Browsing by Author "Sieberhagen, Madeleine"

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    Determining the resistance or susceptibility of grapevine rootstocks used in South Africa towards fungal trunk disease pathogens
    (Stellenbosch : Stellenbosch University, 2017-03) Sieberhagen, Madeleine; Halleen, Francois; Mostert, Lizel; Stellenbosch University. Faculty of Agrisciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: Grapevine fungal trunk diseases are responsible for reduced grapevine production world-wide. Trunk diseases are caused by endophytic, xylem-inhabiting pathogens associated with a wide range of symptoms. Currently the largest concern is the presence of pathogens in rootstock mother vines and propagation material. Very little information is available on the susceptibility of grapevine rootstocks used in the South African industry. The susceptibility of the most popular grapevine rootstocks in South Africa to Petri disease (Phaeomoniella chlamydospora, Phaeoacremonium minimum, Pm. parasiticum, Cadophora luteo-olivacea and Pleurostoma richardsiae), black-foot disease (Ilyonectria liriodendri, Dactylonectria macrodidyma, Campylocarpon fasciculare and C. pseudofasciculare) and Botryosphaeria dieback (Neofusicoccum australe and N. parvum) pathogens were evaluated. Firstly, virulence screenings were conducted by plug inoculating dormant Paulsen 1103 cuttings with 10 isolates of each pathogen. The two most virulent isolates were identified according to lesion lengths that developed after six weeks of incubation at 22 °C. There was significant variability between the isolates of all the pathogens, except Pleurostoma richardsiae. Dormant rootstock cuttings of Ramsey, Ruggeri 140, Paulsen 1103, Richter 99, Richter 110, US 8-7, 101-14 Mgt and SO4 were vacuum inoculated with conidial suspensions (1 x 106 conidia mL) of the two most virulent isolates of each pathogen. After eight months in nursery fields, the dormant plants were uprooted and assessed for un-dried shoot and root weight. The percentage disease severity was determined by cutting the plant 10 cm from the base and assessing the internal vascular discolouration. 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 Mg the largest. The quality of plant material is crucial in the success and longevity of newly established vineyards. By combining existing knowledge of disease management in the propagation process with knowledge obtained from this research, can the quality of plant material be optimized to ensure the sustainability of the South African grapevine industry.