UNIVERSITY OF Cooperative Extension Service
FLO RIDA Institute of Food and Agricultural Sciences
PLANT PROTECTION POINTERS
EXTENSION PLANT PATHOLOGY REPORT NO. 65
GAINESVILLE, FLORIDA, December, 2003
By Pete Weingartner and Tom Kucharek
DISEASE CONTROL PROGRAM FOR POTATOES
Diseases of potato in Florida reduce yield, the quality of tubers, and monetary returns.
Diseases of potato occur in the seed tubers, the growing plant in the field, and the harvested
tubers. Pathogens in seed tubers provide inocula for diseases that occur in the field and
eventually in the harvested tubers. Because potatoes are vegetatively propagated with tubers, the
risk for introducing a pathogen into a field is high. Therefore, emphasis must be placed on
acquiring disease-free planting stock.
Most diseases of potato in Florida are caused by fungi. In addition, eight viral diseases
(corky ringspot, potato virus Y, potato virus X, spindle tuber, potato leaf roll virus, potato virus
S, tomato spotted wilt, potato mop-top) have occurred over the past thirty years. Of these, corky
ringspot, which is limited to the northeast Florida production area, is the most prevalent of the
seven viruses. Two bacterial diseases occur in Florida. Brown rot (bacterial wilt) is a bacterial
disease that is limited to the northeastern production area of Florida, but interestingly strains of
the same species of bacterium cause disease in other crops in all areas of Florida except in the
organic muck soils and the highly calcareous soils near Homestead and Florida City in Dade
County. In addition, a complex of soft-rotting bacteria (primarily Erwinia spp.), cause soft,
mushy rots of tubers and vines. Nematodes are microscopic worms. Southern root-knot, sting
and stubby root nematodes infect potatoes in Florida. In addition, stubby root nematodes carry
and transmit tobacco rattle virus, which causes corky ringspot in potato. A summary of the
common diseases that occur in potato in Florida along with some basic information about the life
cycles of the pathogens are presented in Table 3.
Fungi and fungal-like organisms do not have true roots, leaves, stems, or chlorophyll.
Instead they have hyphae (microscopic threads) of various types or plasmodial (blob-like)
stages, which can grow in the soil or host plant. Some fungi live and reproduce in the host plant
as well as the soil. Fungi reproduce by the growth of hyphae and the production of microscopic
spores. The fungi that cause early and late blights in potato and tomato have spores that are
spread by air currents. In addition, the fungus that causes late blight produces aerially
disseminated sporangia or spores zoosporess) that are mobile in water. Pythium spp. are fungal-
like pathogens that cause root, tuber, and lower stem rots and they move primarily within water
associated with the soil or the plant. Some fungi, such as Rhizoctonia solani and Sclerotium
rolfsii, produce thick, aggregated masses of hyphae that are called sclerotia. These sclerotia are
durable and survive in the soil or in association with tubers for months or even years. Fusarium
spp., which cause tuber rots, and Alternaria solani, the causal agent for early blight, produce
thick walled hyphal cells called chlamydospores that function as survival structures in soil and
The life cycles of pathogens are typically shortest when temperatures and moisture are
ideal for growth and reproduction of the various fungi. For example, life cycles (inoculation to
next generation of spore production) of late blight and early blight can be completed during
favorable weather conditions in 3 tolO days.
Bacteria are microscopic organisms with cells about 1/12,000 of an inch long. They
survive in aggregated colonies within a mucilaginous material. Bacteria may multiply several
times per hour and thus the diseases they cause often seem to develop rapidly (e.g. overnight).
Anyone who has been associated with potatoes has seen how fast bacterial soft rot can occur in a
load of seed tubers or harvested tubers if they are not handled properly. The bacteria that cause
disease in Florida-produced potatoes can survive in the field soil.
Viruses associated with potatoes in Florida require an electron microscope to visualize
their structure. One particle of tobacco rattle virus, which causes corky ringspot in potato, is
approximately 1/121,000 of an inch long. Viruses are considered obligate parasites because
they require a host plant to reproduce. Contrast this with most fungi and bacteria that can
reproduce either in a plant or in the soil in association with dead plant material (organic debris).
Tobacco rattle virus is maintained and spread by stubby root nematodes. Tomato spotted wilt
virus is spread by tiny insects called thrips. Potato virus Y is spread by aphids (plant lice) and it
can be transmitted via seed tubers. Potato virus X is transmitted primarily by infectious sap in
vines or tubers, but it can also be transmitted by biting insects such as grasshoppers. Some claim
certain fungi can transmit potato virus X. Potato mop-top virus, which has been found recently
in association with potatoes produced in Florida, is spread by the fungal-like pathogen,
Spongospora subterranea f.sp. subterranea which causes powdery scab.
Plant parasitic Nematodes are microscopic worms that typically survive in the soil.
Many species of nematodes may become 1/400 to 1/800 inch long so they can be barely visible
with an unaided eye. Life cyles of the various nematodes vary. For example, root-knot
nematodes invade the roots or tubers where they lay eggs. The life cycle can be completed in 20
to 60 days depending upon soil temperature. Stubby root and sting nematodes lay their eggs in
the soil and a typical life cycle is about 45 days in cool soils. Increased levels of nematodes may
predispose potato to other diseases. High levels of root-knot nematode have resulted in
increased incidence of bacterial wilt. Stubby root nematodes transmit the virus which causes
Because of the variation in the types of organisms that cause diseases in potato and
because of variation among potato varieties and production fields, multiple disease management
tactics must be utilized to compensate for the many combinations and interactions that occur
between the host and the many pathogens. Therefore, a sequential disease control program is
presented herein for your consideration.
SEQUENTIAL DISEASE CONTROL PRACTICES FOR POTATO DISEASES
Step 1. Crop rotation and prior crop destruction.
Crop rotation, that is the avoidance of planting potatoes on the same land more than once
every 3-10 years, is desirable, but because of land and logistical constraints on most potato farms
in Florida, adequate crop rotation is often not an option. Two rotational situations exist in
Florida. In the northeastern part of the state where most of the potatoes in Florida are grown,
crop rotation is used on a few farms. For example, some will rotate with a crop of cabbage or
broccoli and then double crop with sorghum/Sudan grass or corn. However, most growers in this
area plant potatoes in the same field each year and double crop in the summer with
sorghum/Sudan grass or corn. In Manatee and Suwannee Counties, potatoes may be rotated with
vegetable crops or peanuts. Regardless of the rotational plan used, double cropping of some sort
is used, often resulting in the build up of soilbome fungal pathogens, such as Rhizoctonia spp.
and Pythium spp., which are capable of infecting many crop species. Legume crops such as
peanut and bean are likely to allow the buildup of Rhizoctonia spp. Although crop rotations as
practiced in most other potato-producing regions of North America are desirable in Florida,
limitations of land, cost, and marketability of other crops restricts the ability of Florida growers
to do so. Interestingly, potatoes have been grown successfully on the same land in the Hastings
area for more than 75 consecutive years. Because of the high potential of soilborne diseases and
nematodes in such situations, every available tactic for pest suppression should be used.
Step 2. Influence of double cropping of summer forage cover crops, weeds,
and other crops on disease.
Potatoes are commonly double-cropped with a summer crop such as a sorghum/Sudan
grass or corn in northeastern Florida. This practice prevents additional buildup of certain
disease-causing organisms such as brown rot (bacterial wilt) and root-knot nematodes by
competing and crowding out broad leaf weeds that would support multiplication of the bacteria
and nematodes. Because these grass crops are suitable hosts for sting nematodes, they should be
cut or harvested by mid-August to mid-September followed by an immediate incorporation of
the remaining stubble into the soil. Early incorporation of the cover crop residue into the soil is
important for several reasons. First, the numbers of sting nematodes which build up on
sorghum/Sudan grass decline rapidly following incorporation. The field must then be
maintained weed-free up to the time of bedding and planting of potatoes. Mowing and growing
sorghum/Sudan grass as a ratoon crop for additional harvests during the summer will result in
increases in sting nematodes. An alternative to planting these grass crops is the maintenance of
the field in a weed-free condition. Another alternative for growers to try is the use of sunn hemp
as a summer cover crop, which is capable of reducing sting nematodes. One should be aware
that sunn hemp, like other legume crops, is likely to maintain high soil populations of
Where vegetables, peanut, or sorghum/Sudan grass are double-cropped with potato, the
crop and associated weeds prior to potato should be chopped and incorporated into the soil at
least 60 days in advance of planting potatoes. This time allows green manure to rot thoroughly;
rotted crop debris is more likely to harbor beneficial soilborne organisms to compete against
Rhizoctonia spp. Fresh, undecomposed green manure is a major source of inocula from
Rhizoctonia spp. and Pythium spp. on many crops in Florida. Rhizoctonia spp. can infect
potatoes prior to or after emergence. This fungus can kill the young potato plant or it may
colonize the young stem without killing the plant. This mild infection persists and provides a
source of inoculum for causing disease in the tubers. Another advantage for incorporating the
double crop early is that old crop debris that is not adequately decomposed reduces the
effectiveness of soil fumigants by trapping the chemicals or providing exit ports to the soil
surface for premature escape of the fumigant.
Step 3. Variety selection.
While demands from the market are likely to heavily influence the choice of varieties to
grow, varieties with resistance to diseases should be used whenever possible. Planting resistant
varieties reduces the need for costly chemical controls. Varieties vary in their response to corky
ringspot and bacterial wilt (brown rot). Information on this issue is present in Table 1. In
addition, some varieties, like Sebago, possess some generalized forms of resistance to late blight,
which means they are not immune, but rather, they incur slightly less disease.
Tablel. Levels of resistance available in different potato varieties to corky ringspot and
Variety Type & skin color Corky ringspot Bacterial wilt
Atlantic* Round white 3-4 4
BelRus Long white russet 3 3
Green Mountain Round white 2 2-3
Hudson Oblong white 1 3
Katahdin Round white 3 3
Kennebec* Oblong white 3 3
LaChipper* Round White 3 3
LaRouge* Round red 5 5
New Superior Round white 1 5
Oceania Round white 1 3
Ontario Oblong white 4 1
Pungo Round white 1 5
Red LaSoda* Round red 5 5
Red Pontiac* Round red 5 5
Sebago Round white 5 2
Superior* Round white 1 5
RelftiVe re~nonn~ to dli~P2~Pb
The higher the number the more susceptible the variety is to that disease.
* Seed pieces for these varieties are readily available. Lack of an indicates that for various
reasons the variety is not widely grown in Florida.
Step 4. Selection of seed potatoes.
The use of certified seed tubers from the United States or Canada is the only option that
should be considered. The bag or bulk load must be accompanied by a certificate of the agency
that certified the seed tubers. The certification process indicates that attempts have been made
to minimize the movement of disease-causing organisms through the seed tubers. The growers
should realize that certification is never a guarantee that the seed tuber he buys will be totally
Seed generation is an important consideration when purchasing seed. Generally, the
incidence of diseases, especially viral diseases, increases as the number of generations from
tissue culture increases. Therefore, the grower should determine the seed generation or coding
system used in the state or province from which the seed pieces are purchased. The coding
system and terminology used for denoting the number of generations from tissue culture and
foundation stock varies. It is best to say that certified seed tubers are likely to carry no higher
percentage of the tubers with specific disease-causing organisms than the seed law allows in the
individual states. However, if weather conditions in seed-producing areas in the northern United
States and Canadian Provinces are favorable for development of bacterial soft rot or late blight or
both, it is possible that infected seed tubers will be shipped to Florida even though a certification
program is in place. Such a situation has happened.
In addition to seed generation, potato growers in Florida should be aware of the
pathogen/disease tolerances stated on the certification label. A tolerance is the incidence or level
at which a pathogen or disease is "tolerated" in the seed. Occasionally, levels of pathogens,
especially aphid-transmitted viruses, "flare up" and make it difficult for seed growers to produce
seed within tolerance. Tolerance levels therefore are sometimes adjusted upward by seed
agencies to enable growers to still have seed within tolerance. Growers should seek certified
seed with the lowest possible tolerances.
Most seed-producing areas perform winter tests in Florida and elsewhere to determine the
incidence of viruses in individual seed lots. It is impossible to complete these tests in time to
accommodate the planting times in Florida so growers in Florida have to purchase seed tubers
without the benefit of the information from these winter tests. Therefore the Florida growers
must communicate with brokers and seed producers to determine what his best options are. This
should be done every year regardless of whether or not wide spread problems exist the seed
production areas. In addition, the seed buyer, according to Florida Seed Law, has the right to
call for a seed inspection prior to accepting the load. However, if seed is accepted without prior
inspection, the risk is transferred to the seed buyer. If you are having unreconcilable problems
with a seed producer or broker, avoid continued business with them. Use mutually understood
contracts, particularly when you are dealing with new contacts.
Step 5. Transit of seed potatoes.
Part of your ability to select the best seed tubers available should be based on your
knowledge of how the seed tubers will be shipped by various seed producers. In order to
minimize diseases such as soft rot and Fusarium dry rot from spreading among seed tubers that
were inspected up north and eventually into your planting, the quality must be maintained during
transit from a cold climate to a warm and humid climate in Florida. The seed tubers must be
loaded with minimal bruising. Free falls from conveyers should be less than 18". Avoid
exposing seed tubers to freezing temperature during loading or transit. Upon receipt of the
tubers, make a personal inspection looking for signs of freezing damage, particularly along the
sidewalls of the carrier. Small electrical heating units in the trailers can be used by the truckers to
prevent cold damage.
Step 6. Acceptance of loads of seed tubers, unloading, establishment of on-
farm storage, and sanitation in storage area.
Every load of seed tubers should be inspected before unloading and placing them into
storage. If a problem exists, a lot can be rejected at that time. When unloading the seed tubers,
rough handling must be minimized. The nature of the operation is such that some bruising will
occur, but minimizing damage will result in fewer problems. For example, the conveyers and
receiving carriers should be padded. Sharp corners on any equipment or carrier will bruise
tubers upon impact and thus be sites of entry for weak pathogens that require wounds such as
bacteria that cause soft rots and the multitude of fungi that cause dry or wet tuber rots..
The storage area should be cleaned and sanitized because bacteria and fungi can survive
at such sites from the previous year, particularly in old tuber debris, vine debris, bags, and soil.
After sweeping or vacuuming the area, the entire holding area and containers should be
disinfested with sanitizers that contain chlorine (e.g. bleach), iodine, or alcohol. Allow the
sanitizers to stay in contact with the materials to be sanitized for at least 10 minutes. Of the three
types, alcohol will not be corrosive to metal. Steam can be used effectively as long as the
sanitized surface area is exposed to 1800 F for at least one minute. Cull piles should not be
established near tuber holding areas or anywhere else. Old, diseased, or unused tubers should be
destroyed by burying or other methods which totally remove potentially infected tubers and their
progeny from the handling or production areas before new tubers are being unloaded for storage.
Because the seed tubers are originating in a cold climate and commonly unloaded in a
warm, moist climate on your farm in Florida, it is likely that condensation will occur
immediately on the seed tubers when they are unloaded. Such condensation creates favorable
conditions for development of bacterial soft rot in the seed tubers. To minimize this problem, the
storage warehouse, should contain enough space for you to space out the bulk or bagged loads to
provide for aeration and drying. To accelerate drying and maintain the dry condition, your
storage area must be equipped with forced air that will reach all areas of stored seed tubers. For
bulk loads, heavy gauge PVC pipes with drilled holes can be permanently mounted at different
levels in a holding area to accept and dispense forced air for drying. Where seed tubers are
stored in pallets, space the pallets apart, allowing for forced air zones in the storage area. If a
load is delivered and unloaded and found to have a disease problem, isolate that load from the
other loads to the extent possible.
Step 7. Cutting the tubers into seed pieces.
By virtue of the natural handling of bulk loads of tubers for cutting, the process will
cause some bruising. However, several procedures can be used to minimize this problem. First,
the seed tubers should be warmed to ambient temperatures before cutting. This will reduce
bruising which in turn will reduce bacterial soft rot and Fusarium rots. Also, bruising can be
minimized by not allowing the tubers or cut seed pieces to fall more than 18", which will reduce
air cracking. Conveyers and receiving containers should be padded. Cutting knives should be
disinfested with chlorine-, iodine-, or alcohol-containing sanitizers prior to use, between seed
tuber lots, and immediately after finding a batch that has obvious rot. Bacterial and fungal
diseases are easily transmitted from one tuber to another in remaining plant sap from an infected
It is best for the cutting operation to be done on a day when temperatures are 500 F or above.
Handling tubers at such temperatures will result in less bruising. Also, temperatures between
50 and 600 F will accelerate healing of the cut tubers, which can seal out infections. If the seed
pieces are to be stored after cutting, maintain a high humidity in the storage area, but do not
allow liquid moisture to condensate on the seed pieces. All these factors will contribute to
healthier seed pieces.
Step 8. Seed piece treatment.
Treatment of seed pieces with a fungicide reduces fungal and bacterial diseases
associated with the seed pieces and young plants (See Table 2). Also, reduction of infection
from Rhizoctonia spp. on young plants is known to reduce the colonization of tubers at the end of
the season with sclerotia (small dark masses of hyphae) formed by this fungus.
Table 2. Some chemicals available for seed piece treatment to suppress Rhizoctonia and
Chemical Formulation Rate/cwt
Maxim 0.5% (fludioxonil) Dust 8.0 oz
Maxim 4F (fludioxonil) Flowable 0.04 to 0.08 fl oz
Maxim MZ 0.5% & 9.6% (fludioxonil & mancozeb) Dust 0.5 lb
Dithane M45, Penncozeb 80, or Manzate 80 (mancozeb) Wettable powder 1 '/#/50 gal. water
Moncoat MZ 6.0% & 1.5% (mancozeb & flutolanil) Dust 0.75 to 1.0 lb
Tops 2.5D (thiophanate-methyl) Dust 1.0 lb
Tops 5 D (thiophanate-methyl) Dust 0.5 lb
Tops MZ-Gaucho 2.5%, 6.0%, & 1.25% (thiophanate- Dust 0.75-1.0 lb
methyl, mancozeb, & imidacloprid)
Evolve 2.5% 2.5%, 6.0%, & 1.0% (thiophanate-methyl, Dust 0.75 lb
mancozeb, & cymoxanil
Amistar 80 DF as an in-furrow treatment Flowable 0.25 ozs.; /1000 ft
of row, in-furrow,
for 40" beds
Silver scurf, caused by Helminthosporium solani, is suppressed by the materials listed in
Table 2 except Moncoat MZ, Tops 2.5 D, and Tops 5.0D. Dry treatments are less likely to be
advantageous for bacterial soft rot and black leg. Regardless of the treatment used, seed
treatment will protect the external portion of the seed piece from invasion by the fungi that are
on the surface of the pieces or in the soil. Seed piece treatments do not reduce inocula already
present in the pieces.
Step 9. Considerations for planting.
Potatoes should be planted in soil with moderate moisture whenever possible. If the soil
is too wet, bacterial soft rots and black leg might increase because of the high moisture situation
coupled with the bruises that are associated with normal handling of the seed pieces. If the soil
is too dry and the seed pieces are planted immediately after cutting, interference with the
suberization process may occur because of a lack of high humidity. Such a situation can lead to
greater infection of the seed pieces by fungi and bacteria. In order to minimize infection from
fungi, such as Rhizoctonia spp., plant at the recommended depth of 4 to 6 inches and not any
deeper. Young sprouts are the most susceptible part of the plant to Rhizoctonia sp.; therefore,
reducing the time of emergence helps minimize damage from this fungus.
Step 10. Chemical control of nematodes, corky ringspot, and bacterial wilt
The control of nematodes, corky ringspot, and bacterial wilt is grouped as an integrated
component of this part of the disease control program because these problems originate in the
soil. In addition, some of the pathogens interact with each other by causing additive damage.
Other pests such as Rhizoctonia spp. and Sclerotium rolfsii can also originate in the soil, but Step
10 will concentrate on the three major pathogens mentioned above.
The complexity of this situation is accentuated because of the following:
A. Three types of nematodes cause the most damage to potato in Florida (root-
knot, sting, and stubby root).
B. Stubby root nematodes carry and transmit the tobacco rattle virus (TRV)
which causes corky ringspot.
C. Several species of stubby root nematodes are present in Florida. However,
only Paratrichodorus minor has been shown to transmit TRV and is the most
prevalent stubby root nematode in potato fields.
D. Stubby root nematodes are usually found deeper in the soil than sting
E. Ralstonia solanacearum, the causal agent for bacterial wilt, is found
throughout the soil profile and it is extremely difficult to suppress with
F. The incidence and severity of bacterial wilt vary widely from season to season
even in the same field.
G. Bacterial wilt is likely to be worse in the presence of high levels of root-knot
H. Chemicals are expensive and prices received for potato are often different for
processed and fresh-market potatoes.
I. Levels of soil moisture influence the effectiveness of the soil-applied
chemicals differently. Temik 15G will not be released from the granules if the
soil is dry. Telone II is more effective in drier soil. Vapam is more effective
if the soil is slightly moist.
J. Several chemicals are available and one must decide what chemical program
is to be used.
The first step in controlling this complex of soilborne pathogens is knowing which
pathogens are present. Past crop history is the best indicator for corky ringspot and bacterial
wilt. Past history is also important for nematodes; however, soil sampling for nematodes will
provide up-to-date information for specific fields. The best time to sample for nematodes is near
the end of the potato season before the plants die-back or following the summer cover crop.
Once the disease or nematode problems have been identified, decisions can be made on the
selection of management options.
Telone II should be applied at least two to four weeks prior to planting using 6 GPA, in
the row, at a depth of 10 to 14" below the top of the bed when the bed is formed. Telone II
applied with this method when the soil moisture is less than 12% by weight will suppress root-
knot, sting, and stubby root nematodes. Higher rates of Telone II or Telone C-17 applied with
multiple chisels per row have also suppressed bacterial wilt in research tests. An alternative is to
apply Telone II with rip chisels (subsoilers) at a depth of 18" which is likely to break the
compaction layer in flatwoods soils. Deep applications have been more effective in suppression
of stubby root nematodes and corky ringspot
Vapam can be injected into the soil at the time of bedding and two to four weeks before
planting. Vapam is an alternative for Telone II. Vapam should be applied with at least two
chisels/row because it does not permeate the soil as well as Telone II. Vapam should be applied
at 20 to 25 GPA (62 to 72.5 lb a.i.). If the soil is extremely dry (less than 8 to 10% moisture by
weight), dilute the Vapam in water to enhance its activity. This addition of water is typically not
needed in northeastern Florida because the flatwood soils tend to have adequate moisture for
Temik 15G can be applied in the furrow at planting at a rate of 21 oz/1000 linear foot of
row. In fields where nematodes are a severe problem, it might be beneficial to supplement the
pre-plant fumigation of Telone II or Vapam with Temik 15G. Temik 15G can also be used by
itself without pre-plant fumigation. Temik 15G should be applied at 21 oz/1000 linear feet of
row in the seed piece furrow at planting time for control of corky ringspot. Lower rates (e.g. 7
oz/1000' of row) of Temik 15 G used for insect control will not adequately suppress nematodes
for good control of corky ringspot. Adequate soil moisture (10-12% by weight) is essential for
optimum effectiveness of Temik 15G.
Vydate L 2EC will suppress corky ringspot by suppressing stubby root nematodes. It
will also suppress sting nematodes. Vydate L 2EC can be applied in the seed piece furrow at 1.5
gallons/acre for this purpose. The Vydate in-furrow treatment can substitute for the in-furrow
treatment with Temik 15G. Vydate can also be applied as sprays three times,7-10 days apart at
the rate of 2 quarts/acre. The three sprays can be used as supplemental control for nematodes and
corky ringspot after using Telone II, Vapam, or Temik 15G. Vydate would be particularly useful
if Temik 15 G is applied to dry soil and does not leach off of the granule. Do not rely solely upon
foliar sprays of Vydate L 2EC for your nematode control program; it should be considered as a
supplemental treatment to soil-applied treatments.
Step 11. Observation after emergence is initiated.
As soon as emergence occurs, you must be vigilant in observing for any problems. Any
unrecognizable problem should be diagnosed accurately as soon as possible so that remedial
action can be taken if possible. The later remedial action (e.g. spray) is taken, the less likely
successful control will be achieved.
Step 12. The interactions of cultivating equipment and disease.
Whenever the soil is used to cover emerging plants for cold protection, the risk of
bacterial soft rot and black leg is increased. This is another reason for having disease-free seed
tubers. These bacteria can originate from the seed tubers or soil. If the plants must be covered,
uncover them as soon as possible as gently as possible and when the soil is dry.
When cultivating, space the chisels far enough from the growing root system of the
widest plants (emerged or not) so that root-pruning is avoided. Root pruning with cultivators or
chisels used for application of liquid nitrogen has caused increases in bacterial wilt (brown rot).
Step 13. Sanitation is the foundation for plant disease control at all stages of
growing and of handling the crop.
Even though the value of sanitation was mentioned earlier in the seed tuber handling
steps, it is so important in reducing primary inocula for diseases such as soft rot, black leg, early
blight, late blight, and other diseases that we want to drive the point home again. Cull piles
should not be allowed to become established around tuber handling areas or fields after planting.
Cull piles are notorious as havens for primary inocula for many diseases on many crops. It is
important to bury cull piles regardless of how busy farm operations become. Assign someone the
task of burying cull piles. Sprouting potatoes left in the field after planting or around the farm
have been found to carry the late blight fungus. Containers and bags used for handling potatoes
should be sanitized before reuse. Sawdust or other materials used in trucks for cushioning tubers
should be buried or burned because sprouts developing on tuber fragments remaining in the saw
dust can serve as sources of inocula. Wash soil from equipment and tractor tires after working in
a field known to have problems with soilbome diseases such as bacterial wilt. This will
minimize the spread of these diseases around the farm.
Step 14. Work in the field when the leaves are dry.
Disease causing organisms, such as bacteria that cause soft rot, are easily spread from
plant to plant by contact, particularly when the plants are wet.
Step 15. Control weeds.
Control of weeds in the off-season was discussed earlier as a major tactic for suppression
of nematodes and bacterial wilt. The presence of weeds during the potato season increases the
number of hours leaves stay wet, which can result in increases in early or late blight. Weeds
interfere with spray deposition of fungicides and insecticides. If potatoes are grown near
tomatoes, volunteers of tomato should be destroyed in the area to minimize the risk from early
or late blight. Abandoned tomato fields can also serve as sources of inocula for these two
Volunteer potato plants in potato fields rotated to other crops can also serve as reservoirs
of inocula, particularly for potato virus Y and late blight.
Step 16. To minimize potato scab, do not over lime the soil.
Step 17. Use of nitrogen to suppress early blight.
Supplemental applications of nitrogen will reduce early blight by delaying maturity. If
this technique is used, it is important to be cognizant of the delayed maturity and possible
overloads in the use of Best Management Practices for nitrogen relative to ground water
contamination. In northeastern Florida, 200 lb of actual nitrogen is the current BMP maximum.
Step 18. Fungicide spraying for early blight and late blight.
Because adequate resistance to early and late blights is not available, spraying with
fungicides is important. While many fungicides are available, currently registered
fungicides will control these diseases provided they are applied in a spray program
beginning before infection occurs. Wide ranges of spray pressure and spray volume are
effective for control of foliar diseases. Probably, lower spray pressure (i.e. < 100 psi) are
best because more fungicide is deposited on the plant rather than allowed to drift. Spray
volumes in the 25 to 50 gpa should be adequate unless you are trying to drench the
fungicide to the soil. Flat fan and hollow cone nozzles of different sizes have performed
well. Nozzle placement is highly important because you want the spray to cover the plant,
not the soil in the row-middles. In recent years, strains of the late blight fungus have been
more aggressive on the stems and therefore using spray application techniques that
penetrate below the outer canopy would be beneficial.
Spray adjuvants, such as spreader stickers, are available for use, but for most fungicides,
they are not needed. They should be useful for the wettable powders formulations of mancozeb
(e.g. Dithane M-45, Manzate 80, Penncozeb 80). Crop oils should not be used in sprays as they
are likely to cause bums in leaves and stems.
For best control of early and late blight in south Florida, begin a spray program after
emergence and maintain a calendar schedule with intervals not extending more than 7 days.
Consult with local University of Florida, Extension personnel.
In the Hastings region, spray advisories based on forecasting for late blight have been
available. Advisories have been available from the Hastings REC or area Extension agents. If
forecasting is not used, begin spraying before plants are 6 to 8 inches tall if late blight fails to
occur earlier. Follow a 5 to 7 day spray interval. An alternative for a more economical method
is to spray plants following eight consecutive days when the average daily temperatures range
from 500 to 770 F and the 10-day rainfall total is 1.02" or greater. Weather data are available for
the Hastings' area by consulting the FAWN network. Spray intervals using this system may vary
from 5 to 14 days or more. SPRAY INTERVALS SHOULD NOT EXCEED 7 DAYS
REGARDLESS OF WEATHER CONDITIONS ONCE LATE BLIGHT IS REPORTED IN
THE REGION. SPRAY APPLICATIONS MORE FREQUENT THAN SEVEN DAY
INTERVALS MAY BE REQUIRED DURING BLIGHT-FAVORABLE PERIODS AND
WHEN PLANTS ARE GROWING RAPIDLY. LABEL RESTRICTIONS MAY REQUIRE
ALTERNATING SPRAYS AMONG CHEMICALS THAT ARE NOT RELATED WHEN
SPRAYING MORE OFTEN THAN EVERY SEVEN DAYS.
For a list of registered fungicides, see Table 4. ONE MUST ALTERNATE BROAD
SPECTRUM FUNGICIDES SUCH AS MANCOZEB AND CHLOROTHALONIL WITH
SPECIFIC MODE OF ACTION FUNGICIDES SUCH AS QUADRIS. IF YOU DO NOT USE
SUCH A RESISTANCE MANAGEMENT PROGRAM, IT IS LIKELY THAT STRAINS OF
THE FUNGI THAT CAUSE EARLY AND LATE BLIGHTS WILL EVOLVE THAT ARE
UNCONTROLLABLE BY AMISTAR OR HEADLINE, TWO RELATED COMPOUNDS.
SOME OF YOU REMEMBER HOW EFFECTIVE RIDOMIL WAS AGAINST LATE
BLIGHT UNTIL RESISTANCE OCCURRED.
IT IS ESSENTIAL TO ADHERE TO LABEL RESTRICTIONS AND INSTRUCTIONS
WHEN APPLYING ANY KIND OF PESTICIDE.
Step 19. Considerations for disease control near and at harvest.
Vine desiccation (vine kill, burdown) is desirable for fresh market potatoes to set the
skin of the tuber. Setting the skin makes the tuber more resistant to infection and bruising.
Adequate skin set also reduces feathering or sloughing of the skin during harvest and results in a
more eye-appealing product. Adhere to the label instructions of vine desiccants because
inappropriate use of these products can lead to crop damage including stem end rot.
For table stock potatoes, vine killing should be done 10 to 18 days prior to digging.
Vine killing can reduce infection of tubers by Phytophthora infestans, the causal agent for late
blight. After vine killing, do not allow potatoes to remain in the soil any longer than necessary
because the black-specked, sclerotia of Rhizoctonia spp. will become more abundant on the skin.
Where a seed piece treatment was used these sclerotia should be reduced in number. Adequate
rotation would also reduce the incidence of Rhizoctonia black scurf at harvest.
Avoid harvesting during situations when soil is too wet. Such conditions are conducive
for post-harvest loss of tubers to bacterial soft rot. Interestingly, if excessively dry weather
prevails prior to vine killing and shortly thereafter, more stem-end rot (Pythium spp.) may occur
Brown rot (bacterial wilt) in tubers can be reduced in fields where this disease has been a
problem by delaying the harvest. This delay allows a higher proportion of the affected tubers to
rot in the field or fall through the digger chain.
Rough handling of tubers during the harvest operation should be minimized to the extent
possible. After loading, keep the tubers as dry as possible and do not allow them to be
sunburned. To enhance suberization of bruises and wounds, maintain the tubers in a moist
setting without free water accumulation.
Table 3. Pathogens that cause disease in potato in Florida
Sources of inocula*
Soil, organic Other
Type of debris, cull mechanisms of
Organism Disease or symptoms organisms Seed pieces piles transmission
Stem rot, stolon
canker, root rot, tuber Growth of
Rhizoctonia spp rot, tuber black scurf Fungi Yes Yes fungus, wind
Stem rot, root rot,
Pythium spp. tuber rot Fungi Yes Yes Soil moisture
subterraneaf Powdery scab on
sp. subterranea tubers Fungus Yes Yes Soil moisture
Streptomyces Common scab on Growth of
scabies tubers Bacterium Yes Yes bacterium
Southern stem rot,
Sclerotium Southern blight, tuber Growth of
rolfsii rot Fungus No Yes fungus
Sclerotinia White mold, stem rot, Wind, growth of
sclerotiorum tuber rot Fungus No Yes fungus
albo-atrum & V. Early dying, Growth of
dahliae Verticillium wilt Fungi Yes Yes fungus
Early blight on leaves,
Alternaria solani stems, and tubers Fungus Yes Yes Wind
Brown spot on leaves
Alternaria and black pit on
alternate tubers Fungus ? ? Wind
Phytophthora Late blight on leaves, Wind, rain,
infestans stems, and tubers Fungus Yes Yes irrigation
carotovora Soft rot of stems and Wind, rain,
pv. carotovora tubers Bacterium Yes Yes irrigation
carotovora pv. Black leg, stem and Wind, rain,
atroseptica tuber rots Bacterium Yes Yes irrigation
sepidonicus Ring rot Bacterium Yes No Rain, irrigation
Ralstonia Bacterial wilt (brown Soil moisture,
solanacearum rot) Bacterium No Yes rain, irrigation
Clostridium spp. Soft rot of tubers Bacterium Yes Yes rain, irrigation
Tobacco rattle Stubby root
virus Corky ringspot Virus Not commonly Yes nematodes
Potato virus Y Rugose mosaic Virus Yes No plant sap
specific soil Mechanical via
Potato virus X Potato virus X Virus Yes fungus plant sap
Table 3 continued
Table 3 continued
Sources of inocula*
Soil, organic Other
Type of debris, cull mechanisms of
Organism Disease or symptoms organisms Seed pieces piles transmission
Aphids for some
Potato virus S Potato virus S Virus Yes No strains
virus Potato leafroll mosaic Virus Yes No Aphids
Tomato spotted mechanical via
wilt virus Tomato spotted wilt Virus Yes No plant sap
Potato spindle plant sap; pollen
tuber viroid Potato spindle tuber Viroid Yes & true seed
Potato mop-top subterranea f
virus Potato mop-top virus Virus Yes sp. subterranea
*Volunteer potatoes or other susceptible hosts near by can be sources of inocula for many
pathogens that infect potato
Table 4. Fungicides available for control of early blight, late blight, and other diseases in
potato in Florida
Maximum Rate per Minimum
Acre per days to
Fungicides Appl. Crop harvest Remarks
Apply at For suppression of
0.42 fl 0.42 fl planting in Pythium diseases
oz/1000' oz/1000' seed piece Do not tank mix
Ridomil Gold 4 EC of bed of bed furrow. with anything else.
Dithane M-45, Penncozeb 80, or
Manzate 80 WPs 2 lbs. 14 lbs. 3
Dithane F 45 or Manex II 4 FLs 1.6 qts. 11.2 qts. 3
Penncozeb, Dithane, or Manzate
75 DFs 2 lbs. 15 lbs. 3
Manex 4 F 1.6 qts. 11.2 ts. 3
Maneb 80 WP 2 lbs. 14 lbs. 3
Maneb 75 DF 2 lbs. 14.9 lbs. 3
Use a resistance
program as outlined
on the label. Limit
Amistar 80 DF 5 ozs. 30 ozs. 14 is 6 appl./crop
Up to 34 pt.
vines close or 18
Bravo Weather Stik 6L 11/2 pts. 16 pts. 7 values occur
Up to 0.7 lbs
vines close or 18
Bravo Ultrex 82.5 WDG 1.4 lbs. 14.5 lbs. 7 values occur
Equus 720 or Chloro Gold 720 Same as Bravo
6 FLs1pts 115.0 pts. 7 Weather Stik 6L
Echo 90 DF or Equus 82.5 DF 1.2 lbs. 12 lbs. 7
Acrobat 50 WP 6.4 fl ozs. 32 ozs. 4 For late blight
Bravo ZN 4.17 FL 2 1/8 pts 23 pts 7
Polyram 80 DF 2 lbs 14 lbs 3 Limit is 7 appl.
Table 4 continued
Table 4 continued
Maximum Rate per Minimum
Acre per days
Fungicides Appl. Crop harvest Remarks
For use with closed
tractor cabs only.
Tank mix with one
of the maneb or
Super-Tin 80 WP 3.75 oz 15 ozs 7 fungicides
Limit is 4 appl.
Effective for early
Rovral 4 F 2 pts. 8 pts. 14 Sclerotinia
Tanos 50 DF 8 ozs. 48 ozs. 14 Limit is 6 appl.
Most isolates of P.
Ridomil Gold Bravo 76.4 W 2 lbs 8 lbs 14 Limit is 4 appl.
Limit is 3 appl.
Ridomil Gold MZ 68 WP 2.5 lbs 7.5 lbs 3 (See above remark)
Limit is 3 appl.
Ridomil Gold Copper 64.8W 2 lbs 7.5 lbs 14 (See above remark)
Late blight only.
Limit is 7 appl./
crop. Alternate with
not replant area to
other crops within
30 days of
Curzate 70 DF 3.2 ozs 22.4 ozs 14 treatment
Do not make more
than 2 appl. in a
Limit is 6
Headline 2.08 F 12.0 fl oz 72 fl oz 3 appl./crop
Table 4 continued
Table 4 continued
Maximum Rate per Minimum
Acre per days to
Fungicides Appl. Crop harvest Remarks
Do not replant area
to other crops
within 30 days of
Gavel 75 DF 2.0 lbs 12 lbs 3 treatment
use a proportionally
based on band
Previcur Flex 6F 1.2 pts 6 pts 14 width.
Labelled as an in-
Moncut 70 DF 1.1 lbs. furrow spray.
Topsin 70 WP 1 1/2 bs 4 lbs. 21 For white mold
Topsin 4.5 FL 30 fl. ozs. ozs. 21 For white mold