Common Tomato Diseases

During most summer seasons, fungal diseases occur on tomatoes. Early blight can cause leaf spots, fruit rot and stem lesions; under favorable conditions, it can completely defoliate plants. Small, irregular, dark brown to black spots appear on older leaflets, ranging in size from a pinpoint up to a half-inch in diameter. Concentric rings form as the spots enlarge, yellow ring often forms around each spot and the leaf veins usually border the lesions. If numerous, the spots coalesce, causing the entire leaflet to turn yellow and die. On the stem, infection results in small, dark, slightly sunken areas that enlarge to form circular spots with lighter-colored centers. Concentric markings often develop on the stems, too.

Septoria leaf spot is also prevalent and can cause rapid defoliation when the weather is warm and wet. Symptoms are usually noticed after plants have begun to set fruit in mid-July. Small (1/8” diameter), roughly circular spots, with tan centers and ark margins, are scattered over the leaf. Older leaves near the soil are infected first, but symptoms quickly appear on younger leaves in rainy weather, as water splashes the disease spores around. Leaves will drop prematurely if heavily infected, subjecting the tomato fruits to sun scald.

Rotate tomato plants to allow time for the infested plant debris to decompose in the soil; a four year rotation is suggested. If possible, deeply bury plant refuse. Try to select tomato cultivars with lower susceptibility to these diseases. Also, control weeds in the garden area, especially Jimson weed, horse nettle and nightshade, which also serve as hosts for Septoria. Maintain fertility at optimum levels; deficiencies of nitrogen and phosphorus can increase susceptibility to early blight. Irrigate plants at soil level rather than by overhead irrigation to keep foliage dry.

Source: University of Minnesota

June 2003
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