have been experiencing the right weather conditions for this
devastating disease of tomatoes and potatoes. The closest infection
to Allegheny County to date is in a commercial tomato field in Blair
Warm (70 - 80°F), humid days followed by cool (40 - 60°F), foggy
nights make for ideal late blight weather. Late blight is highly
contagious, and can wipe out tomato and potato crops in short order.
Caused by the fungus Phytopthora infestans, late blight is the
disease responsible for the potato famine in Ireland in the
Late Blight Epidemic
Late blight epidemics often start in home gardens where fungicide
applications are less likely to be made on a regular basis than in
commercial plantings. Whenever the disease develops unchecked, large
quantities of late blight spores are produced and released into the
During moist weather, the spores can survive and be transported
up to 50 miles on air currents to infect other plantings of tomatoes
and potatoes. During favorable weather conditions, unprotected
foliage can be infected in three to six hours; symptoms can appear
within a week. Those symptoms can expand rapidly during cool, wet
weather and cause entire plantings to die within two weeks of
infection. Late blight is held in check by hot, dry weather.
The fungus overwinters in southern areas on winter-grown tomatoes
and potatoes. In northern areas, it overwinters in commercial potato
cull piles, compost piles and potato tubers overlooked during
The late blight fungus requires live tissue to overwinter,
so potato tubers are the most likely source in northern gardens. It
can be introduced to the garden on infected tomato transplants or
seed potatoes. This is one of the reasons it is so important to used
certified seed potatoes rather than saving homegrown potatoes to
grow the following season. Late blight spores are also carried north
on air currents coming from the south.
Check for foliar symptoms on tomato and potato plants by examining
vigorous new growth higher on the plant. This helps avoid confusing
late blight symptoms with less serious problems such as early blight
that often develop on the older, lower leaves. Late blight symptoms
first appear as somewhat circular, water-soaked spots near the edge
of expanded leaflets. These spots expand rapidly during moist
weather to form irregular brown, dead areas.
There is often a light
green margin between the dead tissue in the center of the spot and
the normal green tissue outside the spot. The real diagnostic
feature of late blight is the white, downy-looking mold that
develops at the margin of the spot on the underside of the leaflet.
If the white mold is not obvious, remove the suspicious leaflet and
put it in a plastic bag with a moist (rung out well, not sopping
wet) paper towel for 24 hours to see if this symptom develops. If it
does not develop, late blight is probably not the cause of the leaf
spots you are seeing.
Photo: Robert Donnan
protect your tomato plants, avoid growing tomatoes where potatoes
were grown the previous year, or growing potatoes and tomatoes next
to each other. Late blight is likely to start in the potatoes and
spread to the tomatoes. Protective fungicide sprays are the only
sure way to avoid this devastating disease. Chlorothalonil (Daconil
2787) and maneb are labeled to control late blight in home vegetable
gardens. Organic gardeners can use copper in spray or dust form to
prevent late blight. Copper is not as effective as chlorothalonil or
maneb, but it is better than doing nothing. Applications should
continue as long as weather conditions favor the development of late
blight. Follow label directions as to how often the product you are
using should be applied.
One of the difficult things about plant diseases is that fungicide
sprays are most effective when they are used preventatively.
the disease is present, fungicide sprays will not necessarily “cure”
the problem, especially one as severe as late blight.
Symptoms on a tomato
Photo: Robert Donnan
Plants infected with late blight should be removed from the garden
and destroyed. Do not compost them. Cut potato tubers from infected
plants in half so that they decompose quickly. Bury infected plants
rather than sending them out with the trash. The disease-causing
spores could be released if they are exposed at a later date in the
Mature green fruits from infected tomatoes can be removed and stored
for ripening. Avoid storing them under conditions of high humidity
(plastic bags or containers) since this will promote spore
production. Potato tubers from infected plants can be eaten, but
should not be stored for any length of time. Avoid using infected
tubers for seed potatoes.