Wild beets as an important inoculum source of Alternaria alternata, a cause of leaf blight of cotton in Israel

Abstract

Alternaria alternata, isolated from the leaves of wild beets, infected cotton as well as wild and cultivated beets. Scanning electron microscopy of wild beet and cotton leaves infected by an aggressive isolate of A. alternata revealed that conidiophores of the pathogen emerged only from necrotic areas of leaf tissues. Sporulation occurred on leaves only during periods of high relative humidity (> 95%) and temperatures ranging from 20 to 28 °C. Under low relative humidity (60% at 22–25 °C), mycelium penetrated into internal tissues of the leaf or emerged through the stomata. A less virulent isolate did not develop surface mycelium on inoculated leaves, but sporulation was detected on the leaf veins. Plants in several cotton fields adjacent to the diseased wild beet plants were infected by the pathogen early in the growing season. This study proposes that isolates of A. alternata that are virulent on cotton may overwinter on wild beet plants, making them an important source of the pathogen inoculum in epidemics of alternaria blight of cotton. Key words: Alternaria, beet, cotton diseases, fungi overseason transfer, fungi overwintering, survival.