Browsing by Author "Bester, Andries Johannes"

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    Factors influencing the success or failure of graft unions in grapevine.
    (Stellenbosch : Stellenbosch University, 2020-04) Bester, Andries Johannes; Schmeisser, M.; Deloire, Alan; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture.
    ENGLISH ABSTRACT: This study aimed to compare graft quality evaluation methods for their accuracy and efficiency to detect early incompatibility in grapevines in South African nurseries. The study also looked at the effect of current nursery practices on graft success, considering the noted increase in mortality rates of field planted vines in the South African viticulture industry. Paper 1: Two methods for the objective assessment of graft union quality, namely micro- computed tomography (microCT) and stereo microscopy with longitudinal sectioning, were compared, in terms of their accuracy, rapidity and ease of implementation. Longitudinal graft sections were also compared to hydraulic conductivity measurements. A close match of graft union quality ratings obtained from 2D microCT and stereo microscopy images was observed. MicroCT imaging of consecutive segments, taken at 1µm intervals through the entire graft union, rendered a more accurate 3D depiction of the total xylem connections, thereby providing a true representation of graft quality. Graft quality evaluations from a single plane longitudinal section are subject to over- or underestimations of vascular connectivity in comparison to a 3D model. This is confirmed by the lack of correlation between 2D graft quality measurements and hydraulic conductance. MicroCT is too expensive for large-scale screening of graft incompatibility, therefore 2D stereo microscopy is recommended. Should potential incompatibility symptoms be noted during 2D screening; a detailed 3D microCT study may be used to obtain conclusive evidence. Hydraulic conductivity is a relatively affordable method for detection of potential incompatibility, therefore the correlation between hydraulic conductivity and graft incompatibility should be investigated further. Paper 2 and 3: The effect of standard grapevine nursery practices on graft quality were studied two separate trials. The first trial (Paper 2) investigated the effect of different scion/rootstock combinations; different wax types; and the addition of 2,5-DCBA on the graft quality for the first 33 days after grafting by means of histology and stereo microscopy. Only the rootstock choice showed a clear impact on the development of graft unions. Grafting wax, irrespective of the types used, was shown to infuse the graft union, significantly disrupting graft formation. All treatments displayed a good bonding strength and histologically showed similar tissue development, except for Pinot Gris / Pinot Gris homografts, which demonstrated poor callusing and vascular tissue differentiation across the graft unions. Thus, inherent genetic propensity to form successful graft unions appears to be the most important factor for determining graft success in this study. The second trial (Paper 3) also investigated the effect of nursery practices (different wax types; the addition of 2,5-DCBA to wax, and callusing duration) on the graft quality for Pinot Gris grafted onto commercially important rootstocks. Grafts with the US 8-7 rootstock displayed greater success than Ruggeri 140, after one year of growth, suggesting rootstock choice to be the biggest consideration when grafting with Pinot Gris.