Risk assessment and molecular mechanism of Fusarium incarnatum resistance to phenamacril

Cucumber fruit rot (CFR) caused by Fusarium incarnatum is a devastating fungal disease in cucumber. In recent years, CFR has occurred frequently, resulting in serious yield and quality losses in China. Phenamacril exhibits a specific antifungal activity against Fusarium species. However, no data for phenamacril against F. incarnatum is available.

The sensitivity of 80 F. incarnatum strains to phenamacril was determined. The half maximal effective concentration (EC₅₀ ) values ranged from 0.1134 to 0.3261 μg mL^-1 with a mean EC₅₀ value of 0.2170 ± 0.0496 μg mL^-1 . A total of seven resistant mutants were obtained from 450 mycelial plugs by phenamacril-taming on potato dextrose agar (PDA) plates with 10 μg mL^-1 of phenamacril, and the resistant frequency was 1.56%. Phenamacril-resistant mutants showed decreased mycelial growth, conidiation and virulence as compared with the corresponding wild-type strains, indicating that phenamacril resistance suffered a fitness penalty in F. incarnatum. In addition, using sequence analysis, the point mutations of S217P or I424S were discovered in Fimyosin-5 (the target of phenamacril). The site-directed mutagenesis of the S217P, P217S, I424S and S424I substitutions were constructed to reveal the relationship between the point mutations and phenamacril resistance. The results strongly demonstrated that the mutations of S217P and I424S in Fimyosin-5 conferred phenamacril-resistance in F. incarnatum.

Phenamacril-resistant mutants were easily induced and their resistance level was high. The S217P or I424S substitutions in Fimyosin-5 conferring phenamacril resistance were detected and futherly verified by transformation assay with site-directed mutagenesis. Thus, we proposed that the resistance development of F. incarnatum to phenamacril is high risk. © 2022 Society of Chemical Industry.