The distribution of benomyl-tolerant Sclerotinia fructicola (Wint.) Rehm. in stone fruit orchards in New South Wales and comparative studies with susceptible isolates

Abstract

Tolerance of Sclerotinia fructicola (Wint.) Rehm, to benzimidazole fungicides was first recorded in New South Wales in 1976 and has since been confirmed on 11 orchards, all in the Orange and Bathurst districts. Crops affected include peach, nectarine, cherry and plum. The distribution of benomyl-tolerant strains was mapped in two orchards and found to be present in scattered groups of trees, rather than throughout the block. Distribution of tolerant strains was not uniform within trees, and in most cases both tolerant and susceptible strains were present in the same tree. Tolerance was found to be stable after three transfers of the fungus in fruit and after 33 transfers over 2 years in culture. The fungus persisted over the winter in mummified nectarine fruit and was still present in an orchard 18 months after the application of benzimidazole fungicides ceased. The tolerant strain was as pathogenic to peach fruit as the susceptible strain and competed successfully when peach fruits were inoculated with mixtures of spores from susceptible and tolerant strains. There were no significant differences between groups of tolerant isolates and susceptible isolates in growth, sporulation and germination percentage in vitro. It is suggested that since no competitive disadvantage was associated with tolerance, tolerant strains will exist in high numbers in orchards for a considerable period of time.