Crown and Stem Rot of Alfalfa Caused by Sclerotinia sclerotiorum. R. G. Gilbert, Research Plant Pathologist, Vegetable and Forage Crops Production, Agricultural Research Service, U.S. Department of Agriculture, Prosser, WA 99350. Plant Dis. 71:739-742. Accepted for publication 2 April 1987. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1987. DOI: 10.1094/PD-71-0739.

Sclerotinia crown and stem rot was observed at high incidence in several broadcast-planted alfalfa seed production fields in the Touchet-Gardenia area of southeastern Washington in 1982. The disease cycle and activity of Sclerotinia was characterized in test fields. Apothecia were produced from sclerotia in March and April. Symptoms of Sclerotinia infection and extensive mycelial growth on host tissue occurred from early May through June. Dormant and overwintering sclerotia were produced abundantly in June and July. Sclerotial inoculum density data were obtained from 1-m² canopy-soil profile samples of three fields. Each sample consisted of an individual 1-m² sample of alfalfa stem tissue, surface residue, and surface soil. From 30 to 55% of all sclerotia were detected inside alfalfa stems, and the remaining 45–70% were detected in or on plant residue and soil. Sclerotia ranged from 0.85 to 3.35 mm in diameter, with most of the smaller sclerotia produced within the alfalfa stems. Initially, the disease was assumed to be caused by Sclerotinia trifoliorum. However, ascospore dimorphism and number of ascospore nuclei indicated that the pathogen was S. sclerotiorum, which has not previously been reported on alfalfa.

Keyword(s): forage, soilborne disease.