Control of bottom rot in hydroponic lettuce, caused by strains of Botrytis cinerea with multiple fungicide resistance

Abstract

For two consecutive growing periods, fungicide-resistant Botrytis cinerea strains were detected in high proportions in glasshouse-grown lettuce, but at variable frequencies. Pre-transplanting fungicide sprays applied on two successive occasions reduced disease severity and increased the number of healthy plants without leaving detectable residues above accepted MRLs at harvest. In some instances, the disease was further decreased when pre-transplanting applications were combined with one or two further sprays applied soon after transplanting. The fungicide mixture of fludioxonil + cyprodinil was the most effective against the disease and provided better control of B. cinerea isolates in situ. These treatments gave satisfactory disease control despite the predominance of multi-fungicide resistant B. cinerea populations.