Glasshouse-specific occurrence of basal rot pathogens and the seasonal shift of Rhizoctonia solani anastomosis groups in lettuce

Basal rot is a common disease in Belgian lettuce, which is mainly controlled by fungicides and chemical soil disinfestation. A seasonal appearance of the basal rot pathogens: Rhizoctonia solani , Sclerotinia spp., Botrytis cinerea and Pythium spp. has been reported, but lettuce growers use standard spraying schemes, irrespective of the occurrence of the pathogen. Due to stricter regulations and environmental concerns the superfluous use of fungicides should be omitted. We investigated if the use of fungicides could be reduced by only controlling the active pathogens. Therefore, lettuce was continuously grown in three glasshouses without any fungal disease control and the active pathogens causing basal rot were identified. The occurrence of basal rot pathogens appeared to be glasshouse specific and the different basal rot pathogens were active throughout the year. However, a seasonal appearance of R. solani anastomosis groups and Pythium spp. was observed with AG4-HGI and Pythium ultimum active at higher temperatures and AG2–1, AG-BI, AG1-IB and Pythium sylvaticum at lower temperatures. We report for the first time the isolation of AG-BI from infected plants. Each R. solani anastomosis group had its own optimal growth rate in vitro . Differences in pathogenicity between R. solani anastomosis groups were observed on detached leaves. AG1-IB and AG4-HGI were most pathogenic, followed by AG2–1 and AG-BI. These results show that the fungicide spraying scheme should be adapted to the occurring pathogens in the glasshouse. This information is of high importance in developing a sustainable control strategy for basal rot pathogens.