Reports on Plant Diseases
913 - Late Blight and Buckeye Rot of Tomato
[ Symptoms ]
[ Disease Cycle ][ Control ]
Late blight of tomato is caused by the fungus Phytophthora
infestans. Currently, two physiologic races of
the fungus, designated T-0 and T-1, have been identified. There
are also potato and tomato strains of the fungus; each is capable
of infecting the other host. Buckeye rot is caused by three
species of Phytophthora. P. parasitica is
most commonly associated with the disease in Illinois; in other
areas of the country the disease is attributed to P.
capsici and/or P. drechsleri.
These two diseases may be widespread and destructive in Illinois
during wet seasons when the foliage and fruit are not protected
by fungicides. In cool, moist weather the vines and fruit rot very
rapidly from late blight. During prolonged warm, wet weather a
large percentage of the tomato fruit in contact with the soil may
be affected by buckeye rot.
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On seedlings, small, dark spots form on the stems or leaves followed
by death within 2 or 3 days. On older plants the fungus causes
small to large, irregular, rapidly enlarging, water-soaked, pale
green to greenish black lesions which usually start at the margins
or tips of the leaves. In dry weather, these lesions turn dark
brown, dry, and wither. A pale green “halo” often surrounds
affected leaf areas (Figure 1). The spots may enlarge until entire
leaflets are killed. Lesions can expand rapidly and result in extensive,
if not complete, defoliation within 2 weeks. Severely affected
plants may appear as if damaged by frost. Infection of both green
and ripe fruit starts near the stem-end or the side of the fruit.
It soon spreads over
the entire fruit. Infected areas are dark green, brown, or brownish
black and “greasy,” with a rather firm but slightly
wrinkled surface (Figure 2). Secondary organisms commonly invade
affected fruit causing a soft, wet rot.
In moist weather, the late blight fungus produces a white, downy
mildew mostly on the underside of the leaf lesions and on the surface
of affected fruit. The mildew growth is composed of spore-bearing
structures (sporangiophores) which bear lemon-shaped spores called
sporangia (Figures 3a and b).
A primary source of initial inoculum is often from infected potato
tubers in cull piles or those left near or on the soil surface.
(Potatoes are an alternative host of the fungus). The fungus overwinters
in tubers in storage or in the field and resumes growth when the
tubers sprout by infecting the seedlings. Under favorable environmental
conditions, the fungus produces sporangia. The sporangia are spread
by wind or splashing rain to nearby susceptible host plants, infect
them, and initiate another disease cycle. The fungus is capable
of remaining active during mild winters in southern states on potato,
tomato, eggplant, petunia, and numerous other plants in the potato-tomato
family (Solanaceae). Prevailing winds transport the sporangia
northward. Tomato transplants grown in these areas become infected
and serve as a primary source of inoculum in northern states to
where they are transported. The fungus also may remain active in
northern states during the winter on greenhouse tomatoes. In the
spring, sporangia are carried from the greenhouse to the field.
Ideal environmental conditions that favor epidemic development
include warm days (70° to 85°F or 21° to 29°C)
and a relative humidity near 100 percent, followed by cool nights
(45° to 50°F or 7° to 15°C) with heavy dew, fog,
or a light, drizzly rain that persists through the morning.
Heavy overcast skies during the morning prevent temperatures from
rising rapidly and the foliage remains wet. In moist weather, viable
sporangia may be carried 20 miles or more by strong winds and rain.
The late blight fungus is a water mold that forms large numbers
of microscopic, colorless sporangia (Figures 3a and b). Sporangia
may germinate directly by means of a germ tube or by forming motile
zoospores (Figure 3d) that swim freely in water films before settling
down (encysting) on leaves to cause infection. The encysted zoospores
penetrate a leaf by sending a germ tube (Figure 3e) through a naturalopening
(stomate) in the leaf surface, or usually by forming a cushion-shaped
structure (appressorium) from which an infection peg invades the
Once inside the tomato plant, the fungus hyphae grow rapidly between
the cells. Food is obtained by finger-like projections (haustoria)
that invade the tomato cells and absorb nutrients. After the fungus
has started to colonize the plant, initial symptoms (yellowing
or chlorosis) may appear in 2 or 3 days, but usually are evident
after 5 to 7 days. Soon after symptoms appear, the sporangiophores
emerge through the stomates and produce sporangia which serve as
inoculum for repeating, secondary disease cycles.
The conditions that favor spore production, dissemination, and
infection are those which determine the extent of the epidemic.
At temperatures of 45° to 86°F (7° to 30°C), with
an optimum of 65° to 70°F (18° to 21°C), and a
relative humidity near 100 percent for more than 10 to 15 hours,
the fungus forms sporangia. The temperature and number of hours
of essentially 100 percent humidity determine the rate of spore
production. The sporangia may germinate in one of two ways determined
largely by the temperature. At temperatures of 45° to about
70°F (7° to 21°C), and with 100 percent relative humidity,
each sporangium give rise to 8 to 12 or more motile zoospores (Figures
3c and d) in 1 to 3 hours. The optimum temperature for zoospores
formation is 54°F (12°C).
Each zoospores swims for a few minutes in a moisture film, encysts,
then germinates to produce a germ tube and an appressorium from
which a penetration peg invades the leaf. The optimum for zoospores
germination is 54° to 59°F (12° to 15°C) and for
germ tube development it is 70° to 75°F (21° to 23°C).
Germination of sporangia occurs directly via a single germ tube
at about 70° to 86°F (21° to 30°C) without producing
zoospores. The optimum temperature for direct penetration is 77°F
(25°C) and takes from 8 to 48 hours. Both types of germination
occur at overlapping temperatures. Penetration has occurred, infection
and disease development is most rapid at 72° to 76°F (22° to
24°C). Besides tomato and potato, the causal fungus attacks
eggplant, petunia, ground-cherries (Physalis spp),
and other weeds in the potato-tomato family (Solanaceae).
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1. Late blight first appears as water-soaked, greenish
black areas on leaves and stems that later turn brown. Often
a pale green band surrounds the affected area on the leaf.
(Courtesy R.E. Stall)
2. Late blight attacks both green and ripe fruit. Rotted
areas are firm but slightly wrinkled and dark
green, brown, or brownish black.
3. Phytophthora infestans, the late blight fungus,
as seen under a high-power microscope: (a) two sporangiophores
emerging from stomates on the underside of a potato leaf and
bearing a number of lemon-shaped sporangia; (b) two sporangia;
(c) sporangial contents dividing up to form zoo spores; (d)
sporangium releasing zoospores; (e) encysted zoospore with
two germinating zoospores (drawing by L. Gray).
4. Buckeye rot (Purdue University photo).
5. Phytophthora parasitica, a common buckeye rot fungus:
sporangiophores emerging from stomates with immature and mature,
lemon-shaped sporangia on their tips; (right) sporangia in various
of form ing zoospores (drawing by Lenore Gray).
Symptoms appear on the fruit as a grayish green to greenish brown,
water-soaked spot near the blossom end, or where the fruit comes
in contact with the soil. The infected area enlarges rapidly in
warm weather until half or more of the fruit is affected. When
the rot develops slowly, the decayed area shows definite, pale
brown, somewhat concentric rings (Figure 4). The discoloration
may extend to the fruit center. In staked or caged tomatoes, only
the fruit clusters nearest the ground are affected. Commonly, older,
diseased fruit is colonized by secondary bacteria and fungi that
produce a soft, wet fruit rot. A comparison of the fruit symptoms
of buckeye rot and late blight is given in Table 1.
Table 1. Comparison of symptoms on tomato fruit caused
by Phytophthora spp. (Buckeye Rot)
and P. infestans (Late Blight)
Soft and turgescent
Hard and dry
Large brown lesions
Small brown lesions
Endocarp and seed discolored
Exocarp only brown
|Around the calyx
Not usually infected
|Vascular bundles near hilum
Discolored at an early stage
Discolored only when tissue around the calyx is infected
|Reddening of fruit
|Appearance of epidermis
The buckeye-rot fungi may also cause a pale to dark brown, girdling
stem canker which may be somewhat sunken. Affected plants usually
wilt and die. In very wet weather a sparse white mildew appears
on both the fruit and the stems.
The fungi causing buckeye rot are primarily soilborne and cause
disease only in warm, wet weather. Like the late blight fungus,
two types of spores are formed. The primary spore, or sporangium,
develops first. These lemon-shaped spores are formed at the tips
of simple sporangiophores that emerge through the stomates (Figure
5). The sporangia later give rise to motile zoospores. Sporangiophores
and sporangia are not formed unless the soil is wet and above 65°F
(18°C). At 70°F (21°C) the sporangia may be formed
within 24 hours. The sporangia are spread by surface or irrigation
water and splashing rains.
They may remain viable for days when conditions remain moist.
During wet periods, zoospores are discharged from the sporangia
(Figure 5) and are readily splattered by rain from the soil to
the fruit. The zoospores swim about in a film of water for a time,
encyst, and infect the fruit. Penetration can occur through the
unbroken skin. Visible symptoms of the disease may appear within
24 hours and fruit rot develops rapidly. A temperature of 80°F
(26°C) or above is ideal for infection and development of the
disease (optimum is 75° to 86°F or 23° to 30°C).
Fruit with latent infections may decay during transit.
The fungi may spread from fruit to fruit in transit and storage
if the temperature is 70°F (21°C) or above. The fungi are
carried from one part of a field or garden to another by runoff
water, farm equipment and tools, and by workers. Other hosts of
the buckeye-rot fungi include beans, corn, eggplant, melons, onions,
peppers, potato, pumpkin, rhubarb, squash, and turnip.
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Before planting, destroy potato cull piles and prevent growth
of volunteer potatoes. Plant tomatoes as far as possible from
Purchase only certified, disease-free transplants or seed.
If potatoes are also planted purchase only blue tag tubers
certified as disease-free.
Plant in a well-drained, porous soil. Follow a 3-year rotation
excluding susceptible crops. Where possible, keep tomato fruit
off the ground by staking or caging each plant or mulch the
ground with black plastic, straw, hay, ground corn cobs, and
The soil in plant beds and greenhouses should be disinfested
with steam or a soil fumigant.
When transplanting, discard and destroy all tomato seedlings
with lesions on the stem and leaves.
Control all weeds. In commercial fields and home gardens eradicate
all diseased potato tubers plus nightshades, Jerusalem-cherry,
groundcherries, and other weeds in the Solanaceae.
When the foliage is wet with fog, dew, or rain, do not cultivate
or work with the plants. This spreads fungal spores from plant
The use of protective fungicides is essential in controlling
late blight and buckeye rot. For information on current fungicide
recommendations, refer to Midwest Management Guide for Commercial
Vegetable Growers. Spray applications should begin no later
than 2 weeks after the first tomato flowers are open, and should
continue at 5- to 10-day intervals. Five spray applications
are usually sufficient in a “dry” season, while
6 to 8 sprays may be required in a “wet” season.
When possible, time the applications for maximum deposit on
the foliage and fruit just before rainy periods when most infections
occur. If late blight is reported in the vicinity, county,
or the general area, fungicide applications should be made
at once regardless of the stage of plant development. If late
blight appears in the field or garden, the interval between
applications of fungicide should be shortened to 5 days until
no further spread of the disease is noted.
Buckeye rot is not apt to be a problem if fungicides are applied
for general control of foliar and fruit diseases on a regular
7- to 10-day schedule. It is important, however, to apply fungicides
early, and to uniformly cover all fruit. Take extra care to
see that the center of each plant and the undersides of the
leaves are protected. Follow the manufacturer’s directions
as regards amounts to use, interval between the last spray
and harvest, and the compatibility of fungicides and insecticides.
In the home garden, harvest all ripe fruit at each picking.
If left in the field, such fruit will decay, and serve as a
source of infection for remaining fruit.
After harvest is completed, spade or plow under, compost,
or burn all tomato vines. Destroy all potato cull piles, volunteer
plants, and solanaceous weeds.
Rotate 3 or 4 years before planting tomatoes or potatoes in
the same area. If buckeye rot is a problem, also exclude other
vegetables which are hosts of the causal fungus (see previous
Cultivars are available with varying degrees of resistance
to the late-blight fungus. For more information on recommended
varieties, refer to current seed catalogs and trade publications.
Several late blight-forecasting systems have been developed
to help growers identify periods when the environment favors
disease development and fungicide coverage is most important.
Publications available at your nearest Extension office: C1373
Midwest Vegetable Production Guide for Commercial Growers, and
C1331 Vegetable Gardening in the Midwest. They can also be purchased
from ITCS, University of Illinois P345, 1917 S. Wright St., Champaign,
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Information concerning insecticides, weed control, varieties, and other
recommendations can be found in the Illinois Homeowners' Guide to Pest
Management, available at your nearest Extension office.