Colonisation of detached apple shoots by potential antagonistic fungal endophytes

Abstract

European canker, caused by the fungus Neonectria ditissima, is an important disease of apple worldwide, including New Zealand. There are currently limitations surrounding the use of chemicals for the control of picking wound infections by this fungus. This study aimed to evaluate the efficacy of fungal endophytes isolated from the stem and leaf tissues of apple against N. ditissima. Three hundred and eleven fungal endophytes were primarily screened for their activity against N. ditissima using a dual-culture assay. Thirty-seven isolates which showed the highest antifungal activity were further evaluated using other in vitro bioactivity assays; volatile, culture filtrate, and siderophore assays. Thereafter, in planta assays were conducted to evaluate the colonisation ability and pathogenicity of the 37 endophytes on apple. The pathogenicity tests revealed that 10 isolates were pathogenic to apple shoots while 11 showed infections on apple fruits. Of the remaining 24 isolates, 2 failed to colonise apple shoot tissues, while the others were re-isolated from the stem tissue, ranging from 0–5 cm below and above the inoculation point. Based on the results of their pathogenicity, in vitro bioactivity, spore production ability, and relative length of stem tissue colonised, four isolates (one each Biscogniauxia sp., Neosetophoma sp., Trichoderma sp. and Paraphaeosphaeria sp.) were selected, and tested in in planta for their ability to reduce N. ditissima infection. The detached shoot assay indicated that two isolates (Trichoderma sp. and Paraphaeosphaeria sp.) reduced infection of N. ditissima in detached shoot tissues. The biocontrol efficacy of these two endophyte isolates will be further evaluated in attached apple shoot tissues.