Browsing by Author "Van Coller, Gerhardus J. (Gerhardus Johannes)"

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    An investigation of soilborne fungi associated with roots and crowns of nursery grapevines
    (Stellenbosch : Stellenbosch University, 2004-03) Van Coller, Gerhardus J. (Gerhardus Johannes); Denman, S.; Lamprecht, Sandra, C. ; Crous, P. W.; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: Soilborne diseases of grapevines represent a complex problem with limited information available, both locally and internationally. Previous research in South Africa indicated that Phytophthora and Pythium spp. were the most widespread and devastating pathogens in grapevine nurseries and vineyards in the Western Cape province. The local grapevine industry is currently expanding; new cultivars, methods and agricultural chemicals are being used which can affect soilborne pathogens. It has therefore become necessary to reassess the status of soilborne pathogens in nurseries, since information in this regard is crucial for the development of disease management practices for the expanding local grapevine industry. Soilborne fungal genera associated with roots and crowns of declining nursery grapevines were assessed in surveys conducted at three different grapevine nurseries in the Western Cape province. Cylindrocarpon, Fusarium, Pythium, and Rhizoctonia spp. were consistently isolated from roots and crowns of declining nursery grapevines. Cylindrocladiella spp. and Phytophthora cinnamomi were infrequently isolated from diseased roots, crowns and soil whereas Pythium spp. were abundant in most of the soils. Results suggest that the status of soilborne fungal pathogens in grapevine nurseries in the Western Cape province has changed over the last 30 years. The DNA phylogeny and pathogenicity of the isolates of Cylindrocladiella were determined. Four species of Cylindrocladiella occur on grapevines in South Africa, namely C. lageniformis, C. parva, C. peruviana, as well as a new species, described in this study as C. viticola, which forms part of the C. infestans species complex. Pathogenicity trials were inconclusive. Ten Fusarium spp. were isolated from roots and crowns of declining nursery grapevines, namely F. acuminatum, F. anthophilum, F. chlamydosporum, F. equiseti, F. nygamai, F. oxysporum, F. proliferatum, F. scirpi, F. semitectum and F. solani. The dominant species was F. oxysporum, followed by F. proliferatum and F. solani. In pathogenicity trials F. oxysporum and F. solani significantly reduced root volume, root dry mass, length of new shoots, stem diameter and number of leaves, but increased the percentage of chlorotic leaves and root rot severity. Fusarium proliferatum also caused a significant reduction in new shoot growth, number of leaves and increased root rot severity compared to the controls. Fusarium so/ani seems to be more virulent than F. oxysporum, followed by F. pro/iferatum. This is the first report of F. oxysporum, F. pro/iferatum and F. so/ani as pathogens of grapevines in South Africa, and the first report of F. proliferatum as a pathogen of grapevines in the world. Phytophthora cinnamomi was isolated at low frequencies from declined grapevines, although present in the rhizosphere soil. It is possible that the extensive use of downy mildew chemicals in grapevine nurseries may protect grapevines from infection by P. cinnamomi. The effect of chemicals used to combat downy mildew on Phytophthora root rot of nursery grapevines was evaluated in a glasshouse. There was very little discernable effect of the chemicals tested relative to the control plants for the parameters measured and it was concluded that the inoculation technique needed refinement. However, plants treated with phosphorous acid tended to be taller and have more leaves, greater stem diameter and root volume than controls or plants treated with the other chemicals. The data obtained in this study are not conclusive, but indicated certain trends that more glasshouse trials and field trials would resolve. Results presented in this thesis indicate that a major shift has occurred in the status of soilborne fungi associated with roots and crowns of grapevines in nurseries in the Western Cape since the 1970s when Phytophthora and Pythium were predominant. The prevalence and role of soilborne fungi need to be determined so that new appropriate disease management strategies can be developed to limit losses in grapevine nurseries and ensure the sustainable production of healthy plants for the grapevine industry.