As potatoes begin to near harvesting and tomatoes get redder every day, gardeners and farmers should be on the alert for a deadly fungal pathogen once again making its way through Wisconsin. Late blight is a lethal disease of both potatoes and tomatoes, and it can decimate an entire crop in a matter of days.

Early blight affects the same plants as well as eggplants and peppers (affecting tomatoes most severely), and reduces yield significantly. Once these diseases infect a plant, control is difficult.

Late blight appears with dark, oil patches on leaves and stems and brown-to-black rings of hard tissue on plants. It works from the top of the plant down. Early blight is categorized by large, irregular brown spots on leaves, often with a yellow halo.

Conditions, measured in degree-days, are now perfect for blight to appear in all potato and tomato plantings except late plantings of potatoes. This means that late blight fungicides (copper- or chlorothalonil-containing products labeled for edible plants) should be applied as soon as possible to prevent this disease from taking hold. Fungicides including chlorothalonil should also be applied at this time for early blight.

Hot weather and patches of rain and thunderstorms have created the perfect environment for these diseases, so regular control this season will be necessary. Tomato- and potato-growers can find more information on late blight and its control at hort.uwex.edu/articles/late-blight, the UWEx fact sheet on the disease. The early blight fact sheet can be found at hort.uwex.edu/articles/early-blight.

Both diseases persist after an initial infestation. For early blight, infected plant parts or whole plants should be removed and thrown away, burned, or left in the sun inside a black garbage bag to kill the pathogen. Susceptible plants should be moved the following year to a new location (do not return to the infected area for 3-4 years). Increase space between plants and be sure to allow air flow, preventing the moist, humid conditions favorable to the disease. Do not water from above the leaves, and water in the morning to prevent standing water on leaves.

Late blight lives in plant debris and surrounding soil, and nightshades are a host plant; these should be removed. All infected debris should immediately be removed and thrown away, NOT composted. After infection, fungicides are not effective, and plantings should be moved. Do not use seed potatoes from a crop in which infestation occurred, remove seedlings that sprout on their own after infestation, and consider planting resistant varieties.

Gardeners with questions or possible cases of either disease should contact the Oneida County UW-Extension office at (715) 365-2750.