Host Specificity ofPhytophthora megaspermafrom Douglas Fir, Soybean, and Alfalfa

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Abstract
A total of 28 isolates of P. megasperma from either Douglas fir, alfalfa, or soybean were tested for pathogenicity to Douglas fir, alfalfa and soybean. Isolates from alfalfa and soybean were pathogenic only to their original hosts. One group of isolates from Douglas fir was strongly pathogenic to both Douglas fir and soybean; a 2nd group was less pathogenic to Douglas fir and was not pathogenic to soybean. The 1st group from Douglas fir was virulent on 7 soybean cultivars used to define races of P. megasperma f. sp. glycinea and on cultivar Tracy. Applicability of the formae speciales concept to P. megasperma is discussed.