Late Blight

Found over most of the United States. This fungus disease attacks tomatoes and potatoes at any stage of growth. Irregular gray spots form on leaves. White mold grows on the undersides of these spots. Infected leaves turn brown and dry up. Gray, water-soaked spots on infected fruits later turn dark brown and corky. Infected tubers are covered with brown spots where rot begins. Wet weather with cool nights and warm days favors the spread of the disease. The fungus overwinters in infected plant debris.

• Fertilize judiciously — excess nitrogen favors the disease.
• Eliminate infected plants to prevent further spread of disease.
• Avoid overhead watering.
• After potato vines die at the end of the season, wait a week before harvesting to avoid infecting tubers.

• Serenade Garden Disease Control

  Try using Bacillis subtilis, a soil-dwelling bacterium found in Serenade Garden Disease Control.
• For early harvest, first destroy plant residues with a flame weeder before digging tubers.
• Contact your extension office for information on fungicides to control this disease.
• Copper fungicides are effective as a preventive when applied as directed on the product label. It's best not to depend too heavily upon this method of control, as copper can build up in soil to levels toxic to earthworms and microbes.

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Photo courtesy of University of Wisconsin-Madison, Dept. of Plant Pathology.