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Prediction of Phomopsis Seed Decay by Measuring Soybean Pod Infection. D. C. McGee, Professor, Department of Plant Pathology Seed and Weed Sciences, Iowa State University, Ames 50011. Plant Dis. 70:329-333. Accepted for publication 27 September 1985. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/PD-70-329.

The epidemiological basis for predicting the incidence of soybean seed infection by Phomopsis longicolla, Diaporthe phaseolorum var. sojae, and D. phaseolorum var. caulivora (collectively referred to as PD) by measuring pod infection at the R6 (full seed) growth stage was elucidated. Extensive seed infection by PD could not be induced in pods exposed to 100% relative humidity for 7 days in the laboratory when they were detached from plants before the R7 growth stage (beginning maturity). Pod infection therefore can be measured and a fungicide applied during R6 without risk of seed infection already having occurred. Pods artificially inoculated at various growth stages between R3 (beginning pod) and R8 (full maturity) by an isolate of P. longicolla were susceptible to infection only before R7 under field conditions and in growth chambers held at 5070% relative humidity. They were susceptible, however, after inoculation at R7 and R8 in growth chambers at 8697% relative humidity. Periodic measurements of pod infection between R4 (full pod) and R8 in four soybean fields showed low levels of infection until R7, then they increased markedly. This was attributed to further colonization of pods by inoculum already present and not to new inoculum that might have reached pods since R6. It was concluded that under environmental conditions likely to occur in the Midwest, a predictive measurement at R6 would not be invalidated by inoculum that later reached pods. Pod infection was measured by a quick, inexpensive test that could be done by seed company personnel. The predictive capability of the test was demonstrated by clear relationships between inoculum artificially applied to pods and incidences of pod infection at R6 and seed infection at harvest maturity. Also, a correlation of 0.67 was obtained between natural pod and seed infection in 23 soybean fields in Iowa in 1982. Benomyl, applied during R6, reduced seed infection by PD in plots with 71% pod infection but was of no benefit in plots with 27% pod infection where seed quality was not impaired by infection by PD.