Cecilia Agüero, Zhenhua Cui, and M. Andrew Walker*
*Viticulture and Enology, UC Davis, Davis, CA 95616 (awalker@ucdavis.edu)

Specific strains of Grapevine Leafroll Associated Viruses (GLRaV) severely affect the graft union. Some rootstocks (Freedom and 101-14) react strongly, while others (St. George and AXR1) rarely show graft incompatibility. To understand this response, Freedom, 101-14, St. George, and AXR1 were grafted with Cabernet franc infected with isolates LR131 (GLRaV1) or LR132 (GLRaV1 co-infected with grapevine virus A). Under greenhouse conditions, green grafts of Cabernet franc infected with LR131 or LR132 had mild leafroll symptoms on St. George and AXR1. However, symptoms were severe on Freedom and 101-14, and to a greater extent, with the LR132 isolate. Significant differences in scion dry weight were found between healthy and LR132-Cabernet franc grafted on Freedom or 101-14. Moreover, LR132 markedly reduced Freedom and 101-14 graft survival rate. GLRaV-1 concentration in the graft union was similar in all graftings infected with LR131, but St. George and 101-14 had the highest and lowest levels, respectively, when infected with LR132. We also grafted under in vitro conditions in an attempt to hasten the onset of symptoms. Under these conditions, LR132 infections prevented a graft union. Healthy and LR131Cabernet franc showed similar survival rates, but LR131 infection delayed budbreak and root initiation in Freedom and 101-14 micrografts. Differences in GLRaV-1 concentration in LR131 micrografts were not significant among rootstocks. Histological observations indicated that LR132 infection delayed callus formation between scion and rootstock and limited the vascular connection between them. Obvious callus was observed in both healthy and LR131-infected micrografts regardless of rootstock and there was a strong vascular connection two months after grafting. Overall, St. George exhibited the highest tolerance, followed by AXR1. Both Freedom and 101-14 were very sensitive. miRNA profiles are being studied to evaluate the different green graft combinations.

We gratefully acknowledge research funding from the California Grape Rootstock Improvement Commission.

Funding Support: Calif. Grape Rootstock Improvement Commission