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.
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.
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.
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.
from the Ohio State University Extension, 1999