Fungal pathogens associated with Fruit Tree Decline in Ontario

Fruit Tree Decline (FTD) affecting stone fruit trees has been observed in Ontario, Canada, orchards over the last three years. Common symptoms include branch die-off, cankers, necrosis, leaf discoloration, and increased suckering leading mainly to rapid tree decline. Emergence and development of the disease can be triggered by stress caused by both abiotic and biotic factors. Identifying factors that lead to FTD symptoms are important since they will provide information that may help detect the early onset of the disorder and thereby prevent eventual crop losses. The main objective of this study was to identify fungal pathogens associated with FTD in Ontario, employing both morphological and molecular techniques. Single spore fungal cultures were isolated from wood samples collected from symptomatic trees in stone fruit orchards in Ontario. The fungal isolates were identified by morphological characteristics and phylogenetic analysis of three gene sequence data (internal transcribed spacer (ITS), translation elongation factor 1-alpha (tef1-a), and beta-tubulin (tub2)). The following fungal pathogens were identified in association with FTD symptoms: Cytospora cf. plurivora, Diplodia seriata, Nectria cinnabarina, and Phomopsis velata (Diaporthe eres). Pathogenicity tests were conducted by inoculating mycelial plugs of each fungi into excised nectarine and apricot shoots under laboratory conditions for a 12-day period. According to necrosis length, Diplodia seriata was the most pathogenic in nectarines (5.2 cm) while Cytospora cf. plurivora appeared to be most virulent in apricots (2.2 cm) when compared to a control inoculated plug containing no mycelium. Inoculated fungal cultures were re-isolated to fulfill Koch’s postulate. Future experiments will be conducted using whole-genome analysis and digital droplet PCR to develop molecular-based diagnostic tools for efficient early detection of fungal pathogens identified as either primary causes or key components of the FTD disease complexes.