Alternata blight whose foliar symptoms are often called brown spot is caused by organisms in the same genus as early blight, Alternaria alternata. It can also cause a tuber infection called black pit; see under Tuber Blemishes.
I. Leaf symptoms are small, dark brown lesions of dead cells. Brown spot can occur throughout the season and usually is first seen before early blight. Its lesion look similar to new lesions of early blight; however although brown spot lesions have rings, they are fewer and are irregular than those of early blight and the lesions do not grow as large as early blight. The spots may also be mistaken for ozone injury; see Environmental Effects. The leaves in the middle of the plant are most susceptible to brown spot and they show the highest incidence of the disease. High humidity, leaf wetness such as prolonged dew, and warm temperatures are conducive to infection. Brown spot is most severe under over-head sprinkler irrigation. Brown spot is not considered as damaging as early blight nor as aggressive, however yield losses have been reported due to it. Infections greater than 10% leaf area are uncommon but it has been estimated that there is a half percent yield loss for every 1% of leaf area infected.
II. Spores of A. alternata can travel in the soil and may even live there. They may infect underground tubers late in the season or infect tubers during harvest. Infection results in small black holes in the tuber; referred to as black pit; see Black Pit under Tuber Blemishes.
III. Brown spot and black pit are not considered major disease problems in potato. Often brown spot can be an early indication that early blight infection is also present although symptoms are not seen yet. Alternata blight is controlled by the same chemistries used for early blight so special treatment is rarely required.
Alternata Blight - brown spot Alternata Blight - vine