Different Infection Structures and Point Mutation of Diaporthe citri Showing Resistant against Systemic Fungicides

Infection behaviors of some susceptible and resistant Diaporthe citri isolates against two fungicides such as benomyl and kresoxim-methyl were observed on citrus leaves. On the chemically unsprayed leaves, there were no differences in not only the conidial germination rate but also hyphal growth between the resistant and susceptible isolates. However, on the benomyl or kresoximmethyl-pretreated leaves, both conidial germination rate and hyphal growth of the resistant isolates were not decreased, which was different with those of the susceptible isolates. These unchanging germination rate and hyphal growth of the resistant isolates, resulting in the lack of chemical efficacy, coincided with the previous studies in which strong infection was observed by the resistant isolates after pre-treatment with each chemical. On the other hand, a point mutation occurred at 198th amino acid of β-tubulin gene in the resistant isolate NEL21-2 which may cause the resistant against benomyl. However, no mutation was found in cytochrome b in the resistant isolates indicating other genomic changes may be responsible for the resistant against kresoxim-methyl.