A Soft Rot of Onion Caused by the Yeast Kluyveromyces marxianus var. marxianus. D. A. Johnson, Extension and Associate Plant Pathologist, Irrigated Agriculture Research and Extension Center, Prosser, WA 99350. J. D. Rogers, and K. M. Regner. Professor and Chairman, and Research Assistant, Department of Plant Pathology, Washington State University, Pullman 99164. Plant Dis. 72:359-361. Accepted for publication 16 November 1987. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/PD-72-0359.

The yeast Kluyveromyces marxianus var. marxianus isolated from onion bulbs showing soft rot symptoms from commercial fields, a cull pile, and packinghouse was confirmed as a soft rot pathogen of onion bulbs by completing Koch’s postulates. Resultant soft rot symptoms were significantly (P = 0.05) more severe at 27 and 20 C than at 10 or 5 C. Decay was significantly (P = 0.05) less in bulbs injected with 1 ml of 90 and 1,000 yeast cells per milliliter than bulbs inoculated with 11,000 or more yeast cells per milliliter. This is the first report known to us of a true yeast being pathogenic to onion bulbs.