UF UNIVERSITY of
2007 Florida Plant Disease Management Guide: Potato,
Pamela Roberts, Pete Weingartner, and Tom Kucharek2
Specific Common Diseases
Potato is vegetatively propagated by planting
tubers which can carry many different pathogens.
Therefore, it is especially important to include both
preplant and post harvest strategies to control
diseases in this crop. Many potato diseases during
the cropping season and in the harvested crops are
initiated as inoculum in seed tubers. For this reason
utilization of certified inspected seed tubers is
imperative for effective disease management in
potatoes and a Florida Seed Law is in place to help
ensure high quality seed tubers for Florida growers.
Bacterial Ring Rot (Clavibacter
michiganensis subsp. sepidonicus)
Symptoms: Leaves and stems of affected plants
can wilt anytime after midseason. At first, lower
leaves wilt during the heat of the day and then
recover at night. As symptoms progress, leaves first
become pale green and roll slightly at the margins,
followed by more severe wilting, intervenal
yellowing and necrosis of leaves. Often, one or two
stems of a plant are severely diseased with the
remainder remaining healthy. Stems cut and
squeezed near the base often exude a milky white
Infection of tubers starts at the stem-end and
proceeds through the vascular tissue. Tubers cut
crosswise exhibit rings of a cheesy rot that can be
gray, creamy yellow, or light to red-brown in the
vascular tissue. Severely affected tubers may have
dry, gray pockets of tissue around the vascular ring.
Control: Infected tubers are the most important
source of inoculum. Seed certification is therefore
the most effective means of controlling bacterial ring
rot since there is a zero tolerance for the disease in all
seed producing regions. Introduction of the bacterial
ring rot pathogen into a field by using uncertified seed
can result in severe crop losses. The disease can
spread during the season and infected tubers are
difficult to cull at harvest. The pathogen does not
overseason in soil.
1. This document is PDMG-V3-46, one of a series of the Department of Plant Pathology, 2007 Florida Plant Disease Management Guide, Florida
Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Revised December 2005. Reviewed January 2007.
Please visit the EDIS Web site at http://edis.ifas.ufl.edu.
2. P. A. Roberts, associate professor, Plant Pathology Department, Southwest Florida Research and Education Center, Immokalee, FL; P. Weingartner,
associate professor, retired, Plant Pathology Department; T.A. Kucharek, professor emeritus, Plant Pathology Department; Florida Cooperative Extension
Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611.
The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and
other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex,
sexual orientation, marital status, national origin, political opinions or affiliations. U.S. Department of Agriculture, Cooperative Extension Service,
University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry
2007 Florida Plant Disease Management Guide: Potato, Irish 2
Bacterial Soft Rot, Black Leg, and Aerial
Stem Rot (Aerial Black Leg) (Erwinia
carotovora pv. carotovora and E. carotovora
Symptoms: These diseases are closely related.
Soft rot affects the tubers as seed piece decay or as
rots of daughter tubers. Black leg and aerial stem rot
symptoms develop on potato stems during the season.
The causal bacteria are carried on and in tubers.
Seed piece decay is favored by wet weather and
temperatures exceeding 680F. Rotting of seed tubers
in Florida is especially severe in warm humid weather
when condensation develops on cold seed tubers prior
to or during cutting. Bacteria from rotting seed
pieces infect daughter tubers by entering lenticels or
Black leg can develop at anytime during the
season and stems may rot at any point from the seed
piece to several inches above ground. During humid
weather, affected stems are soft and water-soaked.
Lesions shrivel and harden when dry weather
prevails. The color of lesions varies from light brown
to black. Often the pith above the lesions is decayed.
Infected plants often are stunted and have rolled
terminal leaflets which turn yellow and wilt. In
advanced cases the entire plant dies.
Aerial stem rot is generally limited to above
ground plant parts. It is distinguished from other rots
as a soft rot of stems and petioles which does not start
from the seed piece. Infection is often, but not always
predisposed by plant injury. Symptoms start with a
soft green decay which often turns brown in color.
Tuber rot symptoms vary from dark black decay
emanating from the stolon end, to dark colored
lesions at the lenticels. Rot can also initiate from
wounds. Affected tissue is cream to tan in color and
is soft and granular. Margins of decayed tissue often
develop brown to black margins. Advanced stages of
decay often become slimy and foul smelling due to
secondary organisms. Some infections wall off and
dry up as chalky white spots. See Plant Pathology
Fact Sheet PP-12.
Cultural controls: Use certified seed derived
from tissue culture or stem-cutting programs. Avoid
moisture condensation on seed tubers by air stacking
and maintaining air movement through seed piles.
Sanitize seed cutting equipment between seed lots.
Plant in well-drained soil, but also avoid planting
under excessively dry conditions. During the season
avoid cultural practices which wound stems and when
possible remain out of fields when plants are
excessively wet. To avoid post harvest losses, follow
appropriate vine-killing procedures. Do not harvest
after a rain. Minimize bruising and wounding of
tubers and avoid exposure to excessive heat. Tubers
harvested when soil temperatures exceed 700F are
more susceptible to rot and the decay is speeded up
when tubers are warmed in sunlight. Dry all tubers of
free surface moisture after washing. Clean flumes as
frequently as possible to avoid buildup of organic
matter and soft rot bacterial inoculum. Avoid
packing in film bags. Assure ventilation and air flow
(forced air flow is best) around potatoes held for two
days or longer. Cool internal tuber pulp temperature
to 600F. See Plant Pathology Fact Sheet PP-12.
Chemical Control: See PPP-6 for specific
recommendations for seed piece treatments.
Bacterial Wilt and Tuber Brown Rot
Symptoms: Symptoms resemble those of
bacterial ring rot. Initially terminal leaflets wilt on
hot days. Wilt progresses rapidly during hot weather
and individual branches or entire plants wilt. Leaves
often turn yellow and leaf margins roll. Stem cross
sections exhibit vascular browning and as wilt
advances illustrate bacterial oozing which is more
viscous that that of ring rot. Cut stems suspended in
water will usually exude bacterial streams from the
vascular system. Tuber symptoms progress from the
stolon end and consist initially of mild browning
which progresses to distinct vascular discoloration
coupled with bacterial oozing when tubers are cut in
cross section. Soil often adheres to bacterial oozing
from the eyes of severely affected tubers. Complete
disintegration of tubers often results from secondary
invaders. Tubers from a single plant can vary from
asymptomatic to complete rot.
Cultural Control: Maintaining a dense cover
crop of sorghum/sudan grass or corn which crowd
out broad-leaved weeds can reduce incidence of
2007 Florida Plant Disease Management Guide: Potato, Irish 3
bacterial wilt. Avoid rotations with Solanaceous
crops or planting highly susceptible cultivars such as
Superior, Red LaSoda, LaRouge, or Pontiac. Do not
move soil, water, or equipment from infested to non-
infested fields. Incidence of tuber brown rot in the
crop can be reduced by delaying harvest and allowing
infected tubers to rot in the field.
Chemical controls: Incidence of bacterial wilt is
reduced following control of root-knot nematodes
with soil fumigants.
Soft rot caused by Clostridia (Clostridia spp.)
Symptoms: Clostridia were for many years
considered to be solely secondary invaders of rotting
tissue. Recent evidence, however, has shown some to
be pathogens. Clostridial bacterial soft rots are slimy
and have highly offensive odors which distinguishes
them from other the bacterial soft rots of potato.
Clostridal rots are favored by anaerobic conditions
and relatively high (86 to 990F) temperatures.
These bacteria are present in most soils and rots
caused by them are favored by water films on tuber
surfaces. Even though Clostridia are pathogens, tuber
decay is more rapid in presence of other soft rotting
bacteria such as Erwinias.
Cultural controls: Strategies for managing
Clostridial rots are the same as those for other
bacterial soft rots of potato.
Scab (Streptomyces spp., principally S.
Symptoms: Scab is a disease of tubers. New
lesions are brown and at maturity may be circular or
irregular in shape, rough- textured, and dark brown in
color. These lesions may be superficial on the tuber
surface, raised, or pitted. The pathogen strain, variety,
or soil environmental conditions affect lesion type or
severity. The lesions may be few or many, often
covering the entire tuber surface.
Common scab is normally a disease of alkaline
soils of pH = 7 or higher. Some strains cause scab in
soils with pH below 5.0, but these are less common.
Cultural Controls: Plant certified seed. Planting
infested seed not only results in diseased tubers at
harvest, but also can introduce new more aggressive
strains of the pathogen. When possible, increase the
length of time between successive potato crops.
Maintain optimum soil moisture levels during
tuberization. Avoid soil treatments which raise pH.
Seed treatments with fungicides can reduce incidence
of the disease at harvest.
Powdery Scab (Spongospora subterranea f.
Symptoms: Sponospora is a cool weather
pathogen and it was therefore long believed that
powdery scab would not develop in Florida because
of the subtropical conditions. The disease, however,
has been observed during recent years in north
Florida potatoes. Both tuber and root symptoms are
seen. Initially symptoms on roots consist of light
brown lesions that develop into wart-like galls.
Without magnification early symptoms can be
confused with galling caused by root-knot
nematodes. The galls initially are white, but turn
brown with age. Similar lesions and galls develop at
tuber lenticels and eyes. As lesions mature,
developing "spore balls" or cytosori develop in the
tissue. Fully mature lesions filled with spore balls
have not been observed in Florida, however, the
pathogen has been shown to naturally survive for at
least five years in north Florida commercial potato
fields. This organism also carries and vectors the
potato mop-top virus which has been recently (2001)
found in potatoes shipped from north Florida.
Cultural Controls: Plant only certified seed
tubers. Plant symptomless tubers to avoid introducing
the disease to uninfested areas.
Rhizoctonia stem and stolon canker and
tuber black scurf (Rhizoctonia solani)
Symptoms: Development of Rhizoctonia stem
and stolon cankers is favored by cool, wet soil
conditions between planting and emergence.
Immature sprouts are more susceptible to attack than
are green stems. Early season disease leads to
increased tuber black scurf at harvest. Inoculum is
tuber-borne, soilborne, or both. Amber brown to
black lesions develop on stems, stolons, and roots.
Severe cankers can girdle affected plant parts.
Emergence can be delayed or reduced. In severe
cases aerial plant symptoms develop including
2007 Florida Plant Disease Management Guide: Potato, Irish 4
reddening and rolling of terminal leaflets and
formation of aerial tubers. Tuber symptoms include
scurf and cracking, but conspicuous dark black
sclerotia ( "dirt which wont rub off') on the tuber
surface is the most recognized symptom.
Cultural controls: Rotate to other than crucifer
or solanaceous crops for two to three years. Use
certified, inspected seed to reduce levels of
tuber-borne inoculum. Rhizoctonia is favored by
heavy crop residue. Minimize amounts of cover crop
residues by disking in cover crops several weeks
before planting. Shallow planting can reduce stem
cankering. Development of black scurf is enhanced
by vine desiccation. Harvest as soon as possible after
vine- killing or detach tubers from the mother plants.
Chemical controls: Treat cut seed with
recommended fungicides. See PPP-6.
Early Dying or Vertcillium Wilt (Verticillium
albo-atrum and V. dahliae)
Symptoms: Early dying is difficult to distinguish
from other causes including natural plant maturity.
Symptoms can develop when plants are small, but
usually occur after blossoming. Early season infection
can result in stunting. Initially, lower leaves turn
yellow and wilt, eventually becoming necrotic.
Sometimes symptoms are confined to one side of a
plant. Leaf yellowing and necrosis progress up the
plant which dies and often remains erect. Vascular
browning is observed in stems and at the stolon end
of tubers, however, these symptoms may be due to
other causes. Although severity of early dying is
known to be enhanced by root lesion nematodes, the
species reported are not found in Florida.
Cultural Controls: Plant only certified seed.
Rotate and plant summer cover crops to nonhost
plants. Incidence and severity of the disease are
reduced following use of some nematicides.
Chemical Controls: See Nematode Management
Guide for specific nematode controls.
Early Blight (Alternaria solani)
Symptoms: Early blight is a disease of
senescence and is most severe following blossoming.
Lesions fist appear as small dark brown spots, usually
on the older lower leaves of the plant. The spots
enlarge rapidly in wet weather and develop a series of
rings resulting in a "target board" appearance.
Plants can be rapidly defoliated when conditions
favor early blight development. Stems as well as
leaves can be affected. Tuber infections occur and
wounding of the periderm exacerbates infection.
Small (1/16 to 1/8 inch), sunken round to irregular
lesions develop on the tuber surface. Occurrence of
tuber rot is not common in Florida. See Plant
Pathology Fact Sheet PP-7.
Cultural controls: Employ adequate soil
fertilization and moisture conditions. Utilize
fungicides when disease occurs. After midseason
consider using fungicides which prevent both early
and late blight diseases.
Chemical controls: See PPP-6.
Brown Spot and Black Pit (Alternaria
Symptoms: Brown spot can appear on foliage at
any time during the season and is easily confused
with early blight. Initial lesions are dark brown
circular necrotic spots up to 1/2 inch in diameter.
Spots enlarge, can merge and have necrotic brown
margins and concentric target board zonations similar
to early blight lesions. Black pit symptoms on tubers
are black sunken lesions with defined margins which
can be 1/8 inch deep and up to 1/2 inch in diameter.
Cultural Controls: Provide adequate fertilization
to the crop. Avoid bruising tuber at harvest.
Chemical Controls: Broad spectrum fungicides
which control early blight are also effective for this
disease. See PPP-6 for specific fungicides.
Late blight (Phytophthora infestans)
Symptoms: Lesions first appear as water-soaked
spots on stems, petioles, or leaflets. The spots enlarge
rapidly and on leaflets develop a brown center with a
light green border or halo. When it is humid the
fungus produces in the halo on the underside of the
leaflet a white moldy growth that contains sporangia,
spores or both. Frequently, elongate lesions that
develop on stems under the crop canopy enable the
disease to persist even during dry conditions.
2007 Florida Plant Disease Management Guide: Potato, Irish 5
Tuber rot first appears as a brown or
purplish-black metallic discoloration of the skin. An
amber brown discoloration of the flesh just below the
skin develops, usually not penetrating more that 1/4
inch. Soft rots often follow late blight in tubers
resulting in complete decay. See Plant Pathology Fact
Cultural Controls: Use certified seed. Plant
tolerant cultivars. Avoid lifting tubers in wet weather
which increases likelihood of tuber infection. If
disease is severe, desiccate vines prior to harvest.
Disease forecasts are available in the Hastings area.
Chemical Controls: FOR EFFECTIVE
CONTROL OF LATE BLIGHT, ANY
CURRENTLY REGISTERED FUNGICIDES
MUST BE APPLIED BEFORE INFECTION
Use fungicides to control late blight. In south
Florida, begin spraying after emergence and maintain
a calendar schedule. Consult with local University of
Florida, IFAS, extension personnel.
In the Hastings region spray advisories based on
forecasting used to be available. Contact the
University of Florida, IFAS, Hastings REC or area
extension agents for additional information. If
forecasting is not used, begin spraying before plants
are six to eight inches tall if late blight fails to occur
earlier. Follow a 5-7 day spray interval or an
alternative more economical method of blight control
by spraying plants following 8 consecutive days when
the average daily temperatures range from 50 to
77F and the 10 day rainfall total is 1.02 inches or
greater. Weather data are available for the Hastings
area by consulting the FAWN network. Intervals
using this method may vary from 5-14 days or more.
A MINIMUM OF A SEVEN DAY SPRAY
SCHEDULE IS REQUIRED REGARDLESS OF
WEATHER CONDITIONS ONCE LATE BLIGHT
IS REPORTED IN THE REGION. More frequent
sprays may be needed when weather conditions are
favorable. Please consult PPP-6 for suggested
White Mold (Sclerotinia sclerotiorum)
Symptoms: The disease is also known as
Sclerotinia stalk rot. It is generally more severe in
south Florida potatoes than in the north.
Water-soaked lesions develop on the stems,
beginning in the crotches or at points of injury. The
spots enlarge and turn light to dark brown as they dry
out. Infection can occur through old leaves,
especially when in contact with the soil. Secondary
infection often follows other diseases. Under
favorable conditions, a white growth of mycelium
may cover stems and foliage. Hard black resting
bodies or sclerotia may form on the surface of
infected stems or foliage, but are more common
within the pith of the stems. The sclerotia are
irregular in shape and range in size from 1/8 to 3/4 in.
Aerial infections of leaves and stems may occur from
ascospores produced from tiny mushroom-like
structures emerging from soil-bore sclerotia during
cool, moist weather. The ascospores may be
dispersed some distance by wind.
Cultural Controls: Flooding of fields for several
weeks has been suggested in the past, however, this is
impractical in modem agriculture. Avoid injuring
plant tissue when cultivating. Avoid irrigation
practices such as overhead sprinkling which prolong
wetting of foliage. Utilize cultural practices which
enhance air flow within the canopy and speed up
drying of the foliage. Avoid rotations with other
susceptible crops such as snap beans or cole crops.
Use crop rotation with grass crops and turn soil six
inches to bury viable sclerotia. In fields with a
history of white mold, application of protective
fungicides may be advisable. See Plant Pathology
Fact Sheet PP-22.
Chemical Controls: See PPP-6.
Southern Stem Rot or Southern Blight
Symptoms: The disease generally occurs near
harvest when higher soil temperatures prevail. Stem
rot often appears rapidly near the end of the
production season when rainy periods follow a
prolonged period of drought. Most infections
observed in the field are confined to the base and
under ground portions of the stems and to tubers.
2007 Florida Plant Disease Management Guide: Potato, Irish 6
Initially, wet lesions develop at the soil line. As
lesions age, white mycelial growth and numerous
white to amber brown spherical sclerotia, resembling
mustard seeds when mature, are visible on affected
stems. Sclerotial development and growth of
mycelium is usually more profuse on subterranean
plant parts. Tuber infections usually start at the stolon
end. Lesions are brown to dark brown in color and
threads of mycelium and sclerotia develop on the
tuber surface. Tuber flesh beneath the lesion can rot.
Occasionally, dark brown to black circular lesions
develop beneath tuber lenticels. See Plant Pathology
Fact Sheet PP-4.
Cultural Controls: Bury plant debris by deep
plowing. Attempt to complete harvest before hot
Virus Diseases (Various viruses)
Symptoms: A number of viral diseases affect
potato. Field diagnosis is difficult because symptoms
are often similar for different viruses. Mosaic
symptoms are often associated with strains of Potato
Virus Y, however, accurate diagnosis depends upon
laboratory tests. Fortunately, incidence of these
viruses has generally been low due to effective
control through seed certification programs. A noted
exception is tobacco rattle virus which is soilbome
and common in the Hastings region. See the
discussion on corky ringspot for details on this
Cultural Controls: Plant only certified seed.
Plant resistant cultivars when possible.
Corky Ringspot (Tobacco rattle virus)
Symptoms: Foliar symptoms usually do not
occur in the field and symptoms are limited to the
tubers. Although diagnostic symptoms of necrotic
rings and arcs occur on the tuber surface of many
smooth-skinned cultivars, the tubers of many
cultivars do not have distinguishing external
symptoms. Typical internal tuber symptoms in most
susceptible cultivars are seen as arcs and rings of
necrosis and can occur in tubers of any age
depending upon the time of infection. Tobacco rattle
virus is vectored to potato tubers by stubby root
Cultural Controls: Plant resistant cultivars such
as Superior. Crop rotations are generally ineffective,
however, there is evidence from other regions of the
U.S. that rotations with alfalfa provide effective
Chemical control: Consult the Nematode
Control Guide for current suggestions.
Seed-Piece Rots (Various fungi and
Symptoms: Symptoms are varied. Some
pathogens such as Fusarium cause dry rot whereas
other including soft rot bacteria result in soft, wet
rots. Seed piece decay can result from the use of
poor seed, improper handling and storing of seed
prior to planting, or planting seed in an unfavorable
Cultural Controls: Purchase only certified, low
generation seed. For maximum protection, have seed
inspected on arrival. Segregate truckloads or lots of
seed. Avoid moisture condensation on incoming
seed by air stacking (spacing) to maximize air flow
around the tubers. Allow tubers to warm to 55 to
55F before handling. Avoid bruising of seed tubers.
Sanitize cutters between seed lots or at least daily.
Wound healing is favored by fresh air, 95 to 99%
relative humidities, and temperatures of 55 to 600F.
Lower humidity or a film of water interferes with
normal healing. In Florida, the most favorable
conditions for wound healing are often in the soil.
Many seedbome pathogens can be controlled with
seed treatments applied at the time of cutting.
Chemical Controls: See PPP-6 for specific
Post-harvest tuber rots (Bacterial soft rots,
Pythium spp, Sclerotium rolfsii, Fusarium
spp., Rhizoctonia solani)
Symptoms: Bacterial soft rots, Pythium leak,
Sclerotium rot, Fusarium rots, and Rhizoctonia black
scurf, and occasionally charcoal rot (Macrophomina
phaseolina) are the most frequently observed post
harvest rots in Florida. The symptoms of these rots
vary. Pythium leak and Sclerotium rot are often
initiated at the stolon end of the tuber, but they can
2007 Florida Plant Disease Management Guide: Potato, Irish 7
also occur in wounds, or in the case of Sclerotium, at
lenticels. Similarly, Fusarium can be associated with
the stem end of tubers, but is also often seen
emanating from wounds.
Cultural Controls: Although at harvest and post
harvest handling of tubers can exacerbate soft post
harvest rots, most of these diseases are most
effectively managed through use of the previously
described appropriate practices at planting and
through the season coupled with practices which
minimize tuber injury at harvest.
Chemical Controls: Pythium leak, Fusarium
rots, and Rhizoctonia black scurf can be partially
managed with appropriate fungicide programs. See
PPP-6 for fungicide recommendations.