Grapevine Leafroll Disease: a Detailed, Broad-scope Study of Host and Pathogen Effects

Grapevine leafroll disease causes non-uniform maturation of fruit in Vitis vinifera, including poor color development in red grape varieties. The disease causes losses of as much as 20-40%, with delays of 3 weeks to a month in fruit maturation. To date 5 different viruses, namely Grapevine leafroll associated virus (GLRaV) types -1 through -4, and -7, have been conclusively shown to be associated with leafroll disease. In the case of GLRaV-4, several distinct leafroll disease-associated virus strains have been identified within the virus species. This project was planned as a detailed study of the effects of these viruses on variety Cabernet Franc grapevines. This grapevine produces a readily scored foliar response to leafroll virus infection. The analysis includes challenges with each agromonically significant GLRaV species, including types -1 and -2 (2 isolates each), -3 (3 isolates), -4, -5, -7 and -9 (one isolate each). Also, pairwise combinations of GLRaVs -1, -2, -3, -5 and -7 are being tested. The test vines are grafted onto a broad selection of different rootstock varieties. Nine different rootstocks are involved in the test, including AXR #1, Mgt 101-14, 110R, 3309C, 5BB, 420A, Freedom, St. George 15 and St. George 18. 15 replicates for each treatment are divided into three separate blocks each (5 replicate per treatment per block). The project has thus-far revealed a spectrum of differences in infection symptoms attributable to the different virus species, and to different combinations of these viruses and the grapevine varieties they infected. For example, it was observed that leaf symptoms produced by GLRaV-3 were more severe than those produced by GLRaV-4. In another example, it was found that GLRaV-2 induced more severe reactions on vines propagated specifically on rootstocks Freedom and 5BB. Those test vines exhibited red leaf symptoms, short internodes, and a near-lethal decline in vigor. Detailed analysis of these and other specific aspects of leafroll disease are on-going. Data collected from the experiment in 2011 revealed one particularly severe infective combination. Virus isolate LR132 (which contained both GLRaV-1 and Grapevine virus A) produced a severe infection in Cabernet Franc plants propagated on rootstocks 420A, Freedom, 3309C and 101-14. Many of these plants died a few months after inoculation. Whether the severity is due to a particular strain of GLRaV-1 found in the LR132 isolate, or to a synergy arising from the mixture of GVA with GLRaV-1 in the inoculums is under investigation.