Morphology, Pathogenicity, and Host Range ofPhytophthora megasperma, P. erythroseptica,andP. parasiticafrom Arrowleaf Clover

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Abstract
P. megasperma, P. parasitica and a species provisionally identified as P. erythroseptica were isolated from arrowleaf clover plants with symptoms of root rot in Mississippi [USA]. 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 3 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, P. megasperma f. sp. trifolii f. nov. is proposed for isolates pathogenic to arrowleaf clover but not to alfalfa and soybean.