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Morphology, Pathogenicity, and Host Range of Phytophthora megasperma, P. erythroseptica, and P. parasitica from Arrowleaf Clover. R. G. Pratt, Research plant pathologist, U. S. Department of Agriculture, Science and Education Administration, Agricultural Research, Crop Science and Engineering Research Laboratory, Forage Research Unit, and Department of Plant Pathology and Weed Science, Mississippi State University, P.O. Drawer PG, Mississippi State 39762; Phytopathology 71:276-282. Accepted for publication 2 July 1980. 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, 1981. DOI: 10.1094/Phyto-71-276.

Phytophthora megasperma, P. parasitica, and a species provisionally identified as P. erythroseptica were isolated from arrowleaf clover plants with symptoms of root rot in Mississippi. P. erythroseptica also was isolated from hop clover and P. megasperma from a cruciferous weed. Problems in identification of isolates and the validity of taxonomic criteria are discussed. P. megasperma was highly virulent on arrowleaf clover and less virulent on subterranean, crimson, and white clovers. P. erythroseptica was highly virulent on crimson clover and less virulent on arrowleaf. P. parasitica was weakly virulent on all species tested. Red, berseem, and alsike clovers and alfalfa were resistant to isolates of the three Phytophthora spp. from clovers. Generally isolates of P. megasperma from arrowleaf clover, alfalfa, and soybean were pathogenically specific to original hosts. These results support the use of a forma specialis concept with P. megasperma. A forma nova, Phytophthora megasperma f. sp. trifolii f. nov. is proposed for isolates pathogenic to arrowleaf clover but not to alfalfa and soybean.

Additional keywords: Trifolium agrarium, Trifolium alexandrinum, Trifolium hybridum, Trifolium incarnatum, Trifolium pratense, Trifolium repens, Trifolium subterraneum, Trifolium vesiculosum, Glycine max, Medicago sativa, Rorippa sp.