Question: Our tomatoes have had
blight the last two years. Is there anything we can do to keep
them from getting it again?
Answer: There are three major blights that can
attack your tomatoes: Septoria leaf spot, early blight and late
blight. All are fungal diseases spread by spores, which require
dew or rain to infect the plant. These are most severe in wet weather.
Septoria leaf spot, sometimes called Septoria blight, is caused
by the fungus Septoria lycopersici and usually appears on the lower
leaves after the first fruits set. Fruits are rarely infected.
All the leaf loss reduces fruit yield and quality, and exposed
fruits are more susceptible to sunscald. The fungus is spread by
splashing water and by working among the plants when they are wet.
It overwinters on tomato and weed refuse.
Early blight, caused by the fungus Alternaria
solani, appears on the lower leaves, usually after a heavy fruit
set. The spots are dark brown to black. Concentric rings develop
in the spot forming a bull’s eye. The leaf area around
each target spot turns yellow, and soon the entire leaf turns
yellow and drops. Early blight fungus also infects stems and
may produce stem cankers. It occasionally attacks the fruit,
producing large sunken black target spots on the stem end of
the fruit. Infected fruits often drop before they mature. This
disease is most common late in the growing season. The fungus
overwinters on old tomato vines and on weeds in the nightshade
Late blight, caused by the fungal-like organism Phytophthora
in moist weather with cool nights and moderately warm days. Dark-green
to nearly black wet-looking spots begin spreading in from the leaf
edge. In wet weather, the spots may have a downy, white growth
on the lower leaf surface near the outer portion of the spot. Spots
also develop on the fruits. At first, the spots are gray-green
and water-soaked, but they soon enlarge and turn dark brown and
firm, with a rough surface. When conditions are favorable, the
disease may progress very rapidly.
Avoid these diseases by rotating crops. Plant tomatoes in the
same place only once in three or four years. Remove and destroy
tomato vines in the fall. Plow or rototill to bury the remaining
crop refuse. Use healthy transplants. Remove badly diseased lower
leaves, as these are a source of leaf spot fungus spores that help
spread the disease.
Water at the base of the plants to avoid splashing water, which
spreads the spores. Avoid watering with overhead sprinklers in
late afternoon or evening. If the plants stay wet all night, leaf
spot infections are likely to occur.
Use fungicides when needed. These diseases spread rapidly and
are difficult to control once established. Fungicides must be applied
before the disease first appears and reapplied throughout the growing
season. Chlorothalonil fungicide, sold as Ortho Multi-Purpose Fungicide,
can be applied up to the day of harvest.
-- Beverly Shaw, Advanced Master Gardener Purdue University
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