Disease Note.

First Report of Phytophthora cryptogea Causing Root Rot of Sugar Beet in Wyoming. P. C. Vincelli, Department of Plant, Soil and Insect Sciences, University of Wyoming, Laramie 82071. W. F. Wilcox, and C. M-S. Beaupré. Department of Plant Pathology, New York State Agricultural Experiment Station, Geneva 14456; and Department of Plant, Soil and Insect Sciences, University of Wyoming, Laramie 82071. Plant Dis. 74:614. Accepted for publication 23 March 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0614C.

Root rot was observed in a sample of sugar beet (Beta vulgaris L. 'Hilleshog Mono-Hy R2) taproots submitted to the University of Wyoming's Plant Disease Diagnostic Laboratory during October 1989. The distal 3-5 cm of taproots showed a brown, firm decay, with a sharp demarcation between healthy and diseased tissues. A fungus identified as Phytophthora cryptogea Pethybridge & Laferty sensu lato (A₁ mating type) was consistently isolated from the margin of diseased tissues. Although the closely related pathogen P. drechsleri Tucker has been reported as causing root rot of sugar beet in other states, the isolate obtained in Wyoming produced sporangiophores of uniform width and sporangia with rounded bases and grew poorly at 33 C but not at all at 36 C; these characteristics are typical of P. cryptogea. Pathogenicity was confirmed by inoculating surface-sterilized beets (cv. R2) with 7-mm V-8 agar plugs of actively growing mycelium and incubating beets for 7 days at 28 C. Control beets were treated similarly except that sterile V-8 agar plugs were used. A decay similar to field symptoms was observed around all inoculated wounds, and P. cryptogea was reisolated from decayed tissue; control wounds showed no decay. We were unable to make direct observations of disease incidence in the field, as the sample was submitted after harvest. However, the grower submitting the sample reported that only hectarage on his farm (Worland, Wyoming) planted to the cultivar R2 (>1 20 hal was affected. It is not known whether this association with R2 was coincidental or related to host susceptibility. This is the first report of Phytophthora root rot of sugar beet in Wyoming.