Chemotaxis of the zoospores of Phytophthora megasperma was studied on freshly excised primary root tips of 2-day-old seedlings of nine alfalfa (Medicago sativa L.) cultivars and four other legume species. The highly susceptible cultivars Saranac, Algonquin, and Vernal attracted masses of zoospores within minutes after being placed into fresh zoospore suspensions. The moderately susceptible cultivars Iroquois, Angus, and Thor displayed less severe en masse zoospore accumulation. Resistant cultivars Apollo, Agate, and to a lesser degree WL-318 exhibited minor chemotaxis.Zoospores were strongly attracted to the region of elongation, immediately above the root cap area. Relatively few zoospores, if any, were attracted to the root cap and older regions of the roots. Zoospores were not attracted to root hairs. Within 0.5-1 h, zoospores attracted to the roots began to encyst and germinate. Germ tubes always originated from the side of cysts closest to the root surface, and all showed unidirectional growth towards the root.Very weak or no chemotactic responses of zoospores to nonhost legume plants of white sweet clover (Melilotus alba Desr.), red clover (Trifolium pratense L.), bird's-foot trefoil (Lotus corniculatus L.), and soybean (Glycine max (L.) Merr. cv. Vansoy) were observed. Roots of susceptible alfalfa seedlings pretreated in boiling water did not attract zoospores.Injured, susceptible alfalfa roots displayed a strong preferential attraction around a wounded root area. Varying zoospore densities occurred at different distances from the wound. Wounded, resistant alfalfa roots showed slightly more zoospore accumulation than the uninjured resistant roots.The magnitude of chemotaxis and response time appear to be related to the susceptibility or resistance of the young, primary roots of alfalfa seedlings.

The electrophoretic patterns of the buffer-soluble proteins of 26 isolates of Phytophthora megasperma f. sp. medicaginis from diseased alfalfa plants and soil were studied. On the basis of the protein patterns, the isolates were divided into two groups which correlated with cardinal temperatures for growth, pathogenicity, and oogonia size. Group 1 (seven isolates) has minimum growth at 5 °C, optimum at 25–30 °C, and maximum at 35 °C, and is a highly pathogenic pathotype with small oogonia. Group 2 (19 isolates) has minimum growth at less than 5 °C, optimum at 20 °C, and maximum at 30 °C, and is a less pathogenic pathotype with large oogonia. The electrophoresis technique was useful in identifying differences among the Phytophthora isolates.

Performance of some artificially inoculated Phytophthora root rot (PRR) resistant and susceptible alfalfa cultivars was determined under field conditions at the Central Experimental Farm, Ottawa. Yields of both resistant and susceptible cultivars were reduced after inoculation; however, the resistant cultivars outyielded the susceptible ones. The resistant, inoculated cultivars showed less winterkill. There is still a need for improving PRR resistance in alfalfa, and thus increasing yield potential.

Isolates of Phytophthora from alfalfa and soybean and the type isolate of Phytophthora megasperma Drechsler from hollyhock were compared morphologically, for temperature response, and pathogenically. Highly pathogenic, host-specific isolates from soybean and alfalfa, with relatively small oogonia, were identified as P. sojae Kaufmann and Gerdemann and divided into two formae speciales: f.sp. glycines and f.sp. medicaginis, respectively. Less pathogenic isolates from alfalfa with large oogonia were identified as P. megasperma and were compared favorably with Drechsler's original isolate and description. Isolates of P. sojae differed in their cardinal temperatures from those of P. megasperma. We consider P. sojae to be distinct from P. megasperma and propose that this taxon be reintroduced to the literature.

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