of Rhode Island GreenShare Factsheets
Blight of Tomato and Potato
blight is a very common disease of both potato and tomato. It causes
leaf spots and tuber blight on potato, and leaf spots, fruit rot
and stem lesions on tomato. The disease can occur over a wide range
of climatic conditions and can be very destructive if left uncontrolled,
often resulting in complete defoliation of plants. Despite the name,
it rarely develops early, instead usually appearing on mature foliage.
leaves of both crops, the first symptoms usually appear on older
leaves and consist of small, irregular, dark brown to black,
spots ranging in size from a pinpoint to 1/2 inch in diameter.
As the spots enlarge, concentric rings may form as a result
growth patterns by the organism in the leaf tissue. This gives
the lesion a characteristic "target-spot" or "bull's eye" appearance.
There is often a narrow, yellow halo around each spot and lesions
are usually bordered by veins. When spots are numerous, they
grow together, causing infected leaves to turn yellow and die.
Usually the oldest leaves become infected first and they dry
up and drop
from the plant as the disease progresses up the main stem.
tomato, stem infections can occur at any age resulting in small,
dark, slightly sunken areas that enlarge to form circular or
spots with lighter-colored centers. Concentric markings, similar
to those on leaves, often develop on stem lesions. If infested
are used to start tomato transplants, seedlings may damp off soon
after emergence. When large lesions develop at the ground line
stems of transplants or seedlings, the plants may become girdled,
a condition known as "collar rot." Such plants may die when
set in the field or, if stems are weakened, may break early
in the season.
Some plants may survive with reduced root systems if portions of
stems above the canker develop roots where they contact the
Such plants, however, usually produce few or no fruits. Stem lesions
are much less common and destructive on potato.
drop and spotting of fruit stems, along with loss of young fruit,
may occur when early blight attacks tomatoes in the flowering stage.
On older fruits, early blight causes dark, leathery, sunken spots,
usually at the point of stem attachment. These spots may enlarge
to involve the entire upper portion of the fruit, often showing
concentric markings like those on leaves. Affected areas may be
covered with velvety black masses of spores. Fruits can also be
infected in the green or ripe stage through growth cracks and other
wounds. Infected fruits often drop before they reach maturity.
On potato tubers, early blight results in surface lesions that appear
a little darker than adjacent healthy skin. Lesions are usually
slightly sunken, circular or irregular, and vary in size up to 3/4
inch in diameter. There is usually a well defined and sometimes
slightly raised margin between healthy and diseased tissue. Internally,
the tissue shows a brown to black corky, dry rot, usually not more
than 1/4 to 3/8 inch deep. Deep cracks may form in older lesions.
Early blight is caused by the fungus Alternaria solani, which
survives in infected leaf or stem tissues on or in the soil. This
fungus is universally present in fields where these crops have been
grown. It can also be carried on tomato seed and in potato tubers.
Spores form on infested plant debris at the soil surface or on active
lesions over a fairly wide temperature range, especially under alternating
wet and dry conditions. They are easily carried by air currents,
windblown soil, splashing rain and irrigation water. Infection of
susceptible leaf or stem tissues occurs in warm, humid weather with
heavy dews or rain. Early blight can develop quite rapidly in mid
to late season and is more severe when plants are stressed by poor
nutrition, drought or other pests. Infection of potato tubers occurs
through natural openings on the skin or through injuries. Tubers
may come in contact with spores during harvest and lesions may continue
to develop in storage.
Use a crop
rotation that includes potatoes or tomatoes only every third
or fourth year to allow infested plant debris to decompose
in the soil. Rotations with small grains, corn or legumes are preferable.
Use tillage practices such as fall plowing that bury all
Select cultivars that have a lower susceptibility to early
Use certified disease-free tomato seed and transplants.
If producing tomato transplants, disinfect soil in
plant beds and control humidity in cold frames or greenhouses.
Practice good sanitation throughout the transplant production operation.
Use appropriate measures to control weeds.
Maintain fertility at optimal levels--nitrogen and
phosphorus deficiency can increase susceptibility to early blight.
Time applications of overhead irrigation to allow plants
to dry before nightfall.
Although the above measures are important to minimize
infection, it is usually necessary to apply fungicide sprays to
fully protect plants from early blight. Applications to tomato are
usually begun 2-3 weeks following emergence or soon after transplanting.
For potatoes, fungicide applications should be initiated when plants
begin to flower. Thorough coverage is important.
from the Ohio State University Extension, 2000
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
of Rhode Island Cooperative Extension provides equal program