Diversity and Characterization of Resistance to Pyraclostrobin in Colletotrichum spp. from Strawberry

Strawberry crown rot poses a significant menace to strawberry production during the seedling stage, and the main pathogen is Colletotrichum spp. Pyraclostrobin is one of the main fungicides that have been registered to control anthracnose caused by Colletotrichum spp. The diversity of pathogens and the risk of fungicide resistance may change from year to year. In order to explore the diversity of pathogens causing crown rot and evaluate the resistance risk of pathogens to pyraclostrobin in different years, crown rot samples were collected in Jiande, Zhejiang Province in 2019 and 2021, and the pathogens were identified. Based on the morphological identification and phylogenetic analysis based on ACT, CAL, CHS, GAPDH, and ITS, all 55 strains were identified as C. gloeosporioides species complexes, including 23 C. siamense isolates and 2 C. fructicola isolates in 2019, and all isolates were identified as C. siamense in 2021. C. siamense was the dominant pathogen of strawberry crown rot in 2019 and 2021. The resistance frequencies of the isolates collected in 2019 and 2021 to pyraclostrobin were 69.57% and 100%, respectively. In general, compared to that in 2019, the resistance frequencies of the pathogen to pyraclostrobin increased in 2021. In terms of fitness, there was no significant difference between resistant strains and sensitive strains in the mycelium growth rate, sporulation and spore germination rate. In addition, the resistant mutants exhibited positive cross-resistance to kresoxim-methyl and azoxystrobin. A sequential analysis of cytochrome b gene showed that C. siamense resistance to pyraclostrobin is linked to the G143A point mutation. Our study indicated that the risk of resistance a fungicide gradually increases with the increase in use years, and in order to reduce the emergence and spread of resistant populations, we should choose fungicides of different mechanisms of action for rotation to reduce the risk of resistance development.