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Disease Note.

First Report of Powdery Mildew on Kenaf Caused by Leveillula taurica in the Lower Rio Grande Valley of Texas. C. G. Cook, USDA-ARS, Subtropical Agricultural Research Laboratory, Weslaco, Tex. 78596 . J. L. Riggs, Department of Plant Pathology and Microbiology, Department of Entomology, Plant Pathology and Weed Science, New Mexico State University, Las Cruces 88003. Plant Dis. 79:968. Accepted for publication 18 July 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0968D.

In the fall of 1992, severe losses occurred in seed production fields of kenaf (Hibiscus cannabinus L.) in the Lower Rio Grande Valley of Texas. Signs included extensive growth of white, superficial mycelium producing colonies on the upper leaf surface, followed by partial to total leaf defoliation and abortion of pre-bloom floral structures and early-stage seed pods. Infected plants became more sensitive to cold temperature and symptoms were observed on all cultivars. The fungus produced abundant cylindrical conidia that were borne singularly on simple, septate conidiophores that emerged through stomata. Conidial dimensions averaged 19 x 62 μm. Dark cleistothecia with numerous lighter colored appendages and multiple asci were observed on senescent leaves. Cleistothecia (165 μm) contained between 10 and 14 asci. The measurements of the cleistothecia and number of asci agree with reports of cleistothecia of Leveillula taurica (Lev.) G. Arnaud observed on cotton (Gossypium sp), which range from 157 to 175 μm in diameter and contain up to 14 asci. To confirm pathogenicity, diseased leaves were pressed against adaxial sides of leaves of 12 1-month-old kenaf (cv. Everglades 41) plants grown in a greenhouse. After 3 to 4 weeks, powdery mildew developed on all plants and symptoms were identical to those of diseased plants collected from seed production fields. Leveillula taurica, the causal agent of powdery mildew, was initially reported as a pathogen of kenaf in Florida in 1952 (1,2). This disease can cause significant losses in seed yield and reduce seed quality in susceptible kenaf cultivars. This is the first report of powdery mildew on kenaf in Texas.

References: (I) W. W. Diehl. Plant Dis. Rep. 36:52, 1952. (2) W. N. Stoner. Plant Dis. Rep. 36:121, 1952.