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 cultivar 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, -7 and -4 strains 5 and 9 (one isolate each). Also, pairwise combinations of GLRaVs -1, -2, -3, -4 strain 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.

In 2015, the vine performances were evaluated by measuring the trunk diameter, cane length, pruning weight, yield and fruit composition. Trunk diameter analysis showed that the plants propagated on rootstocks Freedom and 3309C were the most affected ones. For cane length measurements, the most affected rootstocks were 5BB and 3309C and the least affected ones were AXR and St. George 18. However, the two different isolates of GLRaV-2 (LR 103 and LR 119) had significant impact on cane length of plants propagated on rootstocks 101-14, 3309C, 5BB and Freedom. For pruning weight, the most affected rootstocks were 5BB, Freedom and 101-14 and the least affected were 110R, St. George 15 and St. George 18. For the yield the most affected rootstocks were 3309C and Freedom and no significant yield reduction were found on plant propagated on 110R, St. George 15 and St. George 18. Cluster count analysis was similar to total yield. Average berry weight analysis showed that the most affected rootstocks were 420A, Freedom and 5BB and no significant differences were found on rootstocks 110R, AXR, St. George 15 and St. George 18. The analysis also showed that both GLRaV-2 isolates (LR103 and LR119) in general have been more severely affecting the plants on panel of rootstocks.