Symptoms on tomato leaves
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 air. 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.
The disease 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 harvest. 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.
Symptoms on the tomatoes
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
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.
Tell tale sign
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.
Protecting your Plants
To 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.
Tomatoes look good but eventually get soft spots
One of the difficult things about plant diseases is that
fungicide sprays are most effective when they are used
preventatively. Once the disease is present, fungicide sprays
will not necessarily “cure” the problem, especially one as
severe as late blight.
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 landfill.
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.
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