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Resistance to Brown Stem Rot in Soybean Germ Plasm with Resistance to the Soybean Cyst Nematode

The soybean cyst nematode (SCN) and Phialophora gregata f. sp. sojae, the causal agent of brown stem rot (BSR), are two pathogens of soybean commonly found in the same field throughout the north-central United States. Field experiments designed to study the role of SCN-resistant germ plasm in soybean production have led to data suggesting that some sources of SCN resistance also may provide resistance to BSR. Soybean germ plasm with resistance to SCN was evaluated in greenhouse and field environments for resistance to BSR development based on the percentage of host tissue symptomatic of BSR. Comparison of SCN-resistant cultivars and plant introductions (PI) to standard BSR-resistant and -susceptible checks were conducted in two greenhouse experiments using a root-dip inoculation with a single isolate of P. gregata. For both greenhouse experiments, PI 209332 was the only source of SCN resistance with resistance to BSR similar to standard BSR-resistant checks. Nine other sources of SCN resistance, including PI 88788 and Peking, expressed BSR symptom severity similar to BSR-susceptible checks. Cultivars derived from most SCN-resistant sources, including PI 209332, also were susceptible to BSR development, while four of the five cultivars derived from PI 88788 were highly resistant to BSR development. SCN-resistant cultivars derived from PI 88788, Peking, and PI 209332 were planted along with standard BSR-resistant and -susceptible checks at two field locations naturally infested with P. gregata and SCN or P. gregata alone. As in greenhouse experiments, four of the five cultivars derived from PI 88788 expressed resistance to BSR development equal to or better than standard BSR-resistant checks at both locations. In contrast, cultivars derived from PI 209332 and Peking expressed varying levels of disease development depending on field environment. Yields observed for PI 88788-derived cultivars were higher than BSR-resistant checks regardless of the presence of SCN. Data from both greenhouse and field experiments suggest that cvs. Williams and Williams 82 may contain a gene or genes for BSR resistance that require one or more modifier genes, possibly located in the genome of PI 88788, for complete expression.