous and complex host-parasite inter- I
relationships. These are all matters of not only theoretical interest, but
1 For the opportunity of studying this plant we are indebted to S. B. Fenne of
the Institute of Inter-American Affairs. .
PLANT NEMATODES THE GROWER SHOULD KNOW 17
of great practical importance since they find their application in control
through effective crop rotations.
There are still other complicating factors that make the diagnosis
of root-knot infections and a proper appreciation of their significance
y--
figure 10.-Roots and basal stem of a rose geranium exhibiting galls (at arrows) of
root-knot nematodes.
18 DEPARTMENT OF AGRICULTURE 4
difficult. Observations on the host range of this group of nematodes
in different regions and locations, as well as studies on the resistance
of an identical crop variety planted at different locations, have show r
that plants and crops attacked in one location sometimes are not attacked
in another. Evidence is rapidly developing through systematic studied
to prove that there exist many different species or host nematodes, each
of which has its own host range; some regions or localities appear t
have only a single species, whereas in others two or more occur, often
in a mixed state. A similar situation appears to exist in other nematodea
species, for example, the bulb and stem nematode or certain bud ancal
leaf nematodes (Aphelenchidae species). These matters are mentioned
to emphasize the fact that the diagnosis of a nematode infection anal
the evaluation of its capacity for damage must be made with care and
knowledge. i
In the 1940's a great step forward was made in resolving some o4
the perplexing problems in studies on the root-knot nematodes. First
the experimental work of J. R. Christie and later the taxonomic work ol
B. G. Chitwood proved that the root-knot nematodes constituted many,
rather than a single, species, each having its characteristic behavioil
and plant-food preferences.
Disease symptoms produced by the various species are rarely specific;l
wilting, discoloration of leaves, swollen and distorted shoots, crinkledI
leaves, leaf spots, gall formation on roots, stems and leaves, galled fruits
and seeds, bunching and dwarfing of the entire plant, blinding of buds
and the phenomena termed dieback, sunburn, winter kill and winter
bronzing may be caused by nematodes. General wilting of plants in the
field over a hot period of the day may be indicative of the presence of1
large numbers of root nematodes, as may also bare spots or areas ex-
hibiting retarded or reduced growth. However, all of these symptoms
likewise may be caused by various other organisms and agents. Again,
therefore, external symptoms alone can not serve as a reliable basis fol
the diagnosis of a nemic plant disease. The identification of the organ-
ism is absolutely necessary. I
Nematodes, or eelworms, so far as plant-parasitic forms are con1
cerned, are very small animals, less than 2 mm long, usually only from
0.4 to 1 mm (or 1/64 to 1/25 of an inch). Therefore they can not be
seen with the naked eye and must be studied with the microscope.
Although of such small size, their organization is highly complex and
embraces all organs and organ-systems found in higher animals except-
a circulatory system. It is difficult for the untrained worker to differ-
entiate the various genera and species and some of the thousands o01
soil-inhabiting forms may easily be mistaken for those which are defi-
nitely parasitic to plants. The grower and entomologist as well as theq
plant pathologist should keep this in mind and if there is any doubt
regarding the symptoms, an identification should be requested froni
FLANT NEMAIUULS H111 LU UWV.K bt1UULU NUW l U
A B C
.stomato-stylet.,
odon to-siglet....
Tylenchidae Aphelenchidae Dorylaimidae
tomato -stylet odonto -stylet
stylet a( transformed stylet a
buccul charity transformed tooth
Figure 11.-Schematic drawings to explain the main differentiating characters
of the three groups of nematodes to which most plant-parasitic forms belong. A.
Tylenchidae, B. Aphelenchidae, C. Dorylaimidae. dsl oes gl, dorsal oesophageal
gland; int, intestine; mdl bib, middle oesophageal bulb; nrv r, nerve ring; out dsl
oes gl, outlet of dorsal oesophageal gland; out rt subv oes gl, outlet of right subven-
tral oesophageal gland; rt subv oes gl, right subventral oesophageal gland.
20 DEPARTMENT OF AGRICULTURE
a nematologist. Considering the comparative youthfulness of this branch
of science, it should be remembered that even the specialists' views1
concerning the relative significance of these forms are subject to aq
great deal of change and modification.
It has already been pointed out that nematodes constitute an ex-1
tremely large and varied group of animals, divided into many different
families. With some exceptions ,the plant-parasitic forms belong to only
three of these families; the Tylenchidae, the Aphelenchidae, and the
Dorylaimidae. The members of all three of these families are provided|
with a stylet with which they feed. This organ appears to be fully effec-
tive in puncturing the plant tissues, and in obtaining the food required l
by these parasites by sucking. Furthermore, many types of plant-parasitic
nematodes appear to induce the host plant to produce their particular
F AB C E
Figure 12.-Drawings of various developmental stages of a root-knot nematode.
A. Unsegmented egg; B. egg containing larva; C. migratory larva free in the soil;
D. sausage-shaped larva living sedentary in the root; E. larval molt containing fully
developed male; F. adult male; G. young female. X 140
t
I
U ,~
I
A -,
Figure 13.-Photomicrographs of developing larvae of a root-knot nematode in
roots of balsam (Impatiens balsamina L.) A. Swollen portion of a root with numer-
aus larval nematodes arranged around its axial cylinder ( X 25); B. single young
larva, its body only slightly swollen but the root tissue cells around its head end
already enlarged to form the so-called giant or nectarial cells. The immigrating root-
knot nematode larva in settling down for its permanent, life-long location injects
secretions of its oesophageal glands into the root tissue. These injections stimulate
the cells to increased growth and, by secretion, to the production of a type of fluid
Food which is directly assimilable by the nematode. It appears that this food ma-
terial is of such comparatively simple structure that a regular digestion in the
intestine of the parasite is not further necessary. The intestine of these parasites
appears to be mainly a storage organ for food reserves and the rectum and anus
are often vestigial (x 63); C. same as B but a later stage (x 100); D. same as
B and C but still a more advanced stage than C. The molt of the larva is seen as
faint mark surrounding its body (x 100).
DEPARTMENT OF AGRICULTURE
food requirements by injecting into the plant tissues the secretions of
their large esophageal or salivary glands. For this purpose the stylet
is used as an injecting needle.
The stylet of the tylenchs and also that of the aphelenchs is called
a buccal stylet or "stomato-stylet." It is assumed to have originated
through the transformation of the buccal cavity of the ancestors of these
two closely related families. The sclerotized walls of the buccal cavity|
of these ancestors are assumed to have been amalgamated and so tol
have formed a perforated tube, or the buccal stylet presently seen in|
these forms. (Fig. 11, A & B). In the dorylaimids, however, which
represent a group of very different origin and relationship, the stylet
is assumed to be a transformed tooth and is therefore termed an "odonto-1
stylet." (Fig. 11 C). I
The tylenchs and aphelenchs include the most outstanding plant
nematode pests, many species of which often occur in very large num-i
bers. The two families are differentiated mainly by one character. In
the tylenchs the dorsal esophageal or salivary gland empties into their
alimentary tract a short distance behind the buccal stylet (Fig. 11),'
while in the aphelenchs it empties into this tract in the middle esophageal q
bulb just in front of its valvulae.
The dorylaims occur in numerous genera and species in soils every-j
where. Many live parasitically on and in plants, even in the leaves andl
other above-ground parts, but they rarely occur in such great numbers|
in an attacked plant as do certain -parasitic aphelenchs and tylenchs.q
All dorylaims appear to lead a migratory mode of life although some|
occur in plant tissues apparently quite sedentary, their body rolled upq
in a more or less tight spiral. I
The most prominent characteristics of the tylenchs and aphelenchs,
in comparing them with dorylaims, is decidedly greater size and more
highly developed function of their oesophageal glands. These charac-
teristics doubtless are directly related to the much more pronounced
pathogenicity of the group. For instance, in many species of tylenchs'
and aphelenchs, the secretions of these glands are known to have a|
poisonous effect on the host in addition to inducing the host tissues to
produce directly assimilable fluid for use as food by the parasite. Thea
presence of these enlarged and strongly functional oesophageal glands
in the tylenchs and aphelenchs appears, in general, to indicate an extra-i
oral digestion and, correlated with it, a transformation of the intestines
into an organ designed mainly for storage of food reserves. It is on
this account that they are able to feed by inducing the host tissues,q
through glandular injections, to produce directly assimilable food. The
dorylaims, on the other hand,. suck the cell contents directly from thel
host tissues; these cell contents, in turn, are digested and broken down
to simple assimilable forms of food in their intestine.
In reviewing the more important plant-parasitic nematodes, it appears4
d
22
PLANT NEMATODES THE GROWER SHOULD KNOW 23
best to begin with those types capable of attacking the growing roots
of plants. Certain species, of course, are of far greater economic im-
portance than others. It is with these species that we will first deal.
THE ROOT-KNOT NEMATODE
Root-knot nematodes (Meloidogyne spp., formerly Heterodera marioni
.[Cornu] Goodey) are considered the most important of this group.
Their extended distribution through the tropics, sub-tropics, and tem-
,perate regions and their occurrence in greenhouses everywhere, make
them one of the most widely distributed and common agricultural pests.
They have long been known in Florida. Neal states that the disease, as
such, was known to occur here as early as 1805, although the causitive
organism, the nematode, was not discovered until 1879, and was first
definitely established as occurring in Florida in 1889. While root-knot
doubtless is now present in all sections of the State it is particularly
bad in those areas where sandy and peaty soils prevail, where it causes
damage in various ways that may well be estimated at several million
dollars annually. It also has been reported to occur in almost every State
of the Continental U.S.A. Although these organisms were formerly
thought incapable of surviving the winter in the northern part of our
country, this is a misconception, for experiments and observations show
some species fully capable of withstanding extreme cold when in the
soil, and of surviving temperatures as low as 100F. if exposed. Root-
knot does not cause such extensive damage farther north, however, as
it produces under warmer climatic conditions apparently because north-
ern summers are short and cool, thus preventing the development of
more than one or two generations.
In the description of the life cycle of the root-knot nematodes we
may best begin with the preparasitic larva (Fig. 12C) after it has hatched
from the egg (Fig. 12 A & B) and migrated through the soil until it
has reached a root tip, where it makes its entry. After migrating to the
axial cylinder of the root, it becomes sedentary, (Fig. 13A) and, with
its short buccal stylet, injects the secretion of its oesophageal or salivary
glands into the tissue of the roots. Thereupon the cells of the root begin
to form from three to five so-called giant cells at the injection point
around the oral opening of the nematode (Fig. 13 B-D). These cells
are also called nectarial cells, from which the nematode absorbs its
food during its entire life. With the intake of food the larva begins to
swell rapidly, first becoming sausage-shaped (Figs. 12 D and 13 B-D)
and then growing to a pear-shaped, whitish body (Figs. 14, 15 & 16).
At this stage of development it may be so large in some instances that
it can be seen with the naked eye (Fig. 17). Under favorable conditions
this development may take three to four weeks, but a much longer time
is often required, particularly when the temperature is suboptimal. The
full-grown female produces eggs (Fig. 18 B) which are deposited in a
yellowish-brown, jelly-like substance which flows from the female geni-
24 DEPARTMENT OF AGRICULTURE
tal opening prior to egg production (Figs. 14 & 16). The number of
eggs produced varies a great deal. The average is considered to be 400 4
to 500, but this number may be much smaller or considerably larger
since over 2,000 eggs have been observed to be produced by a single |
Figure 14.-Drawing of an adult female of a root-knot nematode with attached
egg mass embedded in a protective jelly. x 115
PLANT NEMATODES THE GROWER SHOULD KNOW
female. This variation in the number of eggs is caused not only by
temperature conditions, but also by the kind of host, some plants being -
more suitable than others. Each egg may develop a larva (Fig. 12 A &
Figure 15.-Photomicrograph of a swollen root section of the balsam plant
(Impatiens balsamina) with numerous adult females of a root-knot nematode; the
eggs of one specimen can be clearly seen. X 50
DEPARTMENT OF AGRICULTURE
i), which will break the eggshell and become free in the soil by the
cracking of the root or at the time of its decay. The egg stage is un-
questionably the most resistant, and it is in this stage that the nematode
may survive long periods of adverse conditions. A larva free in the soil
succumbs quickly to adverse conditions such as lack of moisture, exces-
sive heat, direct sunlight, or cold. Males develop under certain condi-
tions and are slender, eel-like organisms of very different shape from
the females (Fig. 12 E and F). Their development is very interesting;
instead of growing to a pear shape like the females, a male larva at the
sausage-shaped stage is transformed into a threadlike, cylindrical nema-
r
Figure 16.-Photomicrograph of two adult female root-knot nematodes in a
balsam plant root (Impatiens balsamina); the giant or nectarial cells are seen as
darkened cell masses in the axial cylinder of the root; the jelly mass secreted by the
females is seen surrounding the broad posterior end of the females, the pointed head
end of which is embedded in (upper specimen) or directed toward (lower specimen)
the giant or nectarial cells. x 135
1" I
PLANT NEMATODES THE GROWER SHOULD KNOW
tode folded and wound up inside the larval molt from which it finally
escapes as a mature male.
It is evident that during its life cycle a root-knot nematode passes
a prolonged period inside the host plant, possibly from three to five
months. The egg and the migratory larvae are the only stages that may
normally occur free in the soil. In control procedures this is an important
point. Fumigants applied to the soil sometimes reach only larvae and
eggs free in the soil, while specimens inside a root, particularly if it is
woody, may not be affected. The complete uprooting of plants will
Figure 17.-Photomicrograph of female root-knot nematodes embedded in the
tissue of a potato of which a thin surface portion was sliced off. In potatoes these
adult females may often be recognized with the naked eye. x 3
DEPARTMENT OF AGRICULTURE
eliminate all stages inside the roots-eggs, larvae, and adults-if the roots
are destroyed. The development of the larvae is interrupted if the root
on which they are feeding is disturbed, and they will perish when the
root dies. In fleshy roots, however, such as peony, and in rhizomes,
corms, tubers, and bulbs development may go on, if these plant parts
Figure 18.-A. Photomicrograph of a larval specimen of a root-knot nematode
in the tissue of the same potato shown in Figure 17. B. Photomicrograph of an
egg mass of the nematode in the tissue of the same potato as shown in Figure 17.
x 85
PLANT NEMATODES THE GROWER SHOULD KNOW 29
are stored or otherwise kept under conditions favorable to the nematode.
This situation explains the many opportunities for the distribution of
this pest through infected plant material as, for example, nursery stock
and root crops such as potatoes, carrots, etc. The spread of the larvae
by active migration is slow.
Over 1700 different plant species are known to be attacked by root-
knot nematodes. However, they exhibit a wide variation in the degree
of susceptibility to attack and in the seriousness of the disease that
follows. Thus some suffer severely from a slight attack (e.g. cyclamen)
while other species are very tolerant (e.g. mulberry tree). It should be
mentioned here that germinating seeds and young seedlings are particu-
larly attractive to root-knot nematode larvae (Fig. 8) including even
seedlings of plants otherwise immune or highly resistant. Seeds should
therefore never be planted in soil badly infested by root-knot nema-
todes, or by any other plant-pathogenic nematode form, nor should such
soil be used for seed-testing purposes. Germination will be poor and
most of the emerging seedlings will be sickly. Thus the seedlings of the
tung oil tree (Aleurites fordii Hemsl.) are very susceptible and may die
from an early infection but after the first year may recover and then
develop a high degree of resistance.
It was mentioned earlier that in some plants the root-knot nematodes
break through the root surface so that the egg-producing females pro-
trude from the root and may then be seen with a hand lens as globular,
whitish bodies with yellowish or brownish egg masses attached. (Some-
times not even swellings are formed.) Under such conditions the para-
site is even more injurious than in those cases where a smooth, un-
cracked gall without necrotic tissues is formed. This is due to the fact
that roots cracked or opened by the action of nematodes are at once
invaded by a whole group of secondary agents including other nema-
todes, fungi, and bacteria and this usually leads to quick decay. Plants
thus attacked naturally suffer much more than those which form
"smooth," uncracked or, if such an expression is permitted, "healthy" galls.
Frequently growers or scientific workers looking for these nematodes
with the conception that galls or knots are its symptoms, have often
considered the nematode absent because there were no galls. Thus they
have missed the root decay initiated by the nematodes which is usually
much more serious than the galls.
SEDENTARY TYPES
This leads us to other sedentary types of plant-parasitic nematodes
of roots, namely the sugar-beet nematode and related species like the
golden nematode of potatoes. In these species even the growing larvae
normally protrude from the root surface and the adult females generally
hang on the outside of a root. In contrast with root-knot nematodes, the
members of the sugar-beet nematode group have a characteristic which
DEPARTMENT OF AGRICULTURE
makes their control much more difficult. The old females, before drop-
ping from the root or tuber, turn from a whitish color to brown as their
cuticle is transformed into a thick, protective cover with a variable
number of eggs and larvae contained inside. Females thus transformed 1
are then called "cysts" and may remain in the soil for years, releasing
active larvae over a period of years. This phenomenon not only compli-
cates control of these pests when crop rotation schemes are used but
also when fumigants are used, since these "cysts" are highly resistant to 1
the action of chemicals. We know of no species of this group occurring
in Florida but are convinced that at least one form, Heterodera weissi,
Steiner, 1949 (Fig. 19) may be present. It is found on various species
of knotweeds and is known to us to occur in numerous states east of the
Rocky Mountains, as far south as the Delta section of southern Missouri
and the region of Ridgeville, Georgia. I
Root-knot nematodes, together with the various species of the sugar-
beet nematode group, represent sedentary root parasites which are all
Figure 19.-A. Male of Heterodera weissi, the knotweed heterodera, located
under the surface of a root of Polygonum pensylvanicum L. B. Female Heterodera
weissi as it hangs on a root of the same knotweed as above; X 100.
PLANT NEMATODES THE GROWER SHOULD KNOW
-.root
- - type of ann
- .pex
- -. pip
Figure 20.-Drawing of a female of the citrus nematode, Tylenchulus semipene-
trns Cobb, attached to a root. The elongated head end is shown inserted deep
into the root tissue so that the cells of the axial cylinder are reached. p ex, excretory
pore; type of ann, annulation of cubicle, only partly drawn; vlv, vulva.
0-.
B..
Figure 21.-The kidney-shaped nematode, Rotylenchulus reniformis Linford. A.
Pieces of roots of Jacquemontia tamnifolia L. Griseb with specimens of the kidney-
shaped nematode attached to them. Their body is covered with small soil particles
which are held together by a glabrous secretion produced by the nematode. Thus,
the uninformed will never suspect that the small globular bodies attached to these
roots are not parts of soil but hide a sedentary root-parasitic nematode. x 24. Sam-
ple collected by A. L. Smith at Cuthbert, Ga. B. Tomato root infected with the
kidney-shaped nematode; the glabrous cover surrounding the nematodes has been
teased off to expose their bodies; their elongated neck, with which they penetrate
the root, may be seen. Tomato plant experimentally infected in a greenhouse. x 170.
PLANT NEMATODES THE GROWER SHOULD KNOW 33
closely related and difficult to differentiate. The adult female has a
swollen, spherical, lemon- or pear-shaped body, unlike most other nema-
todes, while the male retains a threadlike body shape.
But this group is not the only one in which sedentary life has
resulted in transformation of the female by globular inflation. The same
phenomenon is seen in other genera of quite different origin and rela-
tionship; an outstanding example is Nacobbus, a form only recently
described by Thorne and Allen from California. Here the female has a
protruding vulva of such length that it resembles a tail; this enables it
to deposit the eggs outside the root since the protruding tail-like vulva
appears always so located as to reach the root surface.
The citrus nematode (Tylenchulus semipenetrans Cobb) is still an-
other example of a swollen, sedentary type of root parasite. It belongs
to still a different taxonomic group from the two already mentioned. In
this instance the roots are attacked from the surface and only the elon-
gated anterior portion of the body penetrates the root tissue, while the
posterior part swells, as shown in Fig. 20. Here again, however, the
male is of very different shape and appears not to feed at all. In fact
it hardly increases in size while developing from the larval to the adult
stage.
There is still a fourth type of swollen sedentary root parasitic nema-
tode, the kidney-shaped nematode (Rotylenchulus reniformis Linford).
The parasitic female is attached to the root surface but its body is sur-
rounded by a spherical structure which it forms for itself by secreting
a glabrous substance which also surrounds the eggs and cements together
adjacent soil particles. We located this interesting species recently from
Florida on roots of tomato and coffee weed (Cassia tora L.) sent to us
from Quincy. As in the citrus nematode, the anterior portion of the
body of the female is elongated and is inserted into the root while the
reniform, main part protrudes from the surface. The male in this case
also remains small and its body is not swollen. Because of its bubble-
shaped glabrous enclosures covered with soil particles, Rotylenchulus is
very difficult to see and may easily be overlooked or mistaken for globu-
lar soil agglutinations adhering to the root (Fig. 21).
It is obvious that all these sedentary nematodes with inflated and
obese bodies that become fixed in or on a root lose their motility and
are unable to change their position and migrate. If roots containing such
stationary parasites are uprooted and destroyed before the parasites are
fully grown or before they have produced eggs, the source of infection
is removed. Control of certain of these nematodes by a trap- or catch-
crop is based on these considerations. Since only the preparasitic, larval
stage of these sedentary forms is able to migrate, the spread of these
various types by their own means is very restricted. Thus long distance
travel is entirely one of transportation by such carriers as wind, water,
and particularly man, who spreads them with plant material of all kinds
and with soil.
34 DEPARTMENT OF AGRICULTURE
- ( . h t ylO~
-..1w
dr1~
am.'I
Figure 22
PLANT NEMATODES THE GROWER SHOULD KNOW
There exist other sedentary root parasitic nematodes which in con-
trast with the foregoing groups have preserved a certain, though reduced,
motility; they are called ring and scale nematodes and include various
Figure 23.-A ring nematode (undescribed species) attacking the root of a
pummelo (Citrus grandis) grown in a greenhouse at the Plant Industry Station,
Beltsville, Md. Its head is buried in the root surface; several nearby surface cavities
were probably made by the same nematode; x 75.
Figure 22.-Two different types of ring nematodes. A. Criconemoides citri
Steiner 1949, representing a broadly annulated type, infecting roots of the sour
orange in the region of Orlando, Fla. The roots are punctured from the surface with
the remarkably strong buccal stylet and the head itself is frequently buried deep
in the root tissue which becomes necrotic around the organism. B. Criconema ci-
vellae Steiner 1949, representing a very unusual type of nematode with its 8 longi-
tudinal series of fringed scale-like structures on the body surface. It was found
feeding on the roots of Citrus grandis (L.) Osbeck, the pummelo, grown in a
greenhouse of the Plant Industry Station, Beltsville, Md.
36 DEPARTMENT OF AGRICULTURE
genera and numerous species belonging to the Criconematinae and other
related groups (Fig. 22 A & B). They too are ecto-parasites, puncturing
the surface of roots (Fig. 23) and other subterraneous parts with their
stylets which are strong and often extremely long. All members of this
group are short and stout and heavily annulated. In some, the annules
are provided with retrorse scales or spines, or a combination of the two.
This annulation and its armature obviously enables the members of
these groups of rather unusual looking nematodes to get the necessary
support and resistance in the soil while they press their stylets and
heads into the plant tissues. Locomotion also is possible mainly through
the use of the annules and their scales and spines. Certain types of these
groups (e.g. Hemicycliophora deMan) are of pronounced, sedentary
habit (Fig. 24). They retain their moulting skins, which make an addi-
tional protection for their bodies while quite permanently attached to a
NI
S^
Figure 24.-Hemicycliophora sp. attacking the roots of slash pine seedlings
(Pinus caribaea Morelet) near Olustee, Fla. The arrows point to groups of nema-
todes attached to blinded root tips; X 25.
PLANT NEMATODES THE GROWER SHOULD KNOW 37
-70
Figure 25.-Drawing of a spiral nematode (Helicotylenchus sp.) attacking a
rootlet of sweet clover (Melilotus sp.) The anterior end of the nematode has
deeply penetrated the root tissues; dotted region indicates the extent of cell necrosis
evidenced by discoloration; specimen collected by F. R. Jones, Madison, Wis.;
x 333.
DEPARTMENT OF AGRICULTURE
root. Locomotion of these animals with a double sheath around them
would be extremely difficult.
Finally there are root-surface parasites with well developed loco-
motive ability which nevertheless pass a sedentary existence on roots
for extended periods. One group of these are ancestors of the kidney-
shaped nematode previously mentioned. They are forms widely dis-
tributed and because their long body is usually kept in the shape of a
spiral they are called spiral nematodes (e.g. Helicotylenchus Steiner;
Rotylenchus Filipjev). Their buccal stylet is quite long and strong and
well fitted for inserting into plant tissue. Often the head and part of
the anterior portion of the body are also inserted into the root (Fig.
25). It is thought that this procedure is made easier by the retention of
their bodies in a ventrally curved spiral of one to two turns. These
spiral nematodes are still little known but are of wide distribution.
Where numerous they may seriously interfere with plant growth. Helio-
cotylenchus nannus Steiner is a small but very common species in the
southeastern U.S.A., including Florida.
Of similar behavior are the pin nematodes (genus Paratylenchus
Micoletzky). The body of this genus, particularly that of the female,
is venturally curved but to a lesser degree than in the spiral nematodes,
possibly because they are much shorter. All members of this group are
unusually small, and for this reason they frequently have been over-
looked. It now appears that they are rather common, sometimes occur-
ring in enormous numbers. The buccal stylet of the various species
is always long, although the length differs considerably. This organ may
be inserted into the plant tissue for a considerable portion of its length,
a fact well shown in the folds of the esophageal canal when the stylet
is retracted. The males in this group are all of a vestigial character,
in some species very rare or even unknown, and their stylets are usually
weak and degenerate (Fig. 26).
MIGRATORY NEMATODES
In contrast to the various types of fully or partially sedentary root-
parasitic forms, there exists a large group of migratory nematodes be-
longing to several different genera and families. These may be classed
into those forms that enter the tissues, move through the root and live
inside it, as well as others that puncture a root and feed on it from
the outside.
The most important of the first mentioned group, i.e. those of endo-
parasitic habit, are those species belonging to Pratylenchus Filipjev and
related genera, or the so-called meadow nematodes. We know of at
least four different species of Pratylenchus from Florida alone. Meadow
nematodes are also widely distributed elsewhere. They attack the roots,
tubers, rhizomes, corms, or bulbs of various crops. Occasionally they
are also found in stems. A short, stout body and a strong buccal stylet
PLANT NEMATODES THE GROWER SHOULD KNOW 39
make them migratory, tissue parasites, par excellence. From an economic
point of view they are major pests. Unfortunately, their significance,
common occurrence, and widespread distribution have been almost
completely ignored until very recently. The various species are diffi-
cult to distinguish and their classification is still greatly confused. A
form attacking the Irish potato was recorded as one of the first plant
nematode pests in this country. It is Pratylenchus scribneri Steiner
'which was observed in 1889 by F. Lamson Scribner on potatoes in
Tennessee. It was not given a name then, but recently has been estab-
lished as a true species. Observed in different states east of the Rocky
Mountains, it appears to occur mostly in potatoes. A second species,
P. leiocephalus Steiner, 1949, well characterized by the angular contour
of its head, has been found on potatoes in Florida as well as on a va-
riety of other crops. (Fig. 27). While these two species are monosexual
forms, there are other species which are bisexual. In entering and
migrating through root tissues they destroy them by breaking through
cell walls and feeding on their contents with organisms of decay follow-
ing in their wake. Root tissues penetrated by meadow nematodes usually
,exhibit necrotic lesions which frequently are reddish in color at first,
turning dark brown to black later on. The eggs are deposited within the
roots, where, not uncommonly, accumulations of specimens, including
larval stages, adults and eggs, may be seen occurring as "nests." Obser-
vations show these forms to be extremely destructive (Fig. 28 A & B)
since by attacking and destroying a root at a point close to the stem
they render useless the entire distal portion of the organ. Certain plants
try to repair the damage by forming new side roots above the attacked
points. Then as new lesions are developed on these side roots still
more side roots are formed with the result that root systems attacked
by these nematodes frequently exhibit a bearded or matted appearance.
Sometimes the cortex of roots so attacked begins to decay and then
easily slips off. The axial cylinder is rarely entered. On potatoes surface
pimples are formed which later change to blackish depressions. When
very numerous, these pimples and depressions disfigure the tubers to
such a degree that the crop is rendered unmarketable (Fig. 29). Under
certain conditions, particularly in orchards and on perennials where
they remain undisturbed for years, they may multiply to enormous num-
bers. Cases have been observed where large trees lose significant por-
tions of their root systems by the attacks of great numbers of these
meadow nematodes, with the result that branches, limbs, parts of the
crown, and even entire trees show the effect by yellowing foliage, de-
foliation, death of limbs and branches or even of whole trees. It is dif-
ficult to estimate the damage caused by this type of nematode on plants.
They cannot live on dead, decaying tissues and are not generally found
in roots in the process of decay. Such roots abound with a variety of
fungi and bacteria and so-called saprophytic nematodes, while the
primary culprit has escaped to the surrounding soil or to still healthy
tissue. The study of the meadow nematode is still in its infancy. Ex-
40 DEPARTMENT OF AGRICULTURE
pas.. -
Figure 26
PLANT NEMATODES THE GROWER SHOULD KNOW
perimental work with this group is difficult because of their small size
and their migratory way of life.
There exist, of course, many other nematode parasites of similar
habits that belong to the same or related taxonomic groups, but the
meadow nematodes are apparently the most destructive and most widely
distributed.
MISCELLANEOUS TYPES
Since it would not be possible to mention here all these other types,
only a few will be listed. Some are known to occur in Florida and other
southern states. Among them the hoplolaims or lance nematodes espe-
cially should be mentioned. They appear to be mainly a tropical and
subtropical group of migratory root parasites. Various species from
Florida soils are known to us, while farther north only one form, Hoplo-
laimus coronatus Cobb, as yet has been observed. The lance nematodes
are stout, cylindrical, rather large-sized parasites with an extremely
strong buccal stylet, the tail end usually being obtuse and the cuticle
coarsely annulated. They somewhat resemble the meadow nematodes
in their life habits and may also occur in large numbers on roots if
undisturbed for considerable periods of time. In combination with other
pathogenic agents and adverse environmental conditions such as drought,
a killing effect by these nematodes has been observed, particularly on
seedling plants. In Florida they have been noticed as pests in forest
nurseries, parasitizing seedlings of Pinus palustris Mill. Often only sur-
face parasites, the hoplolaims may also enter the roots with the whole
body. They are therefore endo- as well as ectoparasites (Fig. 30).
Other extremely interesting types are the awl nematodes, Dolicho-
dorus heterocephalus Cobb (Fig. 81), and the sting nematode, Belono-
laimus gracilis Steiner, 1949. (Fig. 32). Both are found in Florida and
both feed on roots which they puncture from the surface with their long
needle- or awl-shaped buccal stylets. The former was observed on
celery roots near Sanford. The latter was found on corn roots, also
from Sanford, and on roots of slash and long leaf pine seedlings in
various forest nurseries (Ocala, Brooksville, Valparaiso).
In recent years much progress has been made (through the work of
J. R. Christie, V. G. Perry, and others) in establishing the wide distri-
bution and the virulence of these and still other ectoparasitic nematodes
Figure 26.-Paratylenchus elachistus Steiner, 1949, a pin nematode that was
found attacking the roots of ramie (Boehmeria nivea (L.) Gaud.) in the region of
Clewiston, Fla. Material collected by W. D. Wylie, December 1943. A. Female;
cer ppl, cervical papilla; lat fid, lateral field, x 500. B. Head end of female; dsl
oe gl, dorsal oesophageal gland, ampulla outlet; subv oe gl, subventral oesophageal
gland outlet; x 1800. C. Male; phas, phasmid (?); x 500.
42 DEPARTMENT OF AGRICULTURE
a F i A
Figure 27
PLANT NEMATODES THE GROWER SHOULD KNOW 43
Figure 28.-A and B. Root systems of two dying corn seedlings (Zea mays L.)
heavily attacked by the smooth headed meadow nematode (Pratylenchus leioce-
phalus) (see Fig. 27). These seedlings were collected by R. J. Humphrey near
Sanford, Fla.
Figure 27.-The smooth headed meadow nematode, Pratylenchus leiocephalus
Steiner, 1949. A. Adult females; x 333. B. Anterior end; note the 3 nuclei of
the oesophageal glands and their arrangement; nrv r, nerve ring; x 500. C. Head
end note the angular outline of the head proper and the 3 basal knobs of the buccal
stylet which are so well amalgamated as to appear as a single spherical knob; x
1222. D. Front view of head; amph, amphid, a chemical sense organ; cph ppl,
cephalic papilla, an organ of touch; X 1222. E. The basal knobs of the buccal
stylet as seen in optical cross section; X 1222. F. Tail end of female, drawing to
show the annulation of the cuticle and its interruption by the lateral field; phas,
phasmid, an outlet of a gland; x 500. G. Tail end: anus; phas, phasmid; vlv, vulva;
note the short postvulvar uterus branch; x 500. H. Tail end in ventral view; anus;
phas, phasmid; vlv, vulva; x 500. I. Egg as it is deposited; note the kidney form
which is characteristic for Pratylenchus eggs; x 500. J. Egg containing larva of
which the pear-shaped end bulb of oesophagus is to be seen; x 500. K. Egg with
larva of which tail end is to be seen; X 500.
44 DEPARTMENT OF AGRICULTURE
in the South. Many additional types of root-attacking nematodes could
be mentioned. Usually most of them occur only in smaller numbers
and are not known to cause serious diseases. Future research, however,
may uncover many surprises.
HOST PLANTS OF ROOT-KNOT NEMATODES
Many plants are more or less immune to root-knot, i. e., the worms
do not find their roots suitable medium in which to grow and reproduce.
Probably there are substances in the sap which poison the worms. As
a whole plants of the true grass family, including corn, oats, rye, canes,
sorghums and millets, as well as Crab and Bermuda grasses, etc., are
quite resistant. Still some varieties of corn, oats and cane are sometimes
seriously attacked. Velvet beans and beggarweeds are practically im-
mune. Generally speaking, trees and shrubs are not as apt to be at-
tacked as herbs. There are numerous exceptions to this rule.
Other plants seem to be more or less tolerant, i.e., they are able to
make satisfactory growth and produce a good crop of fruit although
Figure 29.-Potatoes attacked by Scribner's meadow nematode (Pratylenchus
scribneri Steiner). The Figure at left shows a potato with a lighter infection, while
that of the one at right is very heavy. The arrow in the figure at right points to a
single small lesion from which alone 335 nematodes were isolated; thus the nema-
todes in this potato were conservatively estimated at from 12,000 to 15,000.
they are heavily attacked and their roots are badly knotted. Mulberry
trees and sunflowers are such plants.
Unfortunately, most truck and garden crops are severely injured by
the worms. There are, of course, all graduations between highly suscep-
tible and practically immune plants. The following list includes most
of the susceptible plants commonly grown in Florida, given in the
PLANT NEMATODES THE GROWER SHOULD KNOW 45
Figure 30.-Drawing of a root fragment of longleaf pine (Pinus palustris Mill.)
exhibiting the posterior ends of three different specimens (arrows) of Hoplolaimus
coronatus Cobb protruding from the broken root surface; 65 specimens were counted
from a piece of root one inch long. Material collected by the author near Tifton,
Ga., November 1940; x 136.
approximate order of their susceptibility to damage, which is not neces-
sarily that of the abundance of nematodes in their roots:
1. Okra 15. Old World Grapes 30. Rape
2. Tomatoes 16. Irish potatoes 31. Radishes
3. Eggplant 17. Watermelons 32. Sweet potatoes
4. Cucumber 18. Lettuce 33. Asparagus
5. Cantaloupes 19. Careless weed 34. Soybeans
6. Celery 20. Beets 35. Pecans
7. Beans 21. Pineapples 36. Sugar cane
8. Dasheens 22. Cotton 37. Mustard
9. Peppers 23. Cabbage 38. Violets
10. Squash 24. Cauliflower 39. Japanese persimmons
11. Figs 25. Collards 40. Catalpa
12. Peas 26. Sunflowers 41. Kudzu
13. Peaches 27. Carrots 42. Quince
14. Roses 28. Bananas 43. Peanuts
29. Papaya
On land which is heavily infested it is impossible to profitably grow
okra, tomatoes, eggplants and others near the head of this list.
46 DEPARTMENT OF AGRICULTURE
I' !
A D
Figure 31.-The awl nematode, Dolichodorus heterocephalus, Cobb, an interest-
ing type of root parasitic nematode because of its awl-shaped, long, buccal stylet and
the trilobed bursa in the male. A. Anterior end; dsl oe gl, outlet of dorsal oesopha-
geal gland; x 500. B. Cross section through middle region of body to show the
structure of the lateral fields consisting of two longitudinal bands separated by three
longitudinal grooves; gr lat, grooves in lateral field; x 500. C. Tail end of 9 seen
dorsally; anus; phas, phasmid; X 500. D. Tail end of 8 seen dorsally to show the
trilobed bursa; phas, phasmid; sp, spicula, gub, gubernaculum; x 500. The speci-
mens of this parasite here partly sketched were collected by A. L. Taylor on celery
roots near Sanford, Fla. in December 1943. In the "Plant Disease Reporter" of
December 15, 1943, p. 707 they were recorded as a species of a genus closely related
to Paratylenchus; however, it is now certain that Dolichodorus represents a separate
subfamily not directly related to Paratylenchus.
PLANT NEMATODES THE GROWER SHOULD KNOW
Figure 32.-Belonolaimus gracilis Steiner, 1949, the sting nematode. It has been
observed attacking the roots of slash pine and longleaf pine seedlings in various forest
nurseries in Florida. A. Head end; o gl, outlet of dorsal oesophageal gland; X 450.
B. T il end of a in ventral view; phas, phasmid; x 450. C. Tail end of 9 in side
view; |phas, phasmid; x 450.
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