Young vine decline fungi, from nursery to vineyard

Young vine decline (YVD) fungi colonize the xylem of grapevines resulting in their decline and eventual death within few years after planting. Ready-to-plant nursery material has been reported to be a source of infection; however, material sold in Canada has not yet been evaluated. Four nurseries selling grapevines in Canada were selected for identification and quantification of the most prevalent YVD fungi, including Botryosphaeriaceae spp., Cadophora luteo-olivacea, Dactylonectria macrodidyma, Dactylonectria torrensensis, Phaeoacremonium minimum, and Phaeomoniella chlamydospora with droplet digital PCR. Results revealed 99% of plants infected with at least one YVD fungi, and a mean infection of three fungi per plant. Fungal abundance of all YVD fungi varied significantly among sections of plants, individual plants, cultivars, and nurseries. This research indicates plants are likely to already be infected when planting; however, most grapevines are not symptomatic in young vineyards. It has been hypothesized that some YVD fungi are latent pathogens, transitioning from an endophytic to pathogenic lifestyle under plant stress. Two greenhouse experiments were conducted to determine the effect of drought stress and the ring nematode Mesocriconema xenoplax infestation on P. chlamydospora fungal abundance. Phaeomoniella chlamydospora spores were vacuum inoculated into the base of dormant ‘Merlot’ canes at a high (25,000 spores), medium (5,000 spores), and low (1,000 spores) inoculum to evaluate whether initial fungal abundance would impact disease development. Phenotypical measurements were conducted throughout the experiment to monitor the effect of drought stress and nematodes on plant health. Drought stress had a negative phenotypical impact on plants, but nematode infestation did not. Fungal abundance was higher in drought stressed plants after two years, but was not in ring nematode infested plants. Field trials are currently underway to evaluate the effect of plant stress on disease development under natural conditions.