Late blight and early blight

This is a year to watch for late blight (LB ) and early blight (EB) in potatoes and tomatoes, and to a lesser degree in peppers and egg plants. For a disease to get established you need a susceptible host, the disease organism and proper environmental conditions. The past three or so years there have not been any spores of LB and EB detected in Idaho. This year is different.

University of Idaho specialists have a state-wide monitoring system for LB and EB. LB and EB can infect leaves, stems, fruits, and tubers. The spores have been detected. But so far we have not had the right environmental conditions. Cooler, fall weather brings conditions that favor the development of LB and EB.

With the local economy dependent on potatoes, it is very important that gardeners do not become the source of these blights getting a foothold in the area.

Late Blight

LB is caused by a fungus-like microorganism that can overwinter on crop residue, cull potato piles, volunteer plants, or nightshade weeds. It can also be brought in by infected seed potatoes or blown in on the wind, and then spread by rainfall or overhead irrigation. Environmental conditions that favor disease development are high humidity (>80%) with cool temperatures over a period of 10 hours or more, several days in a row.

The symptoms of LB begin on the younger leaves. Large, sunken, dark green or brown lesions develop on leaves, and can eventually kill entire leaves. The margins of the lesions may be green-yellow, or have a water-soaked appearance. The lesions on the stems will be brown to black.

Early Blight

EB is caused by a fungus that overwinters as spores and mycelium in infected crop residue or on solanaceous weeds, such as hairy nightshade and cutleaf nightshade. It can also be carried on the seed. Reduced fertility, along with wet leaves (or high humidity, > 90%) at temperatures of 75°F to 84°F favor the development of this disease. EB shows up on older leaves that are touching the ground first, then spread to the rest of the plant.

The symptoms of EB start out as small, dark spots. As the spots get larger, they have definite concentric rings, looking like a target. The tissue around the spots is usually yellow. The spots will grow and eventually fuse together, forming irregular dead patches on the leaves. The stems develop brown, dry, sunken lesions.

Integrated Pest Management (IPM) Control Options

Preventative:

• Plant resistant cultivars.
• Sanitation—clean up crop residues, especially if they have diseases. Compost crop residues to temperatures above 140°F for at least a week, then turn the pile and compost again.
• Use clean seed, or don’t save seed from infected plants and always use certified potato sets.
• Inspect transplants for symptoms of disease before planting.

Cultural:

• Rotate ground out of solanaceous crops for at least two years.
• Remove volunteer potato or tomato plants and solanaceous weeds.
Use drip irrigation or run sprinklers in early morning so leaves can dry quickly.

Chemical:

• Apply fungicide listed for LB or EB as soon as the disease shows up to protect uninfected plants.

Late blight and early blight may appear at the same time and there are other blights that will infect potato and tomato plants. If you are unsure, bring a fresh sample to the University of Idaho Extension office in Idaho Falls and we will get it to the lab if necessary.