Browsing by Author "Meyer, Andre Harold"

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    Arbuscular mycorrhizal root colonisation and the subsequent host plant response in young grapevines in a South African commercial vineyard
    (Stellenbosch : Stellenbosch University, 2003-03) Meyer, Andre Harold; Botha, Alfred; Valentine, A. J.; Archer, E.; Stellenbosch University. Faculty of Science. Dept. of Microbiology.
    ENGLISH SUMMARY: Arbuscular mycorrhizal (AM) fungi facilitate the uptake of nutrients, improve growth and alleviate drought stress in grapevines. Consequently, AM fungal root colonisation contributes to the optimum performance of grapevines. It is for this reason that young grapevines are sometimes inoculated with commercial AM fungal strains to reduce environmental stresses during transplant. In the past, soil fumigation has often been considered as a prerequisite for soil conditioning with commercial AM fungal strains. However, grape growers opting to inoculate with these fungal strains will have to do so in unfumigated soils, since the use of fumigants in South African agricultural soils is currently being phased out. Since little is known about the nature and scope of indigenous AM fungi that may be present in SA vineyard soils, it is difficult to predict the grapevine's response to artificial inoculation in soils already containing adequate concentrations of these fungi. In the first part of the study, commercially available AM inocula were tested under field conditions that would prevail on a typical farm. This entailed measuring vine growth, nutrition, drought stress resistance and percentage root colonisation, over two consecutive seasons, from the onset of planting new commercial grapevines. The field trial carried out at Groenland, a commercial farm in the Stellenbosch Region. Merlot grafted onto 101-14 Mgt, 110 Richter (110 R) and 99 Richter (99 R), was planted in December 1998. These rootstocks were selected to accommodate different soil forms: 101-14 Mgt and 110 R on a Westleigh soil form, which was ridged and 99 R on an unridged Fernwood soil form. Vine roots were inoculated during planting with different AM inocula, i.e. Biocult®, Vaminoc" and Glomus sp. 1054. One treatment was left uninoculated and treated with a combination of the fungicides Benlate" (active ingredient: benomyl) and Rovral Flo® (active ingredient: iprodione). The control received neither fungicides nor inocula. Microscopic examination of the vine roots revealed that, apart from a significantly higher level of root colonisation observed in Biocult-treated 99 R vines during the first season, the level of AM root colonisation was similar in both the uninoculated (control) and inoculated vines. Infected control vines indicated that indigenous AM fungi were present in the vineyard soil. This level ranged between 40% and 85%, and 70% and 90% in the first and second season, respectively. Apart from the significant growth improvement observed in 110 R vines inoculated with Glomus sp. 1054 during the first season, no growth improvement was observed for the other rootstocks or treatments. Furthermore, generally no alleviation of water stress and nutritional benefits could be detected for both the seasons. Despite this, less than 1% dieback was recorded for the vines. In the second part of the study, additional information on the diversity of indigenous AM fungal species was obtained, which included the quantification and identification of these fungi present in the soil. The AM fungal spore numbers in the vineyard soil ranged from 1000 to 3779 spores/100 g dry soil. The results confirmed that the majority of AM fungal species found in the soil was not part of the commercial inocula, but originated either from the vineyard and/or the nursery where the vines were obtained. The uncovered AM fungal species belong to the genera Acaulospora, Gigaspora, Glomus, Sclerocystis and Scutellospora. This is similar to the AM fungal genera recorded in vineyards by other workers. To the best of our knowledge, this study provided the first documented evidence on the diversity of indigenous AM fungi present in SA vineyard soils. Although it may be preliminary in nature, the results clearly showed that a wide diversity and abundance of indigenous AM fungal populations may occur in a typical SA vineyard. Depending on the superiority and possible masking effects on the part of the indigenous AM fungal populations, positive responses to inoculation with commercial AM fungal strains in grapevines grown in such vineyard soils may consequently be unlikely. Thus, before reconditioning of vineyard soils with these fungi can commence, it is essential for farmers to first assess the mycorrhizal status of their soils and nursery vines. Since the majority of SA farmers are not yet familiar with inoculation practices and are still unacquainted with the mycorrhizal status of their soils, the findings from this study could be of great benefit to particularly wine grape growers opting to inoculate with commercial AM fungal strains on a large-scale.