Disease Note.

First Report of Whitefly-Associated Squash Silverleaf Disorder of Cucurbita in Arizona and of White Streaking Disorder of Brassica Species in Arizona and California. J. K. Brown, Departments of Plant Sciences and Plant Pathology, University of Arizona, Tucson 85721. H. S. Costa, and F. Laemmlen. Departments of Plant Sciences and Plant Pathology, University of Arizona, Tucson 85721, and University of California Cooperative Extension Service, El Centro, CA 92250. Plant Dis. 76:426. Accepted for publication 11 October 1991. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-0426C.

Squash silverleaf disorder was observed in zucchini plants in 1989 in Tucson, Arizona, and by summer 1990 was widespread in Cucurbita species in the state. Leaves of affected squash were entirely silver gray or dotted with silver patches. In winter 1990, a disorder in which stems and florets of broccoli, cauliflower, and rappini were whitened or showed green and white streaks was observed for the first time. Both disorders were associated with high populations of the whitefly Bemisia tabaci Gennadius. B. tabaci was also documented, on the basis of abundant empty whitefly pupal cases, as reproducing on cole crops for the first time in these areas. In greenhouse experiments, B. tabaci collected from zucchini with squash silverleaf syndrome induced symptoms of the disorder in zucchini, and B. tabaci collected from greenhouse-grown poinsettia plants induced white streaking symptoms in cabbage. In contrast, a colony of B. tabaci originating in Arizona cotton fields in 1982 and maintained serially on pumpkin since that time did not induce symptoms of either disorder in zucchini or cabbage. Further, pumpkin colony whiteflies fed on symptomatic zucchini or cabbage source plants did not acquire the ability to induce symptoms of squash silver leaf or white streaking disorder. This is the first report of these disorders, the etiologies of which have not been determined and which appear to be associated exclusively with a new biotype of B. tabaci that differs from the populations indigenous to the region.