In vitro evaluation of fungicides against Cylindrocarpon destructans, Rhizoctonia fragariae and Pestalotiopsis spp. in strawberries

Root diseases, in particular black root rot and crown rot have become a severe problem amongst strawberry nurseries and production fields in Belgium and the Netherlands. Affected plants show decline in runner production, stunted growth and an overall lack of vigor leading to reduced yields. Cultivars ‘Jive’, ‘Elegance’, ‘Sonata’ and ‘Malling Centenary’ appear to be particularly susceptible. The most commonly isolated pathogens from infected root and crown tissues were Pestalotiopsis spp., Rhizoctonia fragariae and Cylindrocarpon destructans. Current chemical treatments in the nursery and after planting in the production field fail to give adequate control. In this study, in vitro applications of several fungicides were evaluated for their efficacy against these pathogens. Of the 7 fungicides tested against Rhizoctonia fragariae mixtures of cyprodonil with fludioxonil and mixtures of dimethomorf with fluazinam were most effective at inhibiting mycelium growth. Of the 8 fungicides tested against Cylindrocarpon destructans, fluopyram combined with trifloxystrobin gave the highest efficacy. Mixtures of cyprodinil with fludioxonil and the combination of pyraclostrobin with boscalid achieved almost comparable mycelium growth inhibition. Further in vitro tests showed that conidial germination was inhibited most with mixtures of pyraclostrobin with boscalid and fluopyram with trifloxystrobin. Of the 4 fungicides tested against Pestalotiopsis spp. fluazinam and the mixture of cyprodinil with fludioxinil were most effective at inhibiting mycelium growth and conidial germination. Fluopyram combined with trifloxystrobin was not effective against conidial germination. The results of these trials provide some direction for the future in vivo testing of fungicides against root pathogens; Pestalotiopsis spp., Rhizoctonia fragariae and Cylindrocarpon destructans in a semi practical scenario.