On-Farm Validation of a Foliar Fungicide Decision Aid in Soybean.

Over 10 years of research in Virginia indicate that foliar fungicides result in a significant soybean yield response only one-third of the time.  Foliar soybean disease development depends on optimum environmental conditions, primarily temperature (daily averages between 18 and 26°C) and relative humidity (>95% for >10hrs/day).  On-farm experiments were conducted to validate a decision aid developed to predict whether or not, and when to make a foliar fungicide application to soybean.  Treatments included an untreated control, R3 stage (early pod development) application, and application based on the decision aid.  Disease ratings were made at 1- to 2-week intervals at multiple locations within a field.  Where yield monitors were used to create yield maps, site-specific disease-yield relationships were obtained.  The decision aid predicted that foliar fungicide was needed at 5 of 7 sites, but recommended spraying 4 to 13 days after R3 in 2014.  In 2015, the decision aid recommended a fungicide at 7 of 8 sites, but recommended spraying 4 to 23 days after R3. Total number of days conducive for disease development during pod development was less in 2015.  Total number of favorable days the week prior to and the week after the beginning of the R3 growth stage was significantly correlated with yield response (R2= 0.66, P=0.0003). Therefore, a revised decision aid that uses 3 instead of 2 favorable days within a week of R3 will be tested in 2016.