These ascomycete fungi are found everywhere and are very important plant pathogens. Various species infect Brassicas, carrots, cucurbits, potatoes and tomatoes (Early Blight of potatoes is one of these species). They are most likely to appear in warm (up to 80 degrees F), humid conditions.
Symptoms of Alternaria Blight first appear on lower, shaded leaves and consist of small, round, yellow, brown or black spots, often with concentric rings. These spots slowly enlarge and join together and may eventually reach 3 inches in diameter. These infected areas may drop out and leave holes in the leaves, or the whole leaf may die and drop off (in extreme cases the whole plant may be defoliated). Flowers often drop off too and cankers may appear on older stems. Infected fruits may develop sunken, mold covered spots and drop off.
One of the commonest ways for plants to be infected with Alternaria blight is from contaminated seed. The spores on the surface of the seed get established on the germinating seedling, though symptoms commonly don't appear until it gets bigger.You can kill spores on seed by treating them with hot water (127 degrees F for 25 minutes) or a bleach solution (1 part bleach to 10 parts water for 30 minutes).
The best way to deal with Alternaria Blight is to keep your plants well fed and watered, as healthy plants are rarely seriously affected. If any leaves show signs of infection, remove and destroy them. The spores overwinter on crop debris, so clean up the beds at the end of the summer to reduce sources of infection for the following year. Increase air circulation around plants by using a wide spacing (tomatoes should be staked) and by removing any tall weeds that could reduce air flow. Rotating crops annually can also help.
Image: Howard F. Schwartz, Colorado State University, Bugwood.org