Evaluation of criteria to assist the selection of good quality grafted grapevines prior to their commercialisation

The production of grafted grapevine plant material is a complex process with many steps running over at least one year, from grafting to final sorting in nurseries. To reach the market in France, grafted grapevines must meet three criteria by law: resistance to a manual graft union test (or thumb test), a minimum number of three roots and a woody, lignified stem which has grown from the bud of the scion wood of at least 2 cm long. This study aimed to evaluate the possibility of using visual criteria to select good quality grafted grapevines, without the need to do the thumb test because the thumb test is manual and therefore very subjective; the test depends on the strength applied by the person who realises it. This study was done on 22 scion/rootstock combinations with different degrees of grafting success, i.e., producing different proportions of marketable plants after one year in the nursery. The three legal criteria currently used to select marketable grafted grapevines in France as well as other external and measurable criteria such as the length of lignified stem and diameter, the number of thin and thick roots, and rootstock wood diameter were measured on the 22 scion/rootstock combinations. Variation in the values for these different criteria was observed and correlations between the criteria and the number of marketable plants were studied. This data was then analysed to determine which visible criteria contribute most to identifying marketable grafts. The percentage of marketable grafts was most strongly correlated with the thumb test and positively correlated with the length of the lignified stem. The variables with the highest predictive effect for identifying marketable plants (other than the thumb test) were the number of large roots and the length of the lignified stem. The possibility of using visible criteria to screen for good quality grafted plants is discussed, but no single, or combination of criteria, was sufficiently strongly correlated with the percentage of marketable plants to replace the thumb test.