Grey mould populations in northern German sweet cherry and plum orchards: selection of fungicide-resistant Botrytis cinerea strains over sensitive B. pseudocinerea by fungicide treatments

Abstract

Second only to brown rot caused by Monilinia spp., grey mould caused by Botrytis spp. is one of the most important pre- and post-harvest diseases of stone fruit in northern Germany. Among 484 Botrytis isolates from sweet cherries produced under integrated pest management (IPM), 60.3% showed resistance to QoI fungicides and 54.6% to boscalid, reflecting the heavy use of these compounds in regional production. More modest resistance frequencies were recorded for fenhexamid (20.7%), fluopyram (8.3%), fludioxonil (9.1%) and cyprodinil (21.9%). A 7.6% share of isolates possessed multiple resistance to at least five of these fungicides. Resistance was also detected for two fungicides no longer registered in cherries, viz. thiophanate-methyl (15.7%) and iprodione (20.9%). Similar results were obtained for 141 Botrytis isolates from IPM plum orchards. In contrast, in organically produced cherries (88 Botrytis isolates) resistance frequencies were < 20% for all fungicides. The dominant species on cherries and plums was B. cinerea. The share of B. pseudocinerea, which is not known to have developed resistance to commercial fungicides, was 18.0% in cherries and 9.9% in plums from IPM, but 50.0% in organically produced cherries. A 10-year annual survey of a sweet cherry orchard under IPM showed the immigration of multi-resistant B. cinerea strains in 2011, followed by their increase and decline in subsequent years. An understanding of the effects of fungicide spray regimes on Botrytis population dynamics in stone fruit has implications for resistance management. In practice, a limitation of the number of pre-harvest sprays to a maximum of two will give a satisfactory control of brown rot in most situations, while a higher number will promote multi-resistant B. cinerea strains.