| A strain of the fungus, Phytophthora infestans,
causes late blight in tomatoes. As the common name
implies, Phytophthora is prevalent on tomato hosts
in late summer, after the plants have bloomed. Late
blight is more common in north central and
northeastern states, but is observed in the Midwest
when the humidity is high and temperatures are
around 68 degrees F late in the growing season.
Watch for the disease when cool, moist nights are
followed by warm, humid days.
Symptoms and Diagnosis
One of the first symptoms of late blight is watersoaked
patches on older leaves. Late blight attacks
the older leaves first, then spreads to the fruit.
Green-black watery patches develop on the upper
surfaces of older leaves. These patches will enlarge
quickly, and in moist weather, a downy growth may
develop on the underside of the leaf. On the fruit,
you will see rough, firm, dark-colored spots.
Life Cycle
The fungus overwinters on tomato and potato plant
debris, including potato tubers. The fungus can
produce spores over a broad range of temperatures
although spores are most infective at temperatures of
68 degrees F with high humidity. The spores are
transmitted by water or are wind blown and may be
introduced from diseased plants in nearby gardens.
The fungus invades the plant through leaf stomata.
Infection rarely occurs when temperatures are higher
than 80 degrees F and humidity is less than 90%.
Integrated Pest Management Strategies
1. Plant resistant varieties. Several tomato varieties
are designated as resistant to late blight.
2. Avoid overhead watering. Avoid working around
plants when they are wet. Both of these practices
can spread the fungal spores from plant to plant.
3. Clean up all garden debris in the fall. Remove
and destroy any affected plants as soon as they
are observed.
4. Practice crop rotation. Do not plant tomatoes,
potatoes, and celery in succession. All of these
crops are susceptible.
5. Use a copper-based fungicide. Apply when
symptoms are first identified. Reapply at
7–10 day intervals. | 
