Fusarium oxysporum f. sp. cepae and an unidentified Fusarium species have been reported to cause bulb rot of onion (Allium cepa L.) in Washington (1). In August and September 2002, a salmon-pink discoloration was observed on the outer three to four layers of dry scales of approximately 20% of white onion bulbs of cv. Cometa F1, in each of two 20-acre fields in the Columbia Basin of central Washington. Isolations from the discolored areas of the dry scales onto water agar and potato dextrose agar (PDA) yielded fungal colonies characteristic of F. proliferatum (3). The isolates formed long, V-shaped chains of microconidia on polyphialides. Pathogenicity of the isolates of F. proliferatum was tested on white onion bulbs purchased at a local grocery store. The outermost dry scales of each bulb were removed, and the bulb was inoculated by one of three methods: (i) a 5-mm3 section of the fleshy scales was removed using a scalpel, the wound was filled with a 3-mm2 plug of PDA colonized by F. proliferatum, the plug was covered with the section of scale that had been removed, and the inoculation site was covered with Parafilm; (ii) the basal plate of the bulb was dipped into a suspension of 106 microconidia per ml; or (iii) the basal plate was dipped into the spore suspension after wounding by inserting a dissecting needle into the bulb to a depth of 1 cm. A noninoculated bulb provided a control treatment. Bulbs were incubated in a moist chamber at 13°C and examined for discoloration of the outer scales and development of bulb rot. After 2 weeks, salmon-pink discoloration of the outer scales was observed at the inoculation site for both methods of dip inoculation, but not for the plug inoculation method. After 3 weeks, water-soaked, tan to golden, shrunken, soft tissue was observed on the remainder of each dip inoculated bulb, but symptoms of basal rot did not develop. Symptoms were similar to those reported in Idaho for a bulb rot of stored onions caused by F. proliferatum (2). One of the nonwounded inoculated bulbs did not develop a bulb rot, but pinkish discoloration was observed beneath the outer scales and in the neck. F. proliferatum was reisolated from the inoculated bulb tissues. The discoloration observed on the white onions raised concern about the potential for infection to develop into bulb rot in storage. However, thorough curing of the bulbs immediately upon storage restricted infection to the outer dry scales. Similar symptoms were observed at harvest on the bulbs of other white onions in a cultivar trial located near Quincy, WA, although symptoms were not observed on yellow or red cultivars in the trial. The same symptoms were later observed on approximately 70% of bulbs harvested from a 32-acre fresh-market crop of the cv. Sterling in the Columbia Basin. These symptomatic bulbs were rejected for the fresh market. To our knowledge, this is the first report of infection of onion bulbs by F. proliferatum in Washington, which in 2001, had the third largest acreage of onions in the United States after California and Oregon (USDA National Agricultural Statistics Service).
References: (1) D. F. Farr et al. Fungi on Plants and Plant Products in the United States. The American Phytopathological Society, St. Paul, MN, 1989. (2) S. K. Mohan et al. (Abstr.) Phytopathology 87:S67, 1997. (3) P. E. Nelson et al. Fusarium species: An Illustrated Manual for Identification. The Pennsylvania State University Press, University Park, 1983.
