Plant Pathology: Extension: Plant Disease Library

Alternaria

Important diseases: Early blight of tomato and potato, Alternaria leaf spot on Bradford pear, Alternaria blight of crucifer, Target spot on tobacco.

Alternaria causes leaf spots and blights on numerous plants including vegetables, fruit tress, and ornamentals, as well as blemishes and storage rots of vegetables. Alternaria can be easily isolated from decaying plant material because it readily colonizes dead and weakened tissues as a saprophyte.

Leaf spots are oval to irregular in shape, brown to black in color, and often characterized by concentric rings ("target spot"). Lesions are often numerous and quickly enlarge to blight leaf tissue. Lower, older leaves are usually attacked first with the disease progressing upward. Affected leaves turn yellow and dry up. Lesions on the stem are sunken and dark. They may girdle young plants at the soil line killing them.

Darkly pigmented, short, simple conidiophores form on the surface of infected tissue. Single or branched chains of spores (conidia) are formed at the apex of conidiophores.

Conidia are distinctive; darkly pigmented, oval and both horizontal and vertical internal walls (septa). Conidia having elongated terminal cells ("beaks" or "tails") are generally pathogenic. Saprophytic Alternaria spp. do not have beaks.