VIEW ARTICLE

Genetics

Isozyme Variability Among Isolates of Phytophthora megasperma. S. L. Nygaard, Department of Plant Pathology, University of Wisconsin-Madison, Madison 53706; C. K. Elliott, S. J. Cannon, and D. P. Maxwell. Department of Plant Pathology, University of Wisconsin-Madison, Madison 53706. Phytopathology 79:773-780. Accepted 21 February 1989. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-773.

Isozyme analysis was used to study over 300 field isolates of the species Phytophthora. Interspecific diversity for isozyme banding was identified among the four species assayed, P. megasperma, P. cactorum, P. cryptogea, and P. parasitica var. nicotianae. Intraspecific isozyme diversity was present among isolates of P. megasperma that were recovered from 10 host plant species and from many geographical origins; these isolates represented many morphological types and included the three formae speciales of P. megasperma (f. sp. glycinea, f. sp. medicaginis, and f. sp. trifolii). Isozyme analysis separated P. megasperma isolates into at least six intraspecific groups: the three P. megasperma formae speciales—glycinea, medicaginis, and trifolii—and the apple/apricot/cherry, Douglas fir, and broad host-range groups. These groups parallel those previously reported in the literature. The broad host-range group was further divided into at least three subgroups by isozyme analysis. All of the 224 isolates of P. m. f. sp. glycinea and 41 of the 45 isolates of P. m. f. sp. medicaginis had identical isozyme banding patterns within their respective formae speciales. Isolates of P. m. glycinea and P. m. medicaginis baited from the same geographical sites had no intra-formae speciales variant or intermediate banding patterns. This, and evidence previously reported, lead us to conclude that the two formae speciales, P. m. medicaginis and P. m. glycinea, are not naturally intermating populations and that they exist as two biological species distinct from other P. megasperma isolates.