Oklahoma Cooperative Extension Service
Part I. Diseases Caused by Fungi
John P. Damicone
Extension Plant Pathologist
Oklahoma Cooperative Extension Fact Sheets
are also available on our website at:
Diseases are a major limiting factor for tomato produc-
tion. Diseases can be classified into two groups. The first
be exposed more to sunscald. Tomatoes irrigated by sprinkler
are those caused by infectious microorganisms that include
systems that wet the foliage and fruit are more likely to de-
fungi, bacteria, viruses, and nematodes. These diseases are
velop disease problems than those watered by drip or furrow
contagious and can spread from plant to plant in a field, often
very rapidly when environmental conditions are favorable.
Planting disease resistant varieties is probably the most
The second group includes those caused by non-infectious
effective and economical method of disease control. Disease
physical or chemical factors such as adverse environmental
resistance can be utilized to solve current problems or to
factors, nutritional or physiological disorders, and herbicide
prevent a disease from increasing. Fortunately, many excel-
injury. Non-infectious diseases cannot spread from plant to
lent tomato varieties have been developed with resistance
plant; however, the distribution of the disease may be quite
to one or usually more of the common tomato diseases. The
uniform and extensive if an entire planting was exposed to
letters “V” (Verticillium wilt), “F” (Fusarium wilt), “N” (root-knot
the adverse factor.
nematode), “T” or “TMV” (tobacco mosaic virus), “A” or “ASC”
It is critical for effective disease control that the difference
(Alternaria stem canker), and “S” or “St” (Stemphyllium-gray
between infectious and non-infectious diseases be recognized
leaf spot) appear in the variety descriptions in seed catalogs or
and that the type of microorganism causing an infectious
on seed packages to designate resistance to the corresponding
disease be determined. For example, use of a fungicide to
diseases. Strains of the Fusarium wilt fungus have developed
control a non-infectious disease, such as blossom-end rot, is
that can overcome previously resistant varieties. “F1” indicates
a wasted expense which will not correct the problem. This fact
resistance to the original strain (race 1) while “F2” indicates
sheet is intended to aid vegetable producers in recognizing
resistance to the new strain (race 2). OSU Extension Fact
the symptoms of common tomato diseases caused by fungi.
Sheet HLA-6032 lists many disease resistant tomato varieties
Fungi are the most common cause of infectious plant diseases
that are known to perform well in Oklahoma.
and can be very destructive. Diseases of tomatoes caused
Sometimes it is necessary to use chemical control because
by bacteria, viruses, and nematodes are described in OSU
resistance is not yet available to some important diseases.
Extension Fact Sheet EPP-7626. Non-infectious diseases of
The registration status, rates, timings, and method of applica-
tomato are covered in OSU Extension Fact Sheet EPP-7627.
tion of fungicides often change. Consult the OSU Extension
Many times symptoms of diseases are non-typical or confus-
Agents Handbook (E-832) or your county Extension educator
ing. It is a good practice to submit samples of diseased plants
for the latest recommendations for chemical control of tomato
to the OSU Extension Plant Disease Diagnostic Laboratory
for an accurate diagnosis, particularly when a new disease
Wilts & Blights
Control of tomato diseases is best if al available methods,
i.e. cultural practices, disease resistant varieties, and use of
chemicals, are integrated into an overal management strategy.
(Fusarium oxysporum f.sp. lycopersici)
Cultural practices are aimed at avoiding disease or delaying its
occurrence. It is critical to start with disease-free transplants
Wilt diseases are caused by pathogens that invade the
because many infectious diseases may be carried on tomato
vascular system (xylem tissue) and disrupt water flow through
seed. Crop rotation is another cultural practice for reducing
the plant. Fusarium wilt is the major wilt disease of tomato in
losses from plant diseases. It is important to remember that
Oklahoma. Verticillium wilt is easily confused with Fusarium
vegetables such as pepper, eggplant, and potato should be
wilt but has not yet been reported in Oklahoma.
avoided in rotations with tomatoes. These crops are suscep-
The first symptom is usually a yellowing of the lower
tible to many of the same diseases. Cultural management
leaves, which gradually wilt and die. Symptoms may at first
of the crop will also influence what diseases may develop.
occur on only one side of the plant (Figure 1). The disease
Tomatoes allowed to spread on the ground will develop more
progresses up the stem until all of the foliage is killed and the
soil rot problems, while fruit on staked or trellised plants will
plant dies. If stems or petioles from wilted areas of diseased
Division of Agricultural Sciences and Natural Resources • Oklahoma State University
plants are cut, a reddish brown discoloration can be seen
The fungus survives in the soil as sclerotia which may
between the pith (center of the stem) and the outer green
build to high numbers when susceptible plants are cropped
part of the stem.
repeatedly. After sclerotia germinate, the fungus must first
The fungus survives and persists indefinitely in field
colonize organic debris near the soil surface before the fungus
soil. The fungus is also seedborne and is thought to spread
can cause infection. The disease is favored by high humidity
long distances in this manner. The disease is most serious
and soil moisture and warm to hot temperatures (85-95oF).
in sandy soils and at temperatures between 80-90oF. Soils
Control: Southern blight is difficult to control when con-
become infested by planting infected transplants and from
ditions favor the disease and when numbers of sclerotia in
movement of infested soil by wind and water erosion or on
the soil are high. Crop rotation with a non-susceptible grass
crop such as corn is the most effective means of reducing
Control: Growing tomato varieties resistant to Fusarium
numbers of sclerotia and resulting incidence of southern
wilt is the most effective means of control. Most modern
blight. Avoid planting tomato following a very susceptible crop
varieties have resistance to race 1 and many to both races
such as cantaloupe or watermelon. Plant residues should be
1 and 2. Resistant varieties can become susceptible to
thoroughly incorporated into the soil prior to transplanting so
Fusarium wilt over time when they are intensively cropped on
that their presence on the soil surface does not encourage
the same site. Crop rotation should be used in conjunction
southern blight development. Side-dressing with ammonium
with varietal resistance to maintain sustainable control and
nitrate rather than other forms of nitrogen and use of a soil
limit the development of new races.
fungicide at transplanting may provide some control.
Southern Blight (Sclerotium rolfsii)
Southern blight can be a devastating disease of tomato
in Oklahoma. The southern blight fungus has a wide host
Early Blight (Alternaria solani)
range attacking more than 200 species of plants that include
common weeds and crop plants.
Early blight is a common leaf-spotting fungal disease of
The initial symptom of southern blight is a rapid wilting of
tomato. Extensive defoliation from early blight exposes fruit
the entire plant. A water-soaked lesion on the stem near the
to sunscald and increases fruit rot. Early blight also attacks
soil line rapidly expands, turns brown, and girdles the stem.
stems and fruit. Foliar diseases are most severe in eastern
A white mold (mycelium) eventually covers the stem lesion
Oklahoma where rainfall and relative humidity levels sup-
and surrounding moist soil. Small, uniformly round structures
port disease development, or wherever sprinkler irrigation is
about 1/16 inch in diameter, called sclerotia, form on the my-
celium. Sclerotia are first white, later becoming brown, and
Dark brown cankers may develop on and girdle stems of
resemble mustard seeds. The presence of the white mycelium
seedlings at the soil line. Stem lesions on older plants usually
and sclerotia at stem base of affected plants are very useful
remain confined to one side of the stem. Leaf symptoms ap-
characteristics for identifying southern blight (Figure 2).
pear on older leaves first and are characteristic of the disease.
Infected leaves show circular spots (lesions) up to 1/2 inch
in diameter that are brown and have a pattern of concentric
rings imparting a “target-like” appearance to the spot (Figure
3). The leaf area surrounding these spots usually turns yellow.
Infected leaves eventually turn brown and drop from the plant.
Defoliation progresses upward from the lower plant. Sunken,
dry lesions occur most frequently on the stem end of the fruit
and also have a zonate or “target-like” appearance.
The fungus survives in the soil by forming resistant spores
Figure 1. Fusarium wilt—Initial symptoms are yellowing
of older leaves which progresses upwards and later wilt
Figure 2. Southern blight—White moldy growth (mycelium)
and die. (Photo courtesy L.L. Black, Louis.State Univ.)
develops at the base of infected plants.
Figure 3. Early blight—Leaf symptoms are large (up to
Figure 4. Septoria leaf spot—Leaf symptoms are small (1/8
1/2 inch diameter), brown, zonate or target-like spots.
inch diameter) spots with gray centers and dark borders.
(Photo courtesy R.X. Latin, Purdue Univ.)
in association with diseased tomato debris that are capable of
persisting for one year and probably longer. Infection occurs
Gray Leaf Spot (Stemphyllium solani)
rapidly under warm, humid conditions. Thousands of spores
are produced in spots of infected leaves that are capable of
This is a disease confined entirely to the leaves of tomato.
causing more infections. Plants under stress from nitrogen
The oldest leaves are affected first. Symptoms first appear as
deficiency, heavy fruit load, or other factors are most suscep-
small brownish-black flecks that extend to both sides of the
tible to the disease.
leaf. Spots may be bordered by a narrow yellow halo. As the
Control: Crop rotation with crops other than eggplant,
spots enlarge, the central portion becomes gray, cracks, and
potato, and pepper should be practiced to reduce and delay
falls out imparting a shot-hole appearance to affected leaves.
early blight development. Avoid prolonged wetting of leaves
The disease closely resembles Septoria leaf spot except that
from irrigation or use drip irrigation. Maintain adequate but not
no small, black fruiting bodies form in the center of gray leaf
excessive soil fertility. A spray program using a recommended
spots. Leaves with numerous spots turn yellow and drop to
fungicide beginning at fruit set and continuing on a 7- to14-day
the ground. The fungus overwinters on infected plant residue.
schedule should be maintained where early blight problems
The disease is favored by warm temperatures, high humidity,
heavy fruit loads, and adequate levels of soil fertility.
Control: Many of the newer tomato varieties recom-
mended for growth in Oklahoma are resistant to gray leaf
Septoria Leaf Spot (Septoria lycopersici)
spot. If a susceptible variety is grown, cultural and chemical
Septoria is a very common foliage disease in Oklahoma
controls are similar to those for early blight and Septoria leaf
that may also attack stems but not fruit. The disease first ap-
pears on the lower leaves after the plant has set fruit. Leaf
spots begin as yellow areas that later become circular with
Late Blight (Phytophthora infestans)
gray centers and dark borders (Figure 4). Spots may reach
1/8 inch in diameter and be surrounded by a yellow halo. Tiny
Late blight an important potato disease, can also oc-
black specks, which are fruiting structures that release spores,
cur on tomato. The warm to hot temperatures that prevail
develop in the center of these spots. Severely infected leaves fall
in Oklahoma during periods of tomato production make the
off. Defoliation progresses from the base of the plant upwards
occurrence of this disease unlikely. However, the disease
and resembles early blight from a distance. However, the larger
could be a potential problem during unseasonably cool and
dark lesions with concentric rings of early blight are clearly
wet conditions on early-planted or fall-cropped tomatoes. All
different from small, speckled lesions of Septoria leaf spot.
parts of the plant are affected and fruit decay can be severe.
Loss of foliage may cause fruits to become sunscalded.
Symptoms appear on the upper surface of leaves as
Most infection early in the season probably arises from
greasy, gray spots that expand rapidly. White mold usually
infested plant debris remaining in the soil from a previous
develops at the margins of affected areas (Figure 5). If stems
tomato crop. Spores of the fungus are spread by splashing
and petioles are infected, areas above these infections wilt and
rain. The disease is favored by moderate temperatures and
die. Entire plants may be rapidly defoliated when conditions
extended periods of high relative humidity.
favor the disease. Fruit infections begin as brown, greasy
Control: Crop rotation and thorough shredding and
spots that rapidly expand to rot the entire fruit.
incorporation of infested plant residue soon after harvest are
The fungus is a wet weather disease favored by cool
recommended to reduce Septoria leaf spot. Weed control
nights and warm days. Temperatures above 86oF are con-
should be maintained because jimsonweed, horse nettle, and
sidered unfavorable for late blight development. The fungus
nightshade are also sources of infection. Drip but not sprinkler
survives mainly in potato seed tubers and in infected tomato
irrigation is recommended to reduce periods of leaf wetness
transplants. Some survival may also occur in dead potato and
and water splashing. Avoid working plants while foliage is wet.
tomato vines. The disease often begins in potato plantings
A fungicide spray schedule for early blight will also be effective
from which spores of the fungus are blown by wind to infect
for control of Septoria leaf spot.
tomatoes when conditions are favorable. Disease develop-
Figure 5. Late blight—Typical foliage symptoms are greasy
Figure 6. Anthracnose—Fruit rot symptoms are circular,
gray spots with white moldy growth at the margins which
sunken spots with dark centers that become enlarged
turn brown with age. (Photo courtesy L.L. Black, Louis.
and more depressed as fruit ripen.
ment is rapid under extended periods of favorable conditions
and ceases when weather becomes hot and dry.
Control: Late blight control centers on use of disease-free
Soil rots are caused by several fungi which reside in
transplants and certified disease-free potato seed to avoid
soil and infect fruit in contact with the soil. Warm, wet soil
introduction of the disease. Tomatoes should be planted as
and growth of tomatoes in poorly drained areas favor soil rot
far as possible from potatoes and potato cull piles. Dead vines
development. Ripe fruit is generally most vulnerable to soil rot
from previous crops should be removed from the vicinity of
development; however, green fruit may also be affected.
new plantings or destroyed. Avoid use of sprinkler irrigation
where possible. A fungicide spray schedule on 7- to 14-day
intervals may be implemented.
Buckeye rot (Phytophthora parasitica)
Buckeye rot is a destructive fruit rot of green or ripe fruit.
The first symptoms are grayish-green or brownish, water-
soaked spots developing where fruit contact the soil or where
Anthracnose (Colletotrichum phomoides)
soil frequently splashes onto fruit. When temperatures are
Anthracnose is a common and widespread rot of ripe
warm (above 80oF) the spot rapidly enlarges to cover up to
or overripe tomato fruit. Symptoms are rare on green fruit.
half of the fruit diameter. The rot then appears brown with
Symptoms on ripe fruit are small, sunken, circular spots that
concentric rings that resemble the markings of a buckeye
may increase in size up to 1/2 inch in diameter. The center
chestnut (Figure 7). The disease is most severe in poorly
of older spots later become blackish (Figure 6). Spots may
become numerous in severe cases and secondary rotting
organisms may invade anthracnose lesions to completely rot
Southern blight (Sclerotium rolfsii)
The southern blight fungus causes a sunken, yellowish
The fungus forms small, dark survival structures called
lesion to develop where ripe fruit contact the surface of in-
sclerotia in the centers of fruit spots. These sclerotia survive
fested soil. The lesion enlarges to become star-shaped and
in soil for up to three years and cause infections either directly
water-soaked. The entire fruit collapses within 3 to 4 days. A
or by producing secondary spores. Green fruit are infected
mass of white mold and developing sclerotia become evident
but do not show symptoms until ripening. The fungus then
on the lower side of rotted fruit.
spreads from infected to healthy fruit as spores are splashed
by rain or overhead irrigation, or by pickers working wet plants.
Pythium rot (Pythium spp.)
Anthracnose is favored by warm rainy weather, overhead ir-
Pythium rot begins as a small water-soaked spot where
rigation, and heavy defoliation from foliar disease.
the fruit contacts the soil. Within 72 hours the entire fruit
Control: Harvest fruit as soon as possible after ripening.
becomes rotted, the skin ruptures, and the watery contents
Avoid excessive overhead irrigation or use drip irrigation to
of the fruit spill out. A white, cottony growth may be observed
reduce moisture levels on fruit and humidity in the plant canopy.
within lesions under humid conditions.
Fungicide sprays used to control leaf diseases reduce losses
from anthracnose when applied on a regular schedule and
Rhizoctonia soil rot (Rhizoctonia solani)
in a manner to achieve thorough fruit coverage. A three-year
Rhizoctonia causes a brown rot with alternating light- and
rotation may also reduce chances for infection.
dark-colored zonate bands and sharply defined margins in
areas where fruit contact soil (Figure 8). The rot may be firm
at first but later affected fruit becomes mushy.
Figure 7. Buckeye rot—Brown concentric rings resemble
Figure 8. Rhizoctonia soil rot—Symptoms are alternating
markings of a buckeye chestnut. (Photo courtesy L.L.
light-and dark-colored zonate bands on fruit surfaces in
Black, Louis. State Univ.)
contact with soil.
Control: Tomatoes should be staked or otherwise held
upright to minimize fruit contact with soil. Avoid planting
1. Sherf, A.F. and MacNab, A.A. 1986. Vegetable diseases
tomatoes in poorly drained soils or in low areas of a field.
and their control, 2nd Ed. John Wiley & Sons, NY, 728pp.
Mulching with plastic film or with organic matter to keep fruit
2. Jones, J.B., Jones, J.P.,Stall, R.E., and Zitter, T.A. 1991.
from contacting the soil and/or to reduce soil splashing onto
Compendium of Tomato Diseases. APS Press, St.Paul,
fruit will reduce the chance of soil rot infection. A fungicide
spray schedule for foliar disease control may also help reduce
infection, provided coverage of the fruit is achieved.
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