Sexual recombination of carbendazim resistance in Fusarium graminearum under field conditions

Abstract

Carbendazim has been the major fungicide for control of Fusarium head blight (FHB) caused by Fusarium graminearum Schwade in China. However, the effectiveness of carbendazim has been threatened by the emergence of resistant pathogen populations in the field. Five isolates, representing three phenotypes of different carbendazim sensitivity levels (S, MR and HR), were randomly selected to study the inheritance of carbendazim resistance by three genetic crosses under field conditions. Each parent in all crosses was marked with a different class of nitrate non-utilizing (nit) mutation. The presence of sexual recombinants indicated that outcrossing had occurred in the crosses. Over 100 putative self-crossing or outcrossing perithecia for each cross were randomly sampled on the surface of the haulms of dead rice for each pair of parents. Results showed that 5.7-20.9% outcrossing frequency occurred in the three crosses and confirmed sexual recombination under field conditions. There were no significant differences in mycelial linear growth and pathogenicity between the selected recombinants and their parents, but they differed in sporulation ability and capacity to produce perithecia. Nevertheless, most of the sexual recombinants possessed fitness levels comparable with those of their parents. Outcrossing between carbendazim-sensitive and -resistant isolates did occur under field conditions, and sexual recombination must play a role in the development of carbendazim resistance in the field.