Can Blight Affect Other Plants in the Garden?

Definition

• Early blight and late blight are two common types of plant disease. Both are fungal diseases that live in soil or stem tissue. Spores are spread through wind and rain, or from plant-to-soil or plant-to-plant contact when transporting infected transplants. Early blight, or Alternaria solani, typically attacks mature plants. Late blight, or Phytopthora infestans, is extremely dangerous to crops of all stages -- this is the same disease that caused the Irish potato famine in the 1840s.

Susceptible Plants

• Both early and late blight affect plants in the Solanaceae family. Tomatoes and potatoes are most often affected, but peppers, tomatillo and eggplant sometimes experience blight as well. Weedy plants in the same family, including jimson weed and deadly nightshade, can also succumb to this disease.

Symptoms

• Dark spots, or lesions, on leaves or on stems may be an indicator of blight. In potatoes, black lesions on leaves with white spores around the edges are a sign, as well as brown lesions on stems and purple-brown spots on tubers. In tomato plants, lesions appear on newer leaves at the top of the plant, and fruit may bear similar dark spots with a greasy appearance. Early blight causes leaves to turn yellow and drop, and late blight will kill plants if left untreated. Blight can spread quickly as spores are transferred to neighboring plants, so it's important to take action immediately if this disease is suspected.

Diagnosis/Treatment

• Blight can be difficult to diagnose, so check with your cooperative extension office or garden center if you suspect it is present in your garden. To treat early blight, remove affected leaves as soon as possible. If the majority of leaves show signs of blight, remove the plant and destroy it. For late blight, the only safe solution is to destroy the plant. Do not compost affected plants; instead, place them in a garbage bag and set the bag in the sun so high temperatures will kill the plant. Spores cannot reproduce in dead tissue, so the plant can then be thrown out with the trash.

Prevention

• Blight sets in when conditions are cool, wet and cloudy, so pay close attention to plants in this type of weather. To prevent blight, plant only certified seed potatoes and disease-resistant plants. For tomatoes, look for "Mountain Magic" and "Legend" varieties. For potatoes, try "Defender" and "Elba." Weed out plants growing from last year's dropped seeds and keep the area clear of weeds that may act as host plants, such as nightshade. Also, rotate crops every three years and be sure not to put plants in the Solanaceae family in the same place. Fungicides can help, but they must be applied before blight symptoms appear. Fungicides must be applied throughout the growing season every two weeks for optimal effectiveness.