The Disease Cycle of Late Blight in Tomatoes

Late Blight Disease

• Though spores can't overwinter in cold-winter climates, they're introduced during the growing season. Spores overwinter in infected ground that doesn't freeze, in plant debris and in potato tubers stored for seed. Southern climates especially host certain long-lived sexual spores in this way. Wind and rain carry fungal spores over long distances in cool, wet weather, with the clouds protecting the spores from destruction by ultraviolet rays. Once in the garden, the fungus spreads readily from plant to plant.

Effects on Foliage

• Late blight rapidly infests and destroys otherwise healthy foliage on tomato plants. First, brown or dark lesions appear on leaves; they're marked by white edges in moist environments. Patches of brown or dark olive green also appear on the stems in the initial stages of the disease. The undersides of the leaves and the stem patches are marked in moist conditions with white fuzzy growth. The white areas indicate active spores, which spread rapidly in wet conditions or during high relative humidity.

Effects on Fruit

• Infected tomatoes display discoloration. Green tomatoes show dark patches. Ripe fruit may display discolored spots at first, progressing to firm brown areas. The fruit appears as if it's been hit with a hard freeze. White fuzz eventually covers some of the tomatoes. Late blight is not toxic to humans, but affected tomatoes should not be eaten since illness-causing, opportunistic microorganisms may proliferate on damaged fruit. The rapid progression of the disease potentially infects an entire crop within days.

Prevention and Treatment

• Eliminate the source of the spores, if possible. Destroy infected potato tubers and their volunteer sprouts. Select disease-free and late blight-resistant nursery stock. Rotate crops year to year. Use fungicides under the guidance of your county extension service when conditions threaten infection. Isolate and destroy infected plants. Vigilance is necessary for early intervention; a single discolored leaflet may be the fist indication of the disease, but the vast number of spores makes explosive, garden-wide infection almost inevitable. Newly emerging genotypes are variously aggressive and resistant to fungicidal treatment.