| Comments |
A4 is a low vigor grapevine rootstock germplasm. The A4 germplasm is an autotetraploid derived from the closely related VR O39-16 grapevine rootstock. Autotetraploid grapevine rootstocks uniformly demonstrate reduced vigor in the scions grafted to them when compared to the diploid rootstock from which they are created.
A4 germplasm was developed from the VR O39-16 grapevine rootstock. VR O39-16 grapevine rootstock is described in the US Patent Plant 6,166, which was issued on May 3, 1988. At the time in 2011 that the research to develop A4 grapevine rootstock began the patent protection of VR O39-16 had expired. Therefore, A4 germplasm is not encumbered by the expired patent that applied to VR O39-16.
A4 rootstock was invented by selecting branches growing on a VR O39-16 grapevine that was cultivated in a USDA ARS research greenhouse in Geneva, New York. The selected branches were chosen because they showed thicker leaves and shorter internodes than is typical for VR O39-16 rootstock. Thicker leaves and shorter internodes are indications of tetraploidy in grapevines. Tissue from the selected branches was tested by the Oregon State University Seed Lab to determine the ploidy of each branch. Several branches were shown by flow cytometry to be autotetraploids. The autotetraploid branches were chosen for propagation. In several cycles of clonal propagation by vegetatively propagated cuttings the autotetraploid A4 rootstock has shown to be stable and distinct. In all cases the A4 germplasm has shorter internodes and thicker leaves than VR O39-16 rootstock and the two varieties can be easily distinguished. This shows that the A4 rootstock is a distinct new variety.
A4 germplasm has not been tested in the field. The closest variety is VR O39-16, which is a diploid, 2n = 2x = 39. A4 is an autotetraploid derivative of VR O39-16 and the anticipated chromosome number is 2n = 4x = 78.
Although the rootstock VR O39-16 provides protection against both the dagger nematode Xiphinema index and fanleaf degeneration disease, the VR O39-16 rootstock is not favored by all growers. VR O39-16 rootstock is very vigorous and causes the scions grafted on it to grow vigorously, producing large vines with high pruning weights. These large vines can be subject to the problems associated with excessively vigorous plants, such as dense canopies and poor fruit quality. A lower vigor rootstock that provides protection against both the dagger nematode Xiphinema index and fanleaf degeneration disease would be useful for grape growers because such a rootstock would allow growers to grow vines with appropriate vigor and canopy size.
Since other autotetraploid grape rootstock selections show low vigor and the same pest resistance as the diploid varieties from which they are derived, A4 low vigor grapevine germplasm also may show desirable low vigor while providing protection against the dagger nematode Xiphinema index and fanleaf degeneration disease. |
