Early blight is primarily a disease of stressed or senescing plants. Symptoms appear first on the oldest foliage. Affected leaves develop circular to angular dark brown lesions 3 to 4 mm in diameter. Concentric rings often form in lesions to produce a characteristic target-board effect. Severely infected leaves turn yellow and drop. Dark brown lesions develop on stems in later stages of the disease. Infected tubers show a brown, corky dry rot.
Life cycle
The early blight fungus can overwinter on potato refuse in the garden, in soil, on tubers, and on other solanaceous plants. Infection occurs when spores of the fungus come in contact with susceptible leaves and sufficient free moisture is present. Spore germination and infection are favored by warm weather and wet conditions from dew, rain, or sprinkler irrigation. Alternate wet and dry periods with relatively dry, windy conditions favor spore dispersal and disease spread. Tubers can be infected as they are lifted through the soil at harvest. If sufficient moisture is present, spores germinate and infect the tubers.
Solutions
Early blight can be minimized by maintaining optimum growing conditions, including proper fertilization, irrigation, and management of other pests. Grow later maturing, longer season varieties.
