A Quick and Simple Method to Demonstrate Resistance Against Fenhexamid in Botrytis

Isolates of Botrytis cinerea collected from Northern German strawberry and raspberry farms were grouped into three categories according to their pattern of resistance against the fungicide fenhexamid. Strains with baseline sensitivity showed EC50 values of 0.06-0.10 ppm (conidial germ-tube elongation) and 0.07-0.16 ppm (mycelial growth assay). For partially resistant isolates, the corresponding EC50 values were similar for germ-tube elongation but higher (> 0.39 ppm) for mycelial growth. Highly resistant isolates produced more than 50% growth in both assays even at 200 ppm fenhexamid, the highest concentration tested. These isolates matched the criteria of resistance group Hyd R3. By means of a simple test consisting of the harvesting of conidia from sporulating field samples and their direct assay by germ-tube elongation, Hyd R3 resistance was identified within 24 h of sampling. If this assay is performed on overwintered samples prior to blossom time, site-specific recommendations about the use of fenhexamid can be made to the farmer during the running vegetation period.