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University
of Rhode Island GreenShare Factsheets
Potato
Diseases: Late Blight
Late blight is one of the most devastating diseases of potato
in the northeast. Responsible for the devastating Irish potato
famine of the 1840s, late blight has continued to be important
today. Since 1990, late blight has caused widespread damage across
the United States and Canada. If left unmanaged, this disease
can result in complete destruction of potato crops.
Late blight appears on potato leaves as pale green, water-soaked
spots, often beginning at leaf tips or edges. The circular or
irregular leaf lesions are often surrounded by a pale, yellowish-green
border that merges with healthy tissue. Lesions enlarge rapidly
and turn dark brown to purplish-black. During periods of high
humidity and leaf wetness, a cottony, white mold growth is usually
visible on lower leaf surfaces at the edges of lesions. In dry
weather, infected leaf tissues quickly dry up and the white mold
growth disappears. Infected areas on stems appear brown to black
and entire vines may be killed in a short time when moist weather
persists.
On potato tubers, late blight appears as a shallow, coppery-brown
dry rot that spreads irregularly from the surface through the
outer 4-15 mm or more of tissue. On tuber surfaces, lesions appear
brown, dry, and sunken, while infected tissues immediately beneath
the skin appear granular and tan to copper-brown. When tubers
are stored under cool, dry conditions, lesion development is
retarded and, upon prolonged storage, lesions may become slightly
sunken and desiccated. Secondary bacteria and fungi frequently
enter late-blight lesions, usually resulting in a slimy breakdown
of entire tubers.
Late blight is caused by the fungus Phytophthora infestans. Unlike
most pathogenic fungi, the late blight fungus cannot survive
in soil or dead plant debris. For an epidemic to begin in any
one area, the fungus must survive the winter in potato tubers
(culls or volunteers), be reintroduced on seed potatoes or tomato
transplants, or be blown in as live spores during rainstorms.
Disease development is favored by cool, moist weather. Nights
of 50 degrees F weather and days around 70 degrees F, accompanied
by rain, fog or heavy dew are ideal. Under these conditions,
lesions may appear on leaves within three to five days of infection,
followed by the white mold growth soon thereafter. Spores formed
are spread readily by irrigation, rain and equipment. They are
easily dislodged by wind and rain and can be blown into neighboring
fields within 5-10 miles or more, thus beginning another cycle
of disease.
Infection of potato tubers arises from spores that develop
on foliage. Tubers exposed by soil cracking or erosion of hills
may come in contact with spores washed down from infected leaves
and stems by rainfall or irrigation. Tubers infected during the
growing season may partially decay before harvest. Tuber infection
may also occur at harvest, when tubers contact living spores
remaining on infected vines. Little if any tuber-to-tuber spread
of late blight occurs during storage if tubers are kept under
cool, well-ventilated conditions.
Besides potatoes, P. infestans can infect only a
few other closely related plants. Tomatoes, peppers and eggplants
can occasionally be mildly infected, as can a few related weeds
such as hairy (but not black) nightshade. Since 1990, there have
been severe outbreaks of late blight in commercial and home garden
plantings of potato and tomato in both the U.S. and Canada. Much
of this has been associated with new strains of the late blight
fungus that have spread to many areas. Some of these strains
may interact and form a type of resistant spore that can survive
for long periods in soil. Others are insensitive to a systemic
fungicide (metalaxyl) that has been widely used in late blight
management. The protectant fungicides commonly used to protect
plants from late blight remain fully effective with all known
strains of the fungus.
• Infected cull potatoes
are a major source of spores of the late blight fungus and
must be disposed of properly--DO
NOT COMPOST OR MAKE CULL PILES. Cull potatoes should be spread
on fields not intended for potato production the following year
so that they will freeze and be destroyed during the winter.
If this is not possible, they must be destroyed in by complete
chopping, burial, burning or feeding to livestock.
• Plant only certified seed potatoes. Use of "year-out" seed or seed
saved from local crops is potentially dangerous with late blight. Seed sources
should be selected very carefully to avoid bringing in late blight on seed potatoes,
especially new strains of the fungus. Look for the characteristic coppery-brown
discoloration of the potato flesh under the skin of seed tubers. Infected tomato
transplants also can be a significant source of the disease.
• Volunteer potatoes can be a significant source of spores of the
late blight fungus. All volunteers should be destroyed as quickly as possible
by herbicides, chopping, or cultivation.
• Growers should scout fields regularly to look for late blight. Special
attention should be paid to early-planted fields, where the disease is likely
to develop first. Scouting should be concentrated in low-lying areas, field edges
along creeks or ponds, near the center of center-pivot irrigation rigs, in areas
near woodlots or any area that is protected from wind, where the leaves tend
to remain wet longer. Any area where it is difficult to apply fungicides such
as edges and corners or under power lines (if using aerial application) should
be examined. Scouts should look for large, black or purplish lesions on stems
or leaves and the telltale cottony, white mold growth, usually on the undersides.
Be sure to check leaves and stems under the crop canopy as that is where the
disease is most likely to begin.
• Use of a good protectant fungicide program is necessary to fully
protect any crop of potatoes.
• Verify that potato vines have been completely dead for 2-3 weeks
prior to harvest. Fungicide applications should be continued until vines are
dead. When foliage dies, spores of the late blight fungus that remain on the
foliage also die. This practice will prevent infection of tubers during harvest
and development of late blight in storage.
Adapted from the Ohio State
University Extension, 1999
Pesticides
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.
For more
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.
University
of Rhode Island Cooperative Extension provides equal program
opportunities.
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