If your part of the country has been abnormally wet and cool during the growing season, watch out. Late blight disease attacks plants in the nightshade family, which includes tomatoes and potatoes. It can wipe out a healthy-looking crop in days. You can control the disease with certain fungus treatments, but you also need to take other aggressive steps to defeat it.
Check your tomato and potato plants for olive-green or brown spots with white fuzz on stems and the underside of leaves. Be sure to look low on the plants, too. These spots can produce thousands of spores that are carried for miles on the wind. The spores can also wash down into the soil and infect tubers. The spots may be tiny, at first. They'll grow fast, though. On tomatoes, look for the spots on the fruit.
If you indentify late blight disease on your plants, you must remove them very carefully. Do not compost them. Instead, cut them down and place the infected leaves and stems in a plastic bag. Seal up the bag and let it "cook" in the sun. This will kill the fungus. After cooking, put the bag in the garbage.
You may want to try a fungicide on your infected plants. Some -- but certainly not all -- fungicides are labeled to control the disease. Be sure to check the label to see if late blight disease is listed before you use it.
Since late blight disease thrives in cool, damp conditions, give your plants a fighting chance. Water the base of the plant only, being careful not to splash the leaves and stems.
Photos courtesy of Dr. Margaret Tuttle McGrath
Department of Plant Pathology and Plant-Microbe Biology
Cornell University
Late Blight Disease (Phytophthora infestans) is an aggressive fungus-like pathogen that thrives in cool, damp weather. It attacks tomatoes, carrots, and potatoes, among other plants. In fact, it was the primary cause of the Irish Potato Famine of the 1840's. Its spores can travel for miles on the wind, infecting gardens and farms far away.
