VIEW ARTICLE

Etiology

Pathogenicity, Distribution, Sources of Inoculum, and Infection Courts of Botryosphaeria dothidea on Pistachio. Themis J. Michailides, Assistant research plant pathologist, Department of Plant Pathology, University of California, Berkeley/Kearney Agricultural Center, 9240 S. Riverbend Avenue, Parlier, CA 93648; Phytopathology 81:566-573. Accepted for publication 8 January 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-81-566.

A panicle and shoot blight disease caused by Botryosphaeria dothidea is reported for the first time on pistachio in California. The disease was prevalent in northern counties (Butte, Tehama, and Glenn) and sporadic in southern counties (Madera, Fresno, Kings, and Kern). In early spring, symptoms of panicle and shoot blight resulted from continued activity of the pathogen in buds infected the previous summer and fall. Shoot blight symptoms included dark brown to black lesions at the base of current-season shoots, followed by the wilting and drying of leaves, which remained attached to their shoots. Panicle and shoot infections caused perennial cankers. The infection of petioles, leaves, fruit, and rachises was evidenced by expanding lesions, which resulted in the death of the structures involved. The infection of panicles usually occurred at the base, causing fruit to shrivel and remain on the tree even after mechanical shaking for harvest. Current-season shoots and panicles were susceptible to artificial inoculations throughout the growing season. Infected rachises, petioles, fruit, and leaf blades were retained longer on the tree than corresponding healthy plant parts and provided inoculum (pycnidiospores) for winter and spring infections. In addition, pycnidia that had formed on current-season infected rachises, fruit, blighted shoots, petioles, and leaf lesions provided inoculum for summer and fall infections. Moreover, the fungus remained active as mycelium in perennial shoot cankers. Light and scanning electron microscopy indicated that germ tubes of conidia of B. dothidea entered through the stomata of the upper or lower surfaces of leaves, rachises, and shoots, and also through the lenticels on pistachio fruit.