Occurrence of Kluyveromyces marxianus var. marxianus Causing Onion Soft Rot in the Columbia Basin of Washington State.

Kluyveromyces marxianus var. marxianus, a causal agent of onion (Allium cepa L.) soft rot, was originally isolated from Walla Walla sweet onion bulbs grown in Oregon and subsequently isolated from onion throughout the southeast Washington onion-growing area, but was not found to be infecting dry bulb storage onions grown in central Washington (1). During September of 2001, a yeast was isolated from dry storage onion bulbs (cv. Teton) grown under sprinkler irrigation in central Washington, exhibiting soft rot symptoms and identified to be K. marxianus var. marxianus (2). Koch's postulate was completed using cv. Teton bulbs surface disinfested with 0.5% NaOCl for 2 min. This isolate and four isolates of Kluyveromyces marxianus var. marxianus (1) were cultured on potato dextrose agar and resuspended to an OD = 0.3 (approximately 10 CFU/ml). One-half of a milliliter of each isolate was inoculated to onion using the cut bulb method with three replicates and incubated in a moist chamber at 25°C for 7 days. Onion slices inoculated with the new isolate exhibited soft rot symptoms similar to those caused by the known isolates while no symptoms were observed for the water control. The yeast reisolated from symptomatic tissue was confirmed to be K. marxianus var. marxianus (2). The identification of K. marxianus var. marxianus infecting dry bulb storage onions grown in the Columbia Basin is of interest because the disease can be confused with bacterial soft rot and could become a serious problem in this important storage onion-growing region. References: (1) D. A. Johnson et al. Plant Dis. 72:359, 1988. (2) N. J. W. Kreger-van Rig, ed. The Yeasts: A Taxonomic Study. Elsevier, Amsterdam, 1984.