Reaction of soybean cultivars to races of Phytophthora megasperma f.sp. glycinea present in Queensland

Abstract

Phytophthora megasperma f.sp, glycinea, the cause of phytophthora root and stem rot of soybeans, was found to be widespread in southern Queensland. Race determinations on 56 isolates were made at 26¦C using the North American differential set. Thirty isolates resembled race 1 (virulent only on Harosoy), while the remaining isolates killed varying proportions of the other six differentials. Two of these isolates most closely resembled race 15. When race determinations were repeated for four isolates at 31¦C, each isolate was more virulent on most of the differentials. One isolate resembling race 1 at 26¦C would have been classified as race 2 at 31¦C. All of the cultivars grown in Queensland were susceptible to hypocotyl inoculation, indicating the absence of genes conditioning resistance to races 1 and 15. Small proportions of plants in some cultivars, including Davis and Bragg were resistant to hypocotyl inoculation. The field resistances of 21 cultivars and breeding lines were assessed in a disease nursery. Progressive assessments of disease incidence were significantly correlated with grain yield and plant height. Davis had the highest level of field resistance while Ross and Semstar had the lowest. A breeding line (HS 1115) was identified as highly resistant to the fungus, both in the field and following hypocotyl inoculation. Three F6 lines from the cross Davis x Bragg showed a range of disease reactions, with the best showing an equivalent level of resistance to that of Davis.