Effect of soybean growth stage at the time of inoculation with Diaporthe phaseolorum var. meridionales on stem canker development and yield

Infections at early stages of plant development followed by a long incubati on period before symptoms appear during reproductive stages is characterist ic of the disease cycle of stem canker of soybean, caused by Diaporthe phas eolorum var. meridionales. To determine the effect of plant growth stage at the time of infection on symptom development and yield components, soybean plants (cv. Waiters) were grown in microplots and inoculated with a suspen sion of ascospores (10(6)/ml) at the V-1, V-4, V-6, V-10, or R-2 growth sta ge. Noninoculated plants served as controls. Development of foliar symptoms was quantified weekly after flowering (R-2), and yields and seed weights w ere determined. The experiment was conducted twice, in 1992 and 1994. Folia r symptoms of stem canker developed for all treatments except the noninocul ated control. Foliar symptoms appeared at the R-2 to R-5 growth stage and r eached 90 to 100% incidence for most treatments. Disease development was de layed for the R-2 inoculation, suggesting that a minimum incubation period of 34 to 41 days is necessary before foliar symptoms develop. Disease (area under the disease progress curve) was greater and yield, seed number, and seed weight were less in 1994 than in 1992. Quadratic equations significant ly related these variables to time of inoculation and reached maximum or mi nimum values between 40.9 and 43.3 days after planting. This corresponded t o the V-6 growth stage. Yield and seed number were affected by disease deve lopment during pod formation (R-2 to R-5) in both years, while yield and se ed weight were affected by disease development during seed formation (R-5 t o R-6) in 1992.