Abstract
Phytophthora root and stem rot of soybean is a water-borne disease caused by Phytophthora sojae. Two types of resistance have been identified; race-specific complete resistance and non-race-specific partial or field resistance. We developed a method to evaluate the field resistance to P. sojae of Hokkaido soybean cultivars, utilizing paddy fields in summer. The percentage of dead plants was highly correlated with the disease severity index (r=0.992** in 2006, 0.981** in 2007, n=35-37) and it was considered to be an appropriate indicator of field-resistance evaluation. The order of dead plant percentage among 16 genotypes susceptible to all known races of P. sojae in Hokkaido was nearly the same in 2006 and 2007. The genotypes were classified into three groups based on their resistance level and six cultivars were selected as checks. The dead-plant percentage in any two years from 2006 to 2009 was correlated with each other (r=0.542** to 0.839**, n=26-57); however, the field-resistance evaluation from 2006 to 2009 varied with the year in 17.1 to 34.6% of the genotypes. Analyses of genotypes with complete resistance specific to races A and D and those used for race-identification showed that race-specific resistance had little effect on the field resistance evaluation. The method developed in this study, together with information on field resistance of major Hokkaido cultivars, is a useful tool for breeding soybean with field-resistance to P. sojae.
