Implication of secondary viral infections on grafting success rated in nurseries

Abstract

Context and purpose of the study

Grapevine grafting is a complex process that since the establishment of phylloxera has become mandatory for grapevine. Grafting success in grapevine nurseries considerably varies among years and batches with most variety/rootstock combinations reach a high success rate (between 75% and 90%), but some combinations show lower success rates of around 40-50%. The causes of this variation are unknown, although biotic stresses like those caused by some viral infections have been demonstrated to affect the process. European certification schemes for the vegetative propagation of the vine include five major viruses (Arabis mosaic virus, Grapevine Fanleaf Virus, Grapevine Fleck Virus, and Grapevine-associated Leafroll Virus 1 and 3). However, other viral infections are ubiquitous in plant material and may affect the grafting process. The present study aimed to assess the presence of these other ‘secondary viruses’, and their influence on the grafting process.

Material and methods

Plant material was collected from commercial mother plant fields located in Larraga (Navarra, Spain). The presence of Grapevine virus A (GVA), Grapevine virus B (GVB), Grapevine-associated Leafroll Virus 2 Pinot noir (GLRaV-2 PN), and Rupestris Stem Pitting-associated Virus (RSPaV) was assessed by RT-PCR to determine the most common secondary viruses in the plant material. Mother fields for 7 scion (Tempranillo, Chardonnay, Airen, Macabeo, Sauvignon blanc, Garnacha, and Mazuelo) and 5 rootstocks (110 Richter, Rafa García 8 (RG8), Rafa García 9 (RG9), Millardet et Grasset 41 B, and 140 Ruggeri) were tested. Considering the prevalence observed for the different viruses, the influence of the virus on grafting success was tested for Tempranillo grafted onto 110R, since for this material it was possible to use combinations of virus-free and virus-infected scion and rootstock wood. The relation between the viral presence and the success rate was tested by Pearson chi-square. 

Results

RSPaV had the highest incidence (55%), and a difference was observed between varieties and rootstocks (85% vs. 25%). GLRaV-2 was detected in 1.3% of the samples and GVA and GVB were not detected. Three sanitary conditions were established based on the detected infections: I) virus-free, II) RSPaV, and III) RSPaV + GLRaV-2. Tempranillo samples had conditions II and III, while 110R had conditions I and II. A minimum of 24 plants were grafted for each combination (variety/rootstock): RSPaV/virus-free, RSPaV/RSPaV, GLRaV2+RSPaV/virus-free, and GLRaV2+RSPaV/RSPaV. The success rate for the RSPaV/virus-free combination was 72%, for RSPaV/RSPaV combination was 54%, GLRaV2+RSPaV/virus-free was 36%, and for GLRaV2+RSPaV/RSPaV was 25%. The Chi-square test determined a significant relationship between the success rate and the sanitary conditions of the graft, which statistical results suggested was determined by the sanitary condition of the variety and not in the rootstock. The residual analysis of the Chi-square test, which related expected success with sanitary status, had the most positive association for RSPaV/virus-free graft combination while the most negative was for GLRaV2+ RSPaV / RSPaV. The results of the study point to widespread secondary viruses such as GLRaV-2 and RSPaV that could be involved in graft incompatibility.