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 strawberry fruit was evaluated with respect to environmental influences and fungicide regimes over four consecutive years. Disease incidence at harvest was correlated with the average daily values of 13 environmental variables during four discrete periods (or combinations thereof); these 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 weighting factor that most accurately accounted for fungicide influence on individual environmental variable x spray period combinations. Two bloom sprays provided the same annual level of control as four to five sprays from bloom through harvest, whereas applications made only after bloom provided relatively little control. Similarly, disease incidence was correlated strongly with environmental variables measured during the bloom period, particularly the durations of relative humidity >80% and >90% and surface wetness at 15-25 C. Environmental factors after bloom were correlated much more weakly with disease incidence, with the exception of vapor pressure deficit (negative correlation) and rainfall during periods defined by the first postbloom spray. Optimum fungicide weighting factors (0.0 = full fungicide effect, complete negation of environmental influence; 1.0 = no fungicide effect, full influence of environmental variable) were 0.5-0.8 for those variables with the highest correlation coefficients during bloom but were 1.0 for the most influential variables during periods after bloom.