Lack of Host Specificity Among Isolates ofPhytophthora megasperma

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Abstract
Mahaleb cherry and Moapa 69 alfala seedlings were transplanted into pasteurized potting mix artificially infested with isolates of Phytophthora megasperma originally recovered from alfalfa, apple, apricot, cherry, Douglas fir, grape, juniper, kiwi, lilac, peach, pear, raspberry, soybean, or walnut and flooded for 48-hr periods at 2-wk intervals. After 17 wk at a soil temperature of 18-22.degree. C, alfalfa and soybean isolates caused moderate root rot, a 40-62% decrease in root weight, and a 37-75% decrease in shoot weight on Mahaleb seedlings relative to uninoculated controls but no crown rot or plant death; all other isolates caused severe root rot, crown rot, and death of most Mahaleb seedlings. In the same experiment, isolates from fruit trees with small (29-34 .mu.m) oogonia and high optimum (30 C) and maximum (36 C) growth temperatures were weakly to moderately virulent on alfalfa, as were isolates from soybean; isolates from fruit plants with large (38-43 .mu.m) oogonia and low optimum (24 C) and maximum (30 C) growth temperatures and isolates from Douglas fir were moderately to highly virulent (23-95% decrease in root weight, 46-94% decrease in top weight relative to uninoculated controls). Alfalfa isolates were all highly virulent. Results were qualitatively similar in an experiment with soil temperatures of 18-29 C, but many isolates were less virulent than at the lower soil temperatures. Confirming this trend, a cherry isolate of P. megasperma caused 81-97% root rot on Mahaleb cherry when seedlings were grown in infested potting mix at constant soil temperatures of 10, 15, or 20 C but caused only negligible root rot at a constant soil temperature of 25 C. When Harosoy and Wayne soybeans were grown in potting mix infested with isolates of P. megasperma from the above hosts, those from soybean caused extensive root necrosis, whereas all other isolates caused varying degrees of minor root necrosis. Only soybean isolates caused expanding necrotic lesions when directly inoculated into hypocotyls of pathogenicity is not a common phenomenon among isolates of P. megasperma, although a given isolate may exhibit differential virulence against different hosts. The results further suggest that edaphic factors must be maintained to provide optimum conditions for disease development when assessing the pathogenicity of an isolate of P. megasperma, or its inherent virulence may be significantly underestimated.