Alternaria species produce diseases whose most common symptom is circular spots on leaves with target like, circular rings. These spots can be yellow, dark brown, and black and can be found on all parts of the plants. Black speckles are often found on these spots which are the spores. The centers of these spots may fall out, giving the leaf spots a shot-hole appearance. Individual spots can connect into large dead areas, and leaves may fall. Sometimes, as with tomatoes or potatoes, it also attacks fruits, producing black round or oval spots and spoiling crops. Although Alternaria normally develops on the leaves, it can also attack the stems of young seedlings, creating spots of grayish-brown color (normally at the base of the stem) causing rot that often kills the plant.Note: Downy mildew can sometimes be confused for powdery mildew or botrytis.
Circular, target-like lesions on tomato leaf
“A tomato leaf showing typical target shaped lesions of Alternaria solani”
“Shot holes” – lesions with centers that fell out, Brassica family
Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org, licensed under a Creative Commons Attribution-Noncommercial 3.0 License.
Lesions and black spores on tomato fruit
Yuan-Min Shen, Taichung District Agricultural Research and Extension Station, Bugwood.org, licensed under CC 3.0.
Alternaria rot on stems young tomato seedlings
Virginia Tech Learning Resources Center, Virginia Polytechnic Institute and State University, Bugwood.org, licensed under CC 3.0