Determination of the resistance / susceptibility of grapevine rootstocks towards grapevine trunk disease pathogens
Objectives & Rationale
The aim of the study was to test the most important rootstocks currently available and used in South Africa against a range of economically important trunk disease pathogens.
Virulence tests were conducted with isolates of the most important trunk disease pathogens to select the most virulent isolates to be used in field trials. Ramsey, Richter 110, Richter 99, US 8-7, 101-14 Mgt, Paulsen 1103, Ruggeri 140 and SO4, the rootstocks available and used for grafting in commercial nurseries at the start of the project, were inoculated with these pathogens and planted in two field nurseries in October 2014 and 2015 (22 080 vines/year). The vines were removed from the nursery fields during June/July 2015 and 2016 and evaluated for root- and shoot mass, vascular discolorations / lesion development and subjected to fungal isolations. The
determination of mean xylem vessel diameter of the eight rootstocks and possible mechanisms of resistance were also investigated.
All of the rootstocks inoculated with Petri disease, black-foot disease and Botryosphaeria dieback pathogens showed a significant reduction in root mass, shoot mass and a significant increase in percentage disease severity. None of the rootstocks were completely resistant to fungal trunk disease pathogens, but differential levels of tolerance did exist. The reduction in shoot and root mass did not show consistent results between the rootstock cultivars, but rootstocks 101-14 Mgt and SO4 were the most susceptible and Ramsey the most tolerant, in terms of percentage disease severity. In an attempt to explain the variable tolerance of rootstocks to infection, suberin production and xylem morphology was investigated. Transversal cuts of Ramsey and 101-14 Mgt were made and stained using two suberin staining techniques. The important role suberin plays in the
compartmentalization of a pathogen in grapevine wood was confirmed. Suberin was located around vessels filled with tyloses, tyloses itself was also suberized, ray parenchyma cells and cells located on the growth boundary ring. The suberized zones form impermeable barriers that restrict pathogen spread to uninfected and newly developed vascular tissue. The mean vessel diameters were determined for each cultivar using 40 μm thick transversal cuts stained with toluidine O. A strong correlation between mean vessel diameter and rootstock
tolerance to fungal trunk pathogens was established. Ramsey had the smallest mean vessel diameter and 101-14 Mgt the largest.
The results show that pathogens associated with the three major fungal trunk diseases (Petri disease, black-foot disease and Botryosphaeria dieback and cankers) all have the ability to reduce percentage root mass, shoot mass and increase percentage disease severity and that none of the rootstock cultivars were completely resistant. The quality of plant material is crucial in the success and longevity of newly established vineyards. Combining existing knowledge of disease management in the propagation process with the knowledge obtained from this research in terms of disease susceptibility will assist in optimizing plant material quality and the sustainability of the South African grapevine industry. This study has shown that Ramsey could form a useful part of an integrated management strategy for fungal trunk disease pathogens.