UC Davis

One of the most destructive grapevine viruses is grapevine fanleaf virus (GFLV), the causal agent of fanleaf degeneration. This virus is vectored from root-to-root by the dagger nematode (Xiphinema index) and can result in crop losses of up to 80% by greatly reducing fruit set and causing formation of ‘shot berries,’ small, seedless berries that do not mature. Currently, fanleaf degeneration is controlled by grafting vines onto the rootstock O39-16, which suppresses the expression of fanleaf degeneration symptoms in the scion. However, the Vitis vinifera parentage of O39-16 raises concerns about the rootstock’s long-term susceptibility to grape phylloxera and other pests and diseases. Additionally, O39-16 is susceptible to root-knot nematodes and induces high vigor to scions grafted on it. Due to these reasons, breeding efforts to produce alternative fanleaf degeneration rootstocks have continued.In 2007, 101-14 Mgt. was crossed with Muscadinia rotundifolia cv. Trayshed and the resulting progeny have been growing at the University of California, Davis. Muscadinia rotundifolia is the source of rootstock-induced tolerance observed in O39-16, and 101-14 Mgt. (V. riparia x V. rupestris) is a popular commercial rootstock commonly chosen for its ease of propagation, moderate nematode and phylloxera resistance, and the ability to control vigor to scions grafted on it. We quantified GFLV resistance and fanleaf degeneration tolerance in the progeny from this cross and studied the inheritance of these traits. Both traits segregated in the 101-14 x Trayshed population as quantitative traits controlled by multiple genes. Additionally, we developed a novel method that utilized digital imaging to obtain fruit set ratios for individual grape clusters as a means to quantify fanleaf tolerance in vines. Utilizing this method, we identified six progeny from this hybrid population (07107-012, 07107-043, 07107-091, 07107-112, 07107-135, and 07107-148) that were able to control fanleaf degeneration as well as O39-16 and exhibited desirable viticultural traits. This study is the first to have extensively quantified fruit set as a method to determine the degree of fanleaf degeneration tolerance induced by a rootstock to GFLV-infected vines. In addition to exploring the 101-14 x Trayshed breeding population, we also studied five recently-released rootstocks with broad and durable nematode resistance. Although these five rootstocks (GRN-1, GRN-2, GRN-3, GRN-4, and GRN-5) possess resistance to X. index, their ability to induce fanleaf tolerance (such as the case in O39-16) is unknown and the overall performance of these rootstocks on a GFLV-infested site has not been thoroughly evaluated. This manuscript presents the first evaluation of these new rootstocks on a fanleaf site in the San Joaquin Valley in California. After ten years of growth, all five GRN rootstocks performed comparably to O39-16. The low GFLV infection rate in vines grafted on GRN-1 shows the potential for GRN-1 to become a viable alternative to O39-16. Although further studies are needed to evaluate the long-term performance of these rootstocks, these initial results are promising.