Effect of Soybean Growth Stage at the Time of Inoculation with Diaporthe phaseolorum var. meridionales on Stem Canker...

Infections at early stages of plant development followed by a long incubation period before symptoms appear during reproductive stages is characteristic of the disease cycle of stem canker of soybean, caused by Diaporthe phaseolorum var. meridionales. To determine the effect of plant growth stage at the time of infection on symptom development and yield components, soybean plants (cv. Walters) were grown in microplots and inoculated with a suspension of ascospores (10⁶/ml) at the V₁, V₄, V₆, V₁₀, or R₂ growth stage. Noninoculated plants served as controls. Development of foliar symptoms was quantified weekly after flowering (R₂), and yields and seed weights were determined. The experiment was conducted twice, in 1992 and 1994. Foliar symptoms of stem canker developed for all treatments except the noninoculated control. Foliar symptoms appeared at the R₂ to R₅ growth stage and reached 90 to 100% incidence for most treatments. Disease development was delayed for the R₂ 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 significantly related these variables to time of inoculation and reached maximum or minimum values between 40.9 and 43.3 days after planting. This corresponded to the V₆ growth stage. Yield and seed number were affected by disease development during pod formation (R₂ to R₅) in both years, while yield and seed weight were affected by disease development during seed formation (R₅ to R₆) in 1992.