WINETECH Technical Yearbook 2020

It remains unknown whether: 1) GLRaV-3 is generally present in Richter 99 when the scion is infected, but at titers sub-detect- able to the detection methods employed, 2) GLRaV-3 has an uneven distribution in Richter 99 resulting in poor detection, 3) if GLRaV-3 has variants or genome components selected for in the Richter 99 rootstock which are less efficiently detected by PCR, or 4) the V. berlandieri X V. rupes- tris interspecific hybrid produces inhibitors to the PCR reaction. Also, it is not known in instances where GLRaV-3 is at detect- able levels, whether it is a) either due to “resistance breaking” variants capable of partially overcoming the Richter 99 de- fence mechanisms, or b) genetic variation amongst various clones Richter 99, some of which may allow replication of GLRaV-3. Any of these possibilities or combinations of them may account for the differences in GLRaV-3 status observed amongst sci- on and Richter 99 rootstocks and further studies to assess these possibilities need to be conducted.

differences in GLRaV-3 infection status were observed between the Vitis vinifera scion and the V. berlandieri X V. rupestris Richter 99 rootstocks of individual leafroll disease-infected vines (figure 1). The scion material of all 69 vines analysed contained GLRaV-3. GLRaV-3 could not be detected by RT-PCR in 66% of Richter 99 rootstocks from these, despite the fact that the cor- responding scions were positive for GL- RaV-3, displayed obvious GLD symptoms, and were a constant source of GLRaV-3 inoculum to the rootstock. Of the 23 Richter 99 samples that did contain GLRaV-3, only five yielded levels of amplicons in end-point PCR reactions comparable to that of the scions, while the remainder all yielded considerably less, yielding only very faint bands in agarose electrophoresis gels. Using next generation sequencing, we found only minor differences in the GL - RaV-3 variant composition of Richter 99 and corresponding scions in those instanc- es where GLRaV-3 was found in the root- stock. We also demonstrated that Richter 99 can also be infected with GLRaV-1, GL- RaV-2, grapevine virus A (GVA), grapevine virus B (GVB) and grapevine rupestris stem pitting-associated virus (GRSPaV).

For more information, contact Gerhard Pietersen at gpietersen@sun.ac.za.

FIGURE 1. Results of a RT-PCR reaction for GLRaV-3 on electrophoresis gel. The presence of the white bands above is clear in the scion of any vine, but not in its corresponding Richter 99 rootstock (each red rectangle represents a single vine).

WINETECH TECHNICAL YEARBOOK 2020 15