This work compared development of soybean rust (SR) on the susceptible cultivar BRS 133 and on the resistant line CB06-953/963 (Rpp4 gene) following different fungicide treatments applied in different developmental stages. The assessed variables were percent plant defoliation, coefficient of damage (CD) and economic threshold level (ETL), the latter two calculated from disease severity ratings and yield. Experiments were conducted in the field during the 2006-2007 growing season with one sowing date (Experiment I) and in 2007-2008 with two sowing dates (Experiments II and III). The experimental design was a randomized complete block with ten treatments and four replications. Overall, the Rpp4 gene in the resistant line was effective in reducing development of SR in all three experiments. The values of CD and ETL were higher for the resistant line than for the susceptible cultivar. The resistant line needed 13.3 days longer than the susceptible cultivar to reach the ETL in Experiment II. Fungicide applications were more effective in protecting yield and minimizing defoliation in the susceptible cultivar than in the resistant line. Under high inoculum pressure (Experiment III), three and four fungicide sprays applied during the season resulted in significantly higher yields (P<0.05) in both soybean genotypes compared with the untreated controls. Late season fungicide applications reduced rust severity and increased the yield of the resistant cultivar.
Glycine max; Phakopsora pachyrhizi; economic threshold level; fungicide; vertical resistance
