Cecilia B. Aguero,* Xuyyn Yang, Liang Zheng, Andy Nguyen, Howard Ferris, and M. Andrew Walker
*Department of Viticulture and Enology, UC Davis, Robert Mondavi Institute, 595 Hilgard Lane, Davis, CA 95616 (cbaguero@ucdavis.edu)

Fanleaf degeneration is a severe disease of grapevines due to its effect on fruit set. It is caused by grapevine fanleaf virus (GFLV) and vectored from root-to-root by the dagger nematode, Xiphinema index. The objective of this research was to study the ability of a closely related dagger nematode, Xiphinema americanum, to vector GFLV. X. americanum is common in California vineyard soils. Experiments were performed to determine whether this nematode can vector GFLV among grapevines. Roots of in vitro plantlets of GFLV-infected Chardonnay were inoculated with 10 X. index or X. americanum. After one week, nematodes were transferred to roots of healthy St. George plantlets for another week and then removed. Three weeks later, RNA was extracted from roots and cDNA was synthesized, followed by determination of viral load by qPCR. In a similar experiment conducted under greenhouse conditions, GFLV-infected Chardonnay plants were grown in 4-L pots filled with autoclaved sand. Each pot was inoculated with 1 mL inoculum containing 50 nematodes. Five pots were inoculated with X. index, five with X. americanum, and three with water. Three weeks later, healthy St. George plants were transplanted to each pot, so that every pot contained one GFLV-infected Chardonnay and one healthy St. George. Four months later, qPCR was used to test all St. George plants. In both experiments, only plants inoculated with X. index were clear positives for GFLV, although inoculation efficiency was low, with three positives out of five under in vitro conditions and only one positive in the greenhouse testing. The latter outcome may be due to the low number of nematodes used as inoculum in proportion to the volume of soil. These results, however, suggest that X. americanum does not transmit GFLV, or at least that it is not as effective as X. index.

Funding Support: California Grape Rootstock Research Foundation