Efficacy of fenhexamid treatments against Botrytis cinerea in grapevine as affected by time of application and meteorological conditions

Grape bunch rot caused by Botrytis cinerea is one of the major fungal diseases of grapevine (Vitis vinifera L.) worldwide. Besides crop cultural measures, botryticides are used on a regular basis for controlling bunch rot. To investigate the impact of (i) the time of application and (ii) the meteorological conditions on the efficacy of a single botryticide treatment per year (active ingredient: fenhexamid), field trials were conducted over a six year period (2011–2016) on the cultivars Riesling and Pinot gris. The median levels of efficacy compared to the untreated control at 100 days after growth stage BBCH 68 were 6.7%, 23.1%, 34.8%, 25.6%, 16.0%, 34.5% or 9.0% for Riesling and 13.9%, 16.5%, 23.7%, 9.8%, 6.6%, 9.3% or 14.1% for Pinot gris for applications at 0, 2, 4, 6, 8, 10 or 12 weeks after BBCH 68, respectively. Generally, the levels of efficacy as well as the delays of the epidemics caused by the fenhexamid treatment (i) were lower than those attainable by cultural measures and (ii) decreased with increasing temporal distance between BBCH 68 and the date of assessment as well as with the level of disease severity. Results indicate that in Riesling, fenhexamid treatments before bunch closure and shortly after veraison were most effective, whereas this was the case for Pinot gris only around bunch closure. The rather low overall efficacies suggest that a limited number of targeted botryticide applications may represent a facultative component in the complex bunch rot control strategy primarily based on efficient crop cultural measures. The delay of the epidemic induced by a single fenhexamid treatment was significantly affected by the annual meteorological conditions in the different stages of grape development even outside the application period suggesting that plant physiological factors also contribute to the overall effect.