Browsing by Author "Shannon, Strydom"

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    Pruning wound protection of rootstock mother vines
    (Stellenbosch : Stellenbosch University, 2019-12) Shannon, Strydom; Halleen, Francois; Mostert, Lizel; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: One of the factors contributing to the decline in grapevine production across the world is diseases commonly known as grapevine trunk diseases (GTD’s). These diseases are typically triggered by xylem-inhabiting pathogens which causes a variety of symptoms and an overall reduction in grapevine production worldwide. To date there is a great concern on the manifestation of these fungal trunk pathogens in rootstock mother vines and their accompanied propagation material. However, there are limited and inadequate information available of the vulnerability of South African rootstocks to these pathogens. Therefore, the objectives of this study were to determine the temporal susceptibility of pruning wounds against Phaeomoniella chlamydospora and to assess the protection of pruning wounds of commercial rootstocks with fungicides and biological agents against Pa. chlamydospora in South Africa. Pruning wound susceptibility was determined in certified mother vine blocks with two rootstock varieties most commonly grafted in South Africa, namely Ramsey and US 8-7. This was achieved by inoculations of spore suspensions directly after pruning, as well as 1, 7, 21, and 42 days after pruning. The trials were evaluated after nine months by determining Pa. chlamydospora incidences from inoculated wounds. A general decline in Pa. chlamydospora incidence was recorded up to 42 days after pruning with wounds challenged 24 hours after pruning being the most susceptible. In vitro mycelial and germination inhibition studies were conducted in order to determine whether selected chemicals which include chemical fungicide groups such as; benzimidazole, triazole, pyridine-carboxamide and strobilurin are effective against Pa. chlamydospora. Mycelial growth inhibition was assessed for six fungicides. The results obtained were used to determine the EC50 values for Pa. chlamydospora (LM310) and it was observed that all the tested fungicides were effective against Pa. chlamydospora. Furthermore, cell viability was assessed using three of the fungicides known to be effective against germination inhibition via a microtiter assay. EC50 values for Pa. chlamydospora was determined and found to be effective. Chemical fungicides that shown mycelial and germination inhibition, including other control agents, were evaluated in a detached shoot assay by pruning, treating and challenging these shoots (Ramsey, US 8-7 and 101-14 Mgt). After a 4-week incubation period, Pa. chlamydospora incidence was recorded and it was found that several of the chemicals were highly effective in lowering Pa. chlamydospora incidence, hence protecting these wounds from infection. Consequently, these control agents were further evaluated in field trials conducted in a rootstock mother vine nursery. Pruning wounds were treated with the selected control agents immediately after pruning and challenged with a Pa. chlamydospora spore suspension at 1 and 7 days after pruning. The trials were evaluated after nine months by determining the incidence of Pa. chlamydospora from inoculated wounds. The study concludes that integrated control treatments where the biological control agent Trichoderma atroviride are applied together with benzimidazole fungicides such as carbendazim or thiophanate-methyl showed to be the most effective in reducing Pa. chlamydospora incidence in rootstock mother vine pruning wounds. Results from this study have provided new information regarding the protection of pruning wounds via the integration of biological and chemical control techniques applied to rootstock pruning wounds at the most susceptible time period. Considering the results obtained in this study it is suggested to prune as early in the dormant season as possible, before the end of May, and the best fungicide chemical group to provide protection for rootstock pruning wounds was benzimidazole in combination with T. atroviride applied within 24 hours after pruning occurred. However, further research would be necessary to develop a better understanding to produce protocols and commercial products. The application of these fungicide together with Trichoderma spp. in the field holds promise to improve control but would require further trials for possible commercialisation. For successful grapevine propagation and subsequent longevity of new established vineyards, the quality of the planted material plays a crucial role. Therefore, integrating existing information of GTD management in the grapevine propagation process with the knowledge attained from this thesis, can the quality of grapevine material be improved ensuring the success of the South African grape industry.