Dark, dusty spores are produced in concentric rings. The spores can be seen if the spot is touched to a light-colored object. The fungus can survive in soil and in infested crop and weed residues. It may be seed-borne and carried by wind, water, insects, workers and farm equipment. The spores that land on tomato plants will germinate and infect the leaves when they are wet.
Spores can enter the leaf, steam or fruit.
The fungus is most active during mild to warm temperatures and wet weather. The disease is worse during the rainy season. Early blight is most severe on plants stressed by a heavy fruit load, nematode attack, or low nitrogen fertility. The best way to manage the disease is on a preventive basis. Once early blight is established in the crop, it is very difficult to control.