Environmental Factors Influencing Infection of Soybean Seeds by Phomopsis and Diaporthe Species During Seed Maturation. A. J. Balducchi, Research Associate, Department of Plant Pathology, Seed and Weed Science, Iowa State University, Ames 50011. D. C. McGee, Professor, Department of Plant Pathology, Seed and Weed Science, Iowa State University, Ames 50011. Plant Dis. 71:209-212. Accepted for publication 10 September 1986. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/PD-71-0209.

The effect of moisture and temperature on infection of soybean seeds during seed maturation (growth stages R7 to harvest maturity) by Phomopsis longicolla, Diaporthe phaseolorum var. sojae, and D. phaseolorum var. caulivora (collectively referred to as PD) was investigated. Pods, grown in the greenhouse, were detached from plants at the R8 growth stage (full maturity), inoculated with a conidial suspension of P. longicolla, and exposed for different times to high (100%) or low (40–60%) relative humidity at 25 C. At least three continuous days at high humidity were needed for extensive seed infection to occur. Periods of high humidity could be interrupted after 1 day by up to 3 days at low humidity, and extensive seed infection would still occur. When 1-day periods at high humidity were continually alternated with low humidity, no significant seed infection took place. At temperatures of 20 and 15 C, it took 4 and 5 days, respectively, at 100% relative humidity to reach the level of seed infection attained within 3 days at 25 C. Higher seed infection occurred in pods incubated at 85–100% relative humidity for 7 days when they were detached at R7 (beginning maturity) than at R8. Field-grown soybeans, either inoculated with P. longicolla or naturally infected with PD, were overhead-irrigated at different growth stages between R6 and R8 in 1982 and 1983. Irrigation and inoculation both were associated with increased seed infection at harvest maturity. High positive correlations (r values ranged from 0.58 to 0.98) were obtained between seed infection and average temperature during irrigation. Lower negative correlations (r values ranged from 0.32 to 0.56) were obtained for relationships between seed infection and plant age during irrigation.