Relationships Between Phytophthora megasperma Isolates From Chickpea, Lucerne and Soybean

Abstract

The relationships between morphologically indistinguishable Phytophthora megasperma isolates from soybean (P . Megasperma f. sp. glycinea), lucerne (P. megasperma f. sp. medicaginis) and chickpea were investigated using the criteria of growth temperature relations. specific pathogenicity, capacity to cross- protect and electrophoretic patterns of buffer-soluble proteins. The isolates from lucerne and chickpea had the same minimum ( <7°C), optimum (24.5-28°C) and maximum (31-34.5°C) temperatures for growth whereas those from soybean had higher minimum (7-10.5°C), optimum (28-31°C) and maximum (34.5-37°C) growth temperatures. The Australian isolates of P. megasperma f. sp. glycinea also had a higher optimum temperature for growth than that reported for North American isolates.

Cross-inoculation studies showed that the isolates from lucerne and chickpea could produce indistinguishable disease symptoms on both hosts whereas those from soybean were pathogenic to this host only. Several Phytophthora spp. provided varying levels of cross-protection in soybean hypocotyls against P. megasperma f. sp. glycinea. P. megasperma isolates from lucerne and chickpea both provided temporary cross-protection, while P . Nicotianae and P. drecllsleri provided complete protection. When buffer-soluble proteins were extracted from 12 isolates of Phytophthora spp. and analysed by SDS-polyacrylamide gel electrophoresis. the resultant protein profiles were identical for nine isolates of P. megasperma from chickpea or lucerne but differed significantly from isolates of P. megasperma f. sp. glycinea, P. vignae and P . Nicotianae var. parasitica. The protein profiles of the last three species also differed significantly from each other. We consider that the 'formae speciales' grouping previously established for soybean and lucerne isolates of P. Megasperma should be maintained.