An investigation of soilborne fungi associated with roots and crowns of nursery grapevines

[摘要] ENGLISH ABSTRACT:Soilborne diseases of grapevines represent a complex problem with limitedinformation available, both locally and internationally. Previous research in South Africaindicated that Phytophthora and Pythium spp. were the most widespread and devastatingpathogens in grapevine nurseries and vineyards in the Western Cape province. The localgrapevine industry is currently expanding; new cultivars, methods and agricultural chemicalsare being used which can affect soilborne pathogens. It has therefore become necessary to reassessthe status of soilborne pathogens in nurseries, since information in this regard is crucialfor the development of disease management practices for the expanding local grapevineindustry.Soilborne fungal genera associated with roots and crowns of declining nurserygrapevines were assessed in surveys conducted at three different grapevine nurseries in theWestern Cape province. Cylindrocarpon, Fusarium, Pythium, and Rhizoctonia spp. wereconsistently isolated from roots and crowns of declining nursery grapevines.Cylindrocladiella spp. and Phytophthora cinnamomi were infrequently isolated from diseasedroots, crowns and soil whereas Pythium spp. were abundant in most of the soils. Resultssuggest that the status of soilborne fungal pathogens in grapevine nurseries in the WesternCape province has changed over the last 30 years.The DNA phylogeny and pathogenicity of the isolates of Cylindrocladiella weredetermined. Four species of Cylindrocladiella occur on grapevines in South Africa, namelyC. lageniformis, C. parva, C. peruviana, as well as a new species, described in this study asC. viticola, which forms part of the C. infestans species complex. Pathogenicity trials wereinconclusive.Ten Fusarium spp. were isolated from roots and crowns of declining nurserygrapevines, namely F. acuminatum, F. anthophilum, F. chlamydosporum, F. equiseti, F.nygamai, F. oxysporum, F. proliferatum, F. scirpi, F. semitectum and F. solani. Thedominant species was F. oxysporum, followed by F. proliferatum and F. solani. Inpathogenicity trials F. oxysporum and F. solani significantly reduced root volume, root drymass, length of new shoots, stem diameter and number of leaves, but increased the percentageof chlorotic leaves and root rot severity. Fusarium proliferatum also caused a significantreduction in new shoot growth, number of leaves and increased root rot severity compared tothe 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 aspathogens of grapevines in South Africa, and the first report of F. proliferatum as a pathogenof 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 mildewchemicals 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 nurserygrapevines was evaluated in a glasshouse. There was very little discernable effect of thechemicals tested relative to the control plants for the parameters measured and it wasconcluded that the inoculation technique needed refinement. However, plants treated withphosphorous acid tended to be taller and have more leaves, greater stem diameter and rootvolume than controls or plants treated with the other chemicals. The data obtained in thisstudy are not conclusive, but indicated certain trends that more glasshouse trials and fieldtrials would resolve.Results presented in this thesis indicate that a major shift has occurred in the status ofsoilborne fungi associated with roots and crowns of grapevines in nurseries in the WesternCape since the 1970s when Phytophthora and Pythium were predominant. The prevalenceand role of soilborne fungi need to be determined so that new appropriate diseasemanagement strategies can be developed to limit losses in grapevine nurseries and ensure thesustainable production of healthy plants for the grapevine industry.

[发布机构] Stellenbosch University