Disease Note.

Outbreak of Avocado Black Streak in Dade County, Florida. R. J. Schnell, USDA-ARS, National Clonal Germplasm Repository, Miami, FL 33199 . R. C. Ploetz, University of Florida, Homestead 33031. Plant Dis. 80:000. Accepted for publication 1 July 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-1079B.

Black streak disease of avocado, Persea americana Miller, is a significant problem in production areas in California (1). To date, the disease has been observed only on cultivars of the Guatemalan race, var. guate-malensis, and no causal agent has been identified. During the fall of 1995, black streak was observed on 43 different accessions of avocado at the USDA's National Clonal Germplasm Repository in Miami. Reddish brown patches on the trunk or scaffold limbs of affected trees were often superficial, but necrosis progressed into the cambium in some cases. Lesions exuded a white, powdery substance that usually developed above the soil-line and was washed off by rain. Trees that were severely damaged by Hurricane Andrew (August 1992) were most apt to be affected. On a selective medium, Phytophthora spp. were not isolated from affected tissue, thus providing evidence that Phytophthora canker did not cause these symptoms. In contrast to reports from California, there was no relationship between host race and the occurrence of the disease; racial hybrids and accessions of the West Indian and Mexican races were affected as often as accessions of the Guatemalan race. Although black streak was previously observed on single trees in the Canary Islands and Florida, this is apparently the first report of a significant outbreak of the disease outside California.