Browsing by Author "Webber, Matthew"

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    Assessment of hot water treatment for control of grapevine trunk diseases in nurseries
    (2020-12) Webber, Matthew; Halleen, Francois; Mostert, Lizel; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: Grapevine trunk diseases (GTDs) have been identified as a major factor contributing towards the decline of grapevines. The main GTDs in South Africa, and the pathogen species commonly associated with them, include Petri disease (Phaeomoniella chlamydospora, Phaeoacremonium minimum, Phaeoacremonium parasiticum, Cadophora luteo-olivacea and Pleurostoma richardsiae), Black foot disease (Campylocarpon fasciculare, Campylocarpon pseudofasciculare, Dactylonectria macrodidyma and Ilyonectria. liriodendri), Botryosphaeria canker and dieback (Neofusicoccum australe and Neofusicoccum parvum) and Phomopsis dieback (Diaporthe ampelina). Although GTDs are commonly associated with mature vines in established vineyards, it is of particular concern that planting material supplied by grapevine nurseries may already contain infections. Unfortunately, once infected, there are limited management strategies available to control GTD infections, with chemical and biological controls largely focusing on protection of pruning wounds. Hot water treatment (HWT) has shown to be effective in controlling a wide range of fungal pathogens from grapevines. Until recently, the HWT protocol recommended to South African nurseries was 50°C for 30 min. Although this protocol is already well studied and has shown to be effective against a wide range of GTD pathogens, it is also known that it does not completely eradicate all infections. A new HWT protocol of 50°C for 45 min has, however, been recommended to South African nurseries for the control of Aster Yellows. The effect of this HWT protocol on controlling fungal GTD pathogens has, however, yet to be determined under South African conditions. The aim of this study was, therefore, to determine the effect of HWT (50°C for 45 min) on fungal pathogens associated with GTDS found in South African grapevine nurseries, firstly in vitro, followed by in artificially inoculated rootstock cuttings of Ramsey, Richter 110, US 8- 7, Paulsen 1103 and 143B Mgt. Pathogen species evaluated in this study include Pa. chlamydospora, Pm. minimum, Pm. parasiticum, Ca. luteo-olivacea, Pl. richardsiae (Petri disease), Camp. fasciculare, Camp. pseudofasciculare, I. liriodendri, D. macrodidyma (Black foot disease), N. australe, N. parvum (Botryosphaeria canker and dieback) and D. ampelina (Phomopsis dieback). In vitro results concluded HWT (50°C for 45 min) was able to cause complete inhibition of conidial germination and mycelial growth of all pathogen species associated with Black foot disease, Botryosphaeria canker and dieback and Phomopsis dieback. Pathogens associated with Petri disease were, however, more tolerant of HWT (50°C for 45 min), with Pl. richardsiae identified as the most tolerant species within the disease complex. Of the Petri disease pathogens, Pa. chlamydospora was seen to be the most sensitive to HWT, followed by Ca. luteo-olivacea. A general trend observed for all pathogen species was that conidial germination was more sensitive to HWT than mycelial growth. Additionally, the effect of HWT using water temperatures greater than 50°C were also investigated using pathogen species able to tolerate 50°C. Pleurostoma richardsiae once again showed the highest tolerance to HWT with temperatures of up to 60°C not able to achieve complete control, suggesting HWT is not an effective means for controlling this pathogen. Results from the in vivo experiments concluded, with the exception to Pl. richardsiae, HWT (50°C for 45 min) was highly effective in reducing the presence of the inoculated pathogens, completely inhibiting the recovery of Pa. chlamydospora and Ca. luteo-olivacea from HWTed material. Although HWT did not completely remove the presence of Pm. minimum and Pm. parasiticum, the incidence and severity of which these species were able to be recovered from HWTed cuttings was significantly reduced. The effect of HWT on recovery of Pl. richardsiae was less consistent. The treatment was not able to significantly reduce the incidence of cuttings from which Pl. richardsiae was recovered from, however, was able to significantly reduce the severity of the infections, although inconsistently. Even though HWT (50°C for 45 min) may not eradicate all internal infections, it still provides nurseries a convenient and effective means of controlling a wide range of GTD pathogens in a single application. HWT of grapevine nursery material remains highly recommended and should be used in an integrated approach combined with all other available management strategies for optimal control of GTDs.