Jared Hrycan,* Pat Bowen, Tom Forge, Miranda Hart, and José Ramon Úrbez-Torres
*Agriculture and Agri-Food Canada – Summerland Research and Development Centre, 4200 Highway 97, Summerland, British Columbia, V0H 1Z0, Canada (jared.hrycan@agr.gc.ca)

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, using droplet digital PCR. Results revealed 99% of plants were infected with at least one YVD fungus, with 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. Some YVD fungi may be 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 infestation by the ring nematode, Mesocriconema xenoplax, on P. chlamydospora fungal abundance. P. chlamydospora spores were vacuum-inoculated into the base of dormant Merlot canes at a high (25,000 spores), medium (5000 spores), and low (1000 spores) inoculum to evaluate whether initial fungal abundance would impact disease development. Phenotypic measurements were conducted throughout the experiment to monitor the effect of drought stress and nematodes on plant health. Drought stress had a negative phenotypic 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.

Funding Support: Agriculture and Agri-Food Canada, Canadian Grapevine Certification Network, British Columbia Wine and Grape Council