Chemical control and responses of susceptible and resistant soybean cultivars to the progress of soybean rust

Abstract

This work compared development of soybean rust (SR) on the susceptiblecultivar BRS 133 and on the resistant line CB06-953/963 (Rpp4 gene)following different fungicide treatments applied in differentdevelopmental stages. The assessed variables were percent plantdefoliation, coefficient of damage (CD) and economic threshold level(ETL), the latter two calculated from disease severity ratings andyield. Experiments were conducted in the field during the 2006-2007growing season with one sowing date (Experiment I) and in 2007-2008 withtwo sowing dates (Experiments II and III). The experimental design was arandomized complete block with ten treatments and four replications.Overall, the Rpp4 gene in the resistant line was effective in reducingdevelopment of SR in all three experiments. The values of CD and ETLwere higher for the resistant line than for the susceptible cultivar.The resistant line needed 13.3 days longer than the susceptible cultivarto reach the ETL in Experiment II. Fungicide applications were moreeffective in protecting yield and minimizing defoliation in thesusceptible cultivar than in the resistant line. Under high inoculumpressure (Experiment III), three and four fungicide sprays appliedduring the season resulted in significantly higher yields (P<0.05) inboth soybean genotypes compared with the untreated controls. Late seasonfungicide applications reduced rust severity and increased the yield ofthe resistant cultivar.