Sensitivity of Colletotrichum acutatum Species Complex from Strawberry to Fungicide Alternatives to Quinone-Outside Inhibitors.

Colletotrichum acutatum is a species complex that causes anthracnose fruit rot and root necrosis on strawberry. The major and minor species within the complex that affect strawberry production are C. nymphaeae and C. fioriniae , respectively. The disease can significantly reduce yield under conducive weather, and its management has greatly relied on quinone-outside inhibitor fungicides (QoI). However, due to the emergence of resistant isolates, such products are no longer effective. Therefore, alternative fungicides were investigated. C. nymphaeae and C. fioriniae isolates were collected from multiple strawberry fields in the United States from 1995 to 2017. The sensitivity of benzovindiflupyr, penthiopyrad, pydiflumetofen, fluazinam, fludioxonil, and cyprodinil was assessed by in vitro and in vivo assays. Both Colletotrichum species were sensitive to benzovindiflupyr, penthiopyrad, fluazinam, and fludioxonil based on mycelial growth assays. Interestingly, of these products, only penthiopyrad did not inhibit conidial germination at 100 µg/ml. For cyprodinil, C. nymphaeae was sensitive based on the mycelial growth, whereas C. fioriniae was not inhibited. Neither species was inhibited by pydiflumetofen in mycelial growth, conidial germination, nor detached fruit assays. The prepackaged mixtures fludioxonil + cyprodinil and fludioxonil + pydiflumetofen were effective in a field trial; however, their use should be carefully considered because of the lack of efficacy of one of the compounds in the mixture. This study sheds light on the potential registration of products alternative to QoIs, such as benzovindiflupyr and fluazinam, which could improve the management of strawberry anthracnose.