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POTATO VIRUS Y IN PEPPERS AND TOMATOES:
History and Importance on the Lower East Coast;
Weed Control and Bacterial Spot Relationships
J. N. Simons, J. R. Orsenigo, R. E. Stall and P. L. Thayer
This report was prepared especially for a meeting of vegetable growers
held in the Mounts Building, West Palm Beach, on May 13, 1959.
SUMMARY
This report discusses the incidence of potato virus Y in tomato and
pepper along the lower East Coast during the period from its discovery in
1956 to the present. This virus is spread from plant to plant through the
feeding of winged aphids; nightshade is the only important native host plant.
Procedures useful in restricting the spread of this disease include:
1) Eliminate the weed host.
2) Use nonsusceptible barrier crops
3) Avoid planting spring crops next
4) Maintain plants free from other
spot.
Sections pertinent to weed control
control are included in the report.
around the field.
to diseased fall crops.
diseases, particulraly bacterial
recommendations and bacterial spot
Everglades Station Mimeo Report 59-31
Belle Glade, Florida
May 13, 1959
I. The Control of Potato Virus Y on Pepper and Tomato
J. N. Simonsi/
This past growing season, 1958-59, has seen the occurrence of what may have
been the most destructive outbreak of virus infection of peppers and tomatoes
in the history of the Lower East Coast farming area. Because of the severe
losses suffered this year, and since it is almost certain that this disease
will be a serious problem next year, the research and extension workers of the
area are making an intensive effort to avoid repetition of this year's disaster.
It is the aim of this report to point out in detail what is involved in the
spread of this disease, the important factors that influence its destructiveness,
and the procedures that are useful in a control program.
History of the Disease
The first case of a pepper or tomato plant infected with potato virus Y
in eastern Palm Beach County was observed in the Spring of 1957. The disease
had been destructive to pepper crops planted along the shore of Lake Okeechobee
for many years but was restricted to tha part of the County. The discovery of
the disease along the East Coast was not unexpected.
It had been observed that this virus occurred in south Florida only where
potatoes had been grown commercially; i.e., Homestead, Belle Glade, Bradenton.
Both the East Coast and Immokalee sections were free of the disease and neither
had a history of commercial potato production. Since seed potatoes carry a
low percentage of this virus it seem reasonable that: 1) the distribution of
the disease was correlated with potato culture; and 2) with the advent of
potato growing in either the East Coast or Immokalee sections virus Y should
be introduced into those areas. Thus, when the virus was found in pepper on
the East Coast the year following the introduction of potatoes, the expected
had happened.
Losses in the Spring of 1957 were negligible, but the virus became firmly
established in certain weeds (mainly nightshade) where it was carried over
during the summer. During the 1957-58 season the virus appeared over a large
area of the East Coast. Losses were generally moderate but a few late pepper
and tomato growers and one hydroponics tomato grower suffered heavy yield reduc-
tions. We all know only too well what happened during this past 1958-59 season.
How Does the Disease Spread?
Like most plant viruses this virus can only spread when certain factors
are present. These include:
1. A source of virus must be available.
2. An insect capable of transmitting the virus (a vector) must be present
and moving.
3. A susceptible crop must be growing.
I/ Assistant Virologist, Everglades Experiment Station, Belle Glade.
The most important naturally infected plant of virus Y is nightshade. It
is in this plant that the virus lives during the summer and it is from this plant
that the virus spreads into the crop during the fall and winter. Once the virus
has been established in the crop the importance of the nightshade host is
greatly lessened. Most of the spread after this time is from plant to plant in
the crop.
The insect vectors of this disease are aphids. No other insects are known
to transmit potato virus Y and it appears very unlikely that any others do.
Certainly the spread of this virus in our area is very well correlated with
the numbers of winged aphids, particularly the green peach aphid, that are pre-
sent. It should be emphasized that so far as is known the virus is transmitted
only by aphids. It is not carried in the wind, in the soil, in the water or in
the seed. It is not spread by handling plants.
We know a great deal about how the aphids transmit the virus. It takes
but a few seconds (10-15) of feeding for an aphid to acquire virus from a dis-
eased plant. Likewise it takes but a few seconds for the aphid to inoculate
virus into a healthy plant. Aphids that have fed for long periods of time
(hours) almost never are infective; thus the vector aphids are those that flit
from plant to plant. The reason that aphids lose their infectivity after several
hours of feeding is that the virus is concentrated in the outer part of the leaf
while in the center of the leaf where the aphid usually feeds there is but little
virus. Once an aphid has acquired virus it begins to lose it immediately. With-
in 5 minutes there is only half the chance of it causing an infection and within
10-mifate's .thereis only 'one*fourth-the probability of'it causing en infection
as compared to when the aphid first became infective. No aphid retains infec-
tive virus for more than one hour.
How Far Can an Aphid Carry Virus?
Practically all of the infections are caused by winged aphids. The
distance that a winged aphid can carry a virus has been the subject of much
investigation. We are certain that most infections come from flights of less
than 100 feet. Yet it is obvious that if an aphid can remain infective for an
hour and if it is caught in a 10 m.p.h. wind it can be blown for ten miles
before losing its infectivity. The points that have to be kept in mind are
these:
1. The greater the distance between a diseased weed and a healthy plant
the less chance the aphid has of landing on the healthy plant.
2. The greater the numbers of aphids the greater the chance of one of
them landing on a healthy plant.
3. The greater the number of diseased weeds or crops the greater the
chance of an aphid picking up virus.
4. Aphids normally fly short distances and spend several minutes on each
plant they land on.
5. Aphids will not fly when the wind is over 5 m.p.h. and do most flying
during winds of 0-3 m.p.h.
6. Aphids do not fly at night.
7. Not every aphid acquires virs when it feeds on a diseased plant.
8. Not every infective aphid transmits virus when it feed on a healthy
plant.
It should be apparent from these facts that it is not enough to say an
aphid can carry virus for a mile. The whole point is -- what are the chances
for this to occur. It could be summed up this way -- there is very little like-
lihood of an aphid carrying virus more than a few hundred feet during the fall
and winter months; there is very little likelihood of an aphid carrying virus
for over a quarter-mile during late spring when aphid populations are at a peak
and the area is well covered with diseased plants. Always keep in mind that
99.9 plus percent of the aphids that come into a field are not caxbying a virus.
Most virus transmission is the result of aphid flights it-iZb~ th field. That
is why you see circular patches of disease and gradientT wofnfection that
lessen as you get into the field.
Control of Virus Y /
There are many possibilities for virus control that can be suggested. The
most obvious are listed below and the reason (s) why they do or do not work are
stated.
1. Kill the aphids after they have some into the field -- This does not
work because the aphids can acquire and transmit virus before the insecticide
takes effect.
2. Kill the aphids before they get into the field -- There is evidence
that spraying around a field (ditch banks, etc.) with an aphicide will reduce
the chance of aphids bringing in virus from ditchbanks. By next fall we will
probably be able to report how useful this may be. One of the biggest draw-
bakes is that insecticides like parathion cannot be sprayed indiscriminately on
neighboring pastures, crops and homes without endangering other peoples' pro-
perty. Also, and just as important, the use of an insecticide like parathion
can upset the natural balance between aphids and their predators and parasites
and result in heavy aphid buildup after.the spray program is discontinued.
3. Treat the crop with a chemical that protects the plant from infection.
There are no such chemicals as yet.
4. Rogue out the diseased plants -- This may be worth doing the first time
you see a diseased plant but fails to give control in the long run. This is
because an infected plant can be used as a source of virus by an aphid before
symptoms appear. Thus, a roguing operation is never 100 percent thorough.
5. Use resistant varieties There are no varieties of pepper or tomato
that are resistant to virus Y. The mosaic resistant pepper varieties are re-
sistant only to tobacco mosaic virus.
*6. Eliminate the weed hosts of the virus -- Since we know that all of
our virus infections originally come from diseased weeds the eradication of
these weeds is the most direct approach possible in eliminating the virus. Because
of this directness it follows that this should be a very effective control method.
2/ These procedures also apply to viruses affecting squash, cucumber and
watermelon.
* Indicates recommended treatments.
The two most important factors in this approach involve timing and completeness
of kill. To be effective the weeds must be controlled when no crops are in the
area. Once the virus is in the crop no amount of weed control will prove effec-
tive. The degree of kill must be high. Simply destroying the top growth of the
nightshade is not enough. The virus is in the roots of the nightshade and re-
growth will be just as infectious as before. The treated area must include not
only ditchbanks but weed and volunteer crop plants in the field itself. One
thorough treatment during the summer should be adequate. Ideally this treatment
should be timed to be as near planting time as possible. Late July and early
August are recommended.
The biggest drawback to the weed control approach is that it will often
involve cooperation among several growers. Having an abandoned field next to
a pepper field can pretty well nullify the efforts of a farmer on his own farm.
The same thing is true to a lesser extent if the farm next door is in operation
but there has not !ez en ny. ditchbank weed control. This is particularly true
during the Spring when large numbers of winged aphids are present.
*7. Use nonsusceptible barrier crops around your field -- If a good weed
control program is in effect there is merit in protecting a crop on the outside
of the field with a nonsusceptible crop of from 50-100 feet in width. For virus
Y such crops as beans, corn, cucumber, eggplant, and squash could be used. Low
crops are as good as tall ones. The reasons that a barrier crop works are three-
fold. First, it actually means there will be a greater distance between diseased
weeds and the susceptible crop. Second, aphids fly short distances and will feed
on the barrier crop plants for a long enough time so that most of them lose virus
before getting to the susceptible crop. Third, aphids lose virus faster while
feeding than while flying.
The principal drawback to this scheme is that many farmers only grow one
crop and cannot easily set up their plantings to utilize a barrier. It should
be emphasized that a barrier crop will not entirely prevent virus-laden aphids
from entering the crop. It will ordinarily delay by several weeks the time it
takes for virus to get into the crop. This can mean a great deal in the effect
on the yield.
Spraying parathion on the barrier crop is worthwhile. The sprays should be
applied on a 4-5 day schedule.
*8. Avoid planting spring crops next to diseased fall crops. -- A sure way
to get into trouble is to plant a late tomato or pepper crop next to a badly
infected acreage of tomatoes or peppers. If at all possible destroy the old
pla-ning before putting in the young one. With a good weed control program
mot; plantings made up to November 1 should escape serious virus Y damage.
Beyond that time the situation can change drastically.
*9. Grow the healthiest crop possible insofar as other diseases are con-
cerned -- Since the greatest loss to virus Y infection is correlated with early
infection it stands to reason that growing a vigorous crop is worthwhile. This
is particularly true insofar as the bacterial spot disease is concerned. Plants
that are badly stunted becease of poor bacterial spot control will suffer more
from virus infection than healthy, rapidly growing ones. These plants are not
more susceptible to the virus but are unable to withstand the ravages of the
disease the way healthy plants can.
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10. When in doubt call on the Extension Service or the Experiment Station --
There is no excuse for "going off the deep end" with this virus problem. Do not
believe the reports that spring up from time to time about miraculous cures, etc.
unless they are verified by the Experiment Station. A 50-cent telephone call
would have saved a lot of growers considerable expense this spring when the "milk-
cure" report came up.
What will be the Effect of A Good Weed Control Program?
There is not much question as to the necessity of an area wide nightshade
eradication program if a repeat of this years' disaster is to be avoided. Almost
every nightshade plant in Eastern Palm Beach County is today infected with potato
virus Y. These plants must be destroyed this summer,
Naturally there will still be some infected nightshade around next fall as
the eradication of this weed is not possible. The extent of this remnant popu-
lation and the time of buildup of the aphid population will determine what
happens to the pepper and tomato crops next year. The best that can be hoped
for is a delay in the appearance of the disease until the crops are nearly mature.
In the case of peppers, fruits already set at the time of infection are generally
not affected. Tomatoes do not show fruit symptoms and plants that remain healthy
until 2 or 3 hands of fruit are set usually get by in good shape.
Almost certainly at the end of next years' growing season there will be a
great deal of virus Y in the crops. If the onset of the disease is held back
sufficiently losses will not be heavy. The situation is fairly optimistic inso-
far as learning to live with this virus is concerned but the grower must never
lose sight of the fact that there is no substitute for the program outlined here
and the time for action must be this summer
II. Suggestions for weed control programs
J. R. Orsenigo V/
As indicated earlier, one particular weed must be eradicated to control
potato virus Y infections; that weed is NIGHTSHADE (Solanum gracile and/or
-S. ig ). Another host plant, the ground-cherry (Physalls sp.) is not
common in the Florida East Coast area and where present is controlled by
treatments intended for nightshade.
Wild cucumber, Melothria pendula, the host of watermelon: mosaic virus
is also controlled by these treatments.
Herbicide suggestions:
Off-Season: (June, July, August); fields and ditchbanks:
1. Flame (weed burner) particularly LP gas burners'operating at 20000
or higher temperatures. A repeat application two to three weeks after
the initial burn will be necessary.
*2. 2, 4-D plus 2,4,5-T: use amine salt or low volatile ester formulations;
each at 3/4 1b/A, acid equivalent 4/ Approximate cost of this 1 1/2 lb/A
acid equivalent treatment: $2.50 A.
*3. 2,4-D plus 2,4,5-T: amine salt or low volatile ester formulations;
each at 1 Ib/A, acid equivalent. Approximate cost of this 2 lb/A acid
equivalent treatment: $3.50/A
*4. 2,4,5-T: amine salt of low volatile ester formulations t 1 1/2
Ib/A acid equivalent. Approximate cost of this treatment is -,40/A.
Application hazards: When applied properly, 2,4-D, and 2,4,5-T "hormone
type" materials should present no problems in the "off-season." See
spraying practices-below.
3/ Assistant Horticulturist, Everglades Experiment Station, Belle Glade.
Indicates recommended treatments
k/ These chemicals are purchased as liquids and several concentrations
are available. Probably the most common contained 4 lb/ gallon, acid
equivalent; therefore, a 1 lb/A rate would require one quart to treat one
acre. Combinations of 2,4-D and 2,4,5-T (brush killer) herbicides are
commercially available. However, it may be considerably more economical
to buy 2, 4-D and 2,4,5-T separately and mix in the tank at time of spraying.
If purchased separately be sure to use the same formulations; preferably
from the same manufacturer or formulatcr.
Residual hazards: 2, 4-D and 2,4,5-T are subject to both chemical and
microbial breakdown in the soil. No crop detectable residue can be expected
in the soil at 30 to 45 days after application of the suggested rates
especially under the temperature and rainfall conditions of the "off-season."
These treatments will not persist into the crop season if accidentally or
deliberately applied to canals or field ditches during the "off-season."
Treatments numbered 3 and 4 in the preceding text should not be used on
crop land less than 45 days before planting is anticipated.
Crop-season (and off-season) ditchbanks only; repeat as needed:
5. Flame: under same conditions as in number 1 above.
*6. "Antmate" at 50 lb/loo gallons of water applied as a foliage wetting
spray should cover more than 1 acre. The addition of a spreader-sticker
to the tank mix should improve effectiveness. Approximate cost of treatment
$15.00/A. One drawback is that this herbicide is somewhat corrosive to
metal' components of spray equipment and thorough washing is necessary
after use.
*7. PCP-foftified diesel oil emulsion; PCP (Penta) emulsifiable con-
centrate should be mixed at the rate of 1 lb. technical PCP equivalent with
7.5 gallons each of diesel oil and water. Use good agitation and apply
as a foliage vetting spray. This treatment will provide rapid topkill
of vegetation with a very short residual effect. The main disadvantage
of PCP is lack of effect on unexposed plant parts and roots.
8. Growers currently satisfied with chlorates may wish to continue to
use them. Other materials are more effective and are inflammable. Those
who wish to use chlorates should investigate chlorate-borate mixtures
which have reduced flammability hazards.
9. Growers experienced and satisfied with arsenicals may wish to
continue with these materials for a ditchbank program. However, as is
true of chlorates, there are more effective and less hazardous chemicals
available.
Application and residual hazards: Essentially no hazard if sprays are
kept from growing crops end aclatd..
Application Equipment
*10.; Separate spray equipment for herbicides (especially 2,4-D and
2,4,5-T) is desirable and undoubtedly economically justified for all but
the smallest operations. This would eliminate time and bother of cleaning
and calibrating sprayers.
11. Where practicable, brush boom equipped sprayers of the conventional
herbicide type with nylon roller pump will be economical and satisfactory.
There is little need for pumps of greater that 5 pmn capacity. The boom
should be kept as low as possible aid still get the desired coverage and
wet the weeds. Low spraying pressure (30 50 psi) and fairly large
orifice, fan-type nozzles will wet all but the most dense vegetation.
With 2,4-D and 2,4,5-T materials 20 to 50 gpa of solution should be
sufficient.
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12. If boom spray equipment is not practical, either hand guns adjusted
for large-droplet, coarse sprays or off-center nozzles may be used.
13. To use row-crop sprayers after cleaning spraying pressure at the
boom or hand gun should be reduced to 50 60 psi; large, coarse orifices
should be used; and, 2,4-D and 2,4,5-T materials should be applied at 20
to 50 gpa of solution while other weed killers should be applied at about
75 to 100 gpa as foliage wetting sprays. Equipment should be cleaned after
use.
14. If weed growth is very dense and coarse and difficult to wet, it
would be best to mow or chop back area and then apply herbicides to lower
and more tender regrowth.
Cleaning row-crop sprayers:
15. Before using herbicides, especially if sprayer has been used to
apply copper-containing fungicides: Use one gallon of commercial acetic
acid per 100 gallons of water. Operate sprayer, agitate thoroughly and
fill hose lines and boom with solutions. Allow solution to stand two
hours then drain and flush system with several rinses of fresh water.
16. After applying herbicides: after salt formulations of 2 4-D and
2,4,5-T; use one of the following per 100 gallons of water: (a.) 2 Ibs.
of lye (sodium hydroxide); (b.) 5 lbs. sal soda (sodium carbonate);
or (c.) 1 gallon of household ammonia. After low volatile ester formulation
of 2,4-D and 2,4,5-T; use one of the following per 100 gallons of water
(d.) 2 Ibs. lye; (e.1 5 Ibs. sal soda plus 1 pint of emulsifier plus 5
gallons of diesel oil.
17. Line strainers, piping, hoses and booms should be cleaned by
operating the sprayer during 15 or 16 above. All cleaning -should be
followed by thorough flushing with fresh water. Waste from cleaning
operations should not be discharged on crop land.
Herbicide spraying practices; 2,4-D and 2,4,5-T especially:
18. Know what you are buying and using: read and observe labels.
19. Use large orifice, coarse droplets sprays with lowest pressure that
will give desired spray coverage and distance. (Atomization of sprays into
fine mist causes drift loss and affords poor wetting.)
20. Do not mount boom excessively high or point spray gun at sky to get
coverage; this causes additional drift loss and possible hazard to adjacent
areas when windy.
21. When using hand guns: spray to wet foliage and then move on.
22. Do not spray when windy Stop if wind is more than 5 mph, especially
if blowing toward cropped areas less than 1 mile away. Do not spray within
one-half mile of floral or other sensitive crops if wind is greater than
2 mph. Under 2 5 mph winds spray if no crops are downwind. Use caution
near sensitive crops.
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23. Do not spray more than 40 50 acres per day in any one unit unless
the operation is isolated (1 mile) from cropped land.
24. Do not apply if rain is expected within 4 6 hours.
25. Research has established that actual physical drift of sprays is
the major cause of 2,4-D damage to adjacent crops; volatilization appears
to play a relatively minor role, especially when salt formulations are
used.
additional references:
Florida Agricultural Experiment Station Bulletin 532.
2,4-D for Post Emergence Weed Control in the Everglades, by Charles
C. Seale, John W. Randolph and V. L. Guzman, 1953.
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III. Control of Bacterial Spot
R. E. Stall and P. L. Thayer2/
In order to obtain maximum benefit from a virus control program, plants
should be protected as much as possible from other diseases. One of the most
important of these is bacterial spot (caused by Xanthomonas vesicatoria (Doidge)
Dows). Therefore, it might be well to review what we know about bacterial spot
and what can be done to control it.
Source and Spread of the Bacterium
Bacterial spot is most commonly introduced into the field by diseased seed.
In certain areas of the country there is evidence that the bacteria can over-
season on plant refuse, however, there has been no proof of this occurring in
Florida. The number of diseased seed in a lot may be very small, but under the
proper conditions the bacteria can spread from one plant over the entire field.
For spread to occur there must be water present to act as a carrier of the
bacteria, and there must be some method of transporting the water. Wind blown
rain may serve in this capacity. Another important carrier is farm implements
brushing from one dew or rain covered plant to another.
Once the bacterium has been deposited on a healthy plant it moves to the
inside of the leaf through tiny openings in the leaf surface called stomata or
through openings in the edge of the leaf called hydrathodes. Inside the leaf
the bacteria grow and increase in number for 5 to 7 days before there is any
outside indication of the disease. Bacterial build-up inside the leaf results
in death of leaf cells and consequently we see a spot of dead tissue called a
lesion. As the bacteria continue to increase in the leaf a viscous material is
secreted around the bacterial cells and some of them are forced to the surface
of the leaf. This is evidenced visually by the greasy appearance of the spot
lesion. It is at this stage of development that a drop of water falling on the
lesion can spatter bacteria to the next plant or wind driven rain and farm imple-
ments moving through wet plants can carry bacteria over most of the field.
Once the bacterial spot pathogen is established in a field its seriousness
depends to a large extent on the weather. A thunder shower accompanied by heavy
winds can spread the spot pathogen to almost every plant in a field. Periods
of rainy, overcast days tend to favor spread and development of the disease. On
the other hand periods of dry sunny weather and light dews tend to suppress
disease development.
Methods of Control
Obviously the time to begin control of bacterial spot is with disease free
seed. California seed should be free of the bacterial spot pathogen, however,
if this seed cannot be obtained, seed treatment with bichloride of mercury will
eradicate the bacteria. This chemical can be purchased in the form of 7.3 grain
blue tablets or as a white powder. The use of tablets is convenient, since one
tablet dissolved in one quart of water gives the desired 1 to 2000 dilution. If
5/ Assistant Plant Pathologists, Indian River Field Laboratory, Ft. Pierce, and
Everglades Experiment Station, Belle Glade, respectively.
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a large amount of seed is to be treated, 1/3 ounce of the powdered chemical
should be used for 10 gallons of solution. The powder is first dissolved in a
small amount of boiling water. Enough cold water is then added to make the 10
gallons of solution. Seed to be treated is placed in a loosely woven cloth bag,
such as cheese cloth. The bag should not be more than 1/3 full. Seed should be
soaked in the solution for 5 minutes and gently stirred during treatment to assure
good coverage. After treatment, the seed are washed for 15 minutes in running
water and immediately spread out in a thin layer to dry. Three times as much
solution should be used as seed by volume. Care should be taken not to use metal
containers as bichloride of mercury is a very corrosive chemical.
Unfortunately disease free seed is no lasting insurance of a disease free
crop because the bacterial spot pathogen can spread from a nearby diseased plant-
ing to a healthy one. For this reason it is necessary to adhere closely to a
preventative spray program. As soon as the plants germinate they should be
sprayed on a 5-day schedule with basic copper at 2 pounds metallic per 100
gallons of water plus streptomycin at 100 parts per million. After the plants
have attained some size and additional gallonage is necessary for coverage of
the plants it may not be economically sound to continue use of streptomycin, how-
ever, regular application of copper should continue. In order to insure complete
coverage of the plants, at least 2 or 3 nozzles should be used initially per row
and increased to 5 or 6 as the plants approach maturity. At 200 to 300 pounds
pressure the speed of the sprayer should be regulated to deliver from 50-75
gallons per acre on young plants and gradually increased to 150-200 gallons per
acre on mature plants.
This spray schedule should also control frog-eye (Cercospora) leaf spot on
pepper.
EES 59-31
300 copies
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