Temperature induced susceptibility to Phytophthora sojae in soybean isolines carrying different Rps genes

Resistance of soybean plants to specific races of Phytophthora sojae is conditioned by a series of host resistance (Rps) genes. Soybean cultivars that are resistant to certain races of the pathogen may nonetheless be susceptible to infection at elevated temperatures. The objective of this study was to determine if certain resistance genes are invariably associated with temperature induced susceptibility. Nine different resistance genes (Rps1-a, Rps1-b, Rps1-c, Rps1-k, Rps2, Rps3-a, Rps4, Rps5 and Rps6) were tested for temperature induced susceptibility in Williams isolines and 10 different resistance genes (Rps1-a, Rps1-b, Rps1-c, Rps1-d, Rps1-k, Rps2, Rps3-a, Rps4, Rps5 and Rps6) in Harosoy isolines. Three other soybean cultivars or lines carrying the Rps1-c gene were also examined. Resistance or susceptibility was determined by inoculating 7-day-old etiolated seedlings with a zoospore suspension isolated from P. sojae race I, and incubating the inoculated plants at 25 or 33 degrees C. The results suggest that temperature induced susceptibility is generally consistent for specific Rps genes, regardless of genetic background. However, for soybean plants that carry more than one Rps gene, temperature induced changes in host-pathogen compatibility may not be readily predictable. We also show that a short pre-inoculation treatment of 44 degrees C may induce susceptibility in plants that are nor considered to be temperature sensitive. This indicates that temperature induced changes in disease resistance may occur through different mechanisms, depending on the severity of the temperature stress. (C) 1996 Academic Press Limited.