Chestnut rot caused by Gnomoniopsis smithogilvyi is considered a severe threat to the production of chestnuts from European chestnut (Castanea sativa Mill.) in Australia and overseas. Currently, most of the control strategies are applied post-harvest and little is known about the use of fungicides to reduce nut infection in orchards early in the season. This research evaluated the effectiveness of various fungicides against the pathogen in vitro and selected the most effective products for field trials. In vitro experiments showed that pyraclostrobin and difenoconazole-based fungicides effectively inhibited conidial germination and mycelial growth, respectively. The field trial showed that both active ingredients combined were more effective than single applications in suppressing the level of nut infection caused by G. smithogilvyi. Based on our results, using the selected fungicides can be an additional tool for growers to complement their current practices in the control of chestnut rot. To the best of our knowledge, this is the first study focused on the effectiveness of chemical treatments against G. smithogilvyi.

Introduction Chestnut rot caused by the fungus Gnomoniopsis smithogilvyi is a disease present in the world’s major chestnut growing regions. The disease is considered a significant threat to the global production of nuts from the sweet chestnut (Castanea sativa). Conventional fungicides provide some control, but little is known about the potential of biological control agents (BCAs) as alternatives to manage the disease. Objective Evaluate whether formulated BCAs and their secreted metabolites inhibit the in vitro growth of G. smithogilvyi. Methods The antifungal potential of BCAs was assessed against the pathogen through an inverted plate assay for volatile compounds (VOCs), a diffusion assay for non-volatile compounds (nVOCs) and in dual culture. Methanolic extracts of nVOCs from the solid medium were further evaluated for their effect on conidia germination and were screened through an LC–MS-based approach for antifungal metabolites. Results Isolates of Trichoderma spp., derived from the BCAs, significantly suppressed the pathogen through the production of VOCs and nVOCs. The BCA from which Bacillus subtilis was isolated was more effective in growth inhibition through the production of nVOCs. The LC–MS based metabolomics on the nVOCs derived from the BCAs showed the presence of several antifungal compounds. Conclusion The results show that G. smithogilvyi can be effectively controlled by the BCAs tested and that their use may provide a more ecological alternative for managing chestnut rot. The in vitro analysis should now be expanded to the field to assess the effectiveness of these alternatives for chestnut rot management.

The fungus Gnomoniopsis smithogilvyi is a significant threat to the production of sweet chestnut (Castanea sativa) nuts in Australia and worldwide. The pathogen causes nut rot, which leads to substantial production losses. Early and accurate diagnosis of the disease is essential to delineate and implement control strategies. A specific and sensitive multiplex PCR was developed based on the amplification of three barcode sequences of G. smithogilvyi. The assay reliability was enhanced by including the amplification of a host gene as an internal control. Primers were thoroughly evaluated in silico before assessing them in vitro. Primer annealing temperature and concentration were optimised to enhance the assay sensitivity and specificity. The assay detection limit ranged between 0.1 and 1.0 pg (5 and 50 fg/μL) of genomic DNA per reaction. No cross-reactivity was observed with genomic DNA from closely and distantly related fungal species. We also characterised Australian G. smithogilvyi isolates phenotypically and genotypically and found significant differences in morphologic and virulence traits of the isolates. An understanding of the virulence of G. smithogilvyi and the availability of a reliable and accurate diagnostic technique will enable earlier detection of the pathogen, which will contribute to effective control strategies for the disease.