Characterization of Grapevine Latent Viruses
This is ongoing research, which has already yielded important benefits to industry in terms of improved field practices and testing capabilities. The preliminary data from the first rootstock field trial demonstrates that viruses can have a significant detrimental effect on survival and growth of grafted plants at a young age. The effect is dependent on the particular rootstock and virus combination. This year data from the first of the large rootstock trials is available for analysis. Virus – infected grafts on the rootstocks Freedom, Kober 5BB, and Teleki 5C had lower survival and total growth rate than healthy grafts; the rootstock St George was not affected by any virus treatment in growth or survival (Table 2) Between the healthy and LR 101 treatments, which is a single virus infection, there were only small differences in growth and survival in all four rootstocks tested. The multiple infections of LR 102 and LR 109 treatments had severe and dramatic effects on growth of Freedom, and Kober 5BB, but not St. George. In Teleki 5C, LR 109 decreased growth, but LR 102 did not. Tests of the same and additional rootstocks will be providing more comprehensive data in the summer and fall of 1998. We found an association between samples collected from vineyards with replant failure and infection with GCBaV and/or GLRaV-2. However, more inoculations of these viruses to rootstocks need to be done to demonstrate that viruses were the cause of replant failure. Almost all of the samples from vineyards affected by replant failure were infected with these two viruses. Half of the samples are infected with these two viruses only. The other half were infected with GCBaV and/or GLRaV-2 and other virus types including: GLRaV -1, -3, and -4, GFLV and Grapevine virus A, which is not often detected in California. Most samples were infected with two or three viruses. Woody index testing to date found a severe stem grooving disorder known as Kober stem grooving (KSG) for the first time in the western hemisphere. Early experiments associate this disease with grapevine virus A, a recently described grapevine virus for which antisera only recently became available. A new ELISA test first used in our lab this summer supported our results. We will be PCR testing the samples with all available primers to further characterize the virus status of the latent virus collection. A field trial of approximately 900 Freedom rootstock cuttings inoculated with 23 virus sources has already provided strong evidence that Freedom is highly susceptible to many latent viruses. These large trials with Freedom have been initiated as both a demonstraton of the potential effects of latent viruses to growers using this important rootstock and to provide scientific information about the effects of individual viruses on a sensitive indicator. Pruning weight and shoot length of the longest shoot, was inversely correlated with percent of plants that had leaf chlorosis, and vein necrosis (Fig. 5).