RELATIONSHIP BETWEEN STRAWBERRY GRAY MOLD INCIDENCE, ENVIRONMENTAL VARIABLES, AND FUNGICIDE APPLICATIONS DURING DIFFERENT PERIODS OF THE FRUITING SEASON

Incidence of gray mold (caused by Botrytis cinerea) on harvested straw berry fruit was evaluated with respect to environmental influences and fungicide regimes over four consecutive years. Disease incidence at h arvest was correlated with the average daily values of 13 environmenta l variables during four discrete periods (or combinations thereof); th ese periods occurred from bloom until harvest and were defined by the timing of fungicide applications in designated treatments. Correlation coefficients in sprayed plots were determined with a variable weighti ng factor that most accurately accounted for fungicide influence on in dividual environmental variable x spray period combinations. Two bloom sprays provided the same annual level of control as four to five spra ys from bloom through harvest, whereas applications made only after bl oom provided relatively little control. Similarly, disease incidence w as correlated strongly with environmental variables measured during th e bloom period, particularly the durations of relative humidity >80% a nd >90% and surface wetness at 15-25 C. Environmental factors after bl oom were correlated much more weakly with disease incidence, with the exception of vapor pressure deficit (negative correlation) and rainfal l during periods defined by the first postbloom spray. Optimum fungici de weighting factors (0.0 = full fungicide effect, complete negation o f environmental influence; 1.0 = no fungicide effect, full influence o f environmental variable) were 0.5-0.8 for those variables with the hi ghest correlation coefficients during bloom but were 1.0 for the most influential variables during periods after bloom.