Morphology and Host Specialization of Sclerotinia trifoliorum from Small Hop Clover. Robert G. Pratt, Research Plant Pathologist, U.S. Department of Agriculture, Agricultural Research Service, Forage Research Unit, P.O. Box 5367, Mississippi State, MS 39762. . Plant Dis. 76:661-664. Accepted for publication 7 February 1992. 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, 1992. DOI: 10.1094/PD-76-0661.

Symptoms of a Sclerotinia disease were observed annually for 8 yr in volunteer stands of small hop clover (Trifolium dubium) near Starkville, Mississippi. Apothecia developed from sclerotia in November and December, and patches of dead plants with sclerotia present in and on tissue appeared from February through March. The pathogen was identified as Sclerotinia trifoliorum by ascospore morphology. Sclerotia formed by S. trifoliorum on small hop clover in the field and apothecia produced from them were smaller than those recovered from berseem and crimson clovers. However, sclerotia and apothecia produced in culture by isolates from the three clover species did not differ significantly in size. Isolates from all three clover species were highly virulent on berseem clover. Isolates from small hop clover were more virulent (P < 0.05) on small hop than were isolates from berseem and crimson clovers, whereas isolates from berseem and crimson clovers were more virulent (P < 0.05) on crimson clover than were isolates from small hop. Disease severity in field plots of crimson clover infested with sclerotia from berseem and crimson clovers was greater than in plots infested with sclerotia from small hop clover. These results document S. trifoliorum as a pathogen of small hop clover for the first time in North America. Isolates of S. trifoliorum from small hop clover show significant and strong specialization in pathogenicity to this host.