Chapter Three Effects of Resistance-Breaking (RB) Field Isolates of Soybean mosaic virus on Resistant and Susceptible Soybeans

Soybean mosaic virus (SMV; Genus Potyvirus; Family Potyviridae), one of the most widespread viruses in soybean (Glycine max [L.] Merr), can be effectively managed by deployment of resistance. Hutcheson, one SMV-resistant cultivar, is widely used in the MidSouth region and carries an Rsv1 allele for resistance to SMV pathotypes G1-G3, the common strains in US soybean fields. Recently, resistance-breaking (RB) isolates of SMV have been detected in soybean cultivar Hutcheson in natural infections. Seven RB isolates of SMV from Virginia were inoculated on susceptible and resistant soybeans in the field in 2002-2003. The RB isolates induced severe foliar symptoms on Hutcheson and induced seed coat mottling. Hutcheson and herbicide-tolerant Hutcheson RR cultivars reacted similarly and herbicide spray had no effect on the plant-virus interactions. RB isolates did not break the resistance of the Essex-Rsv1 or Essex-Rsv4 and did not affect their field performance. Hutcheson and Hutcheson RR dually inoculated with RB isolates of SMV and Bean pod mottle virus (BPMV) showed an increase in severity of symptoms, including plant stunting. SMV accumulation increased in the dually inoculated plants. BPMV titer increased in the dual inoculations with one of the RB isolates of SMV. The results would be catastrophic if Hutcheson is infected with both viruses in the field, a definite threat with the emergence of the RB isolates of SMV and the increased spread of BPMV in soybean-growing areas.