Doctoral Degrees (Plant Pathology)

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Browsing Doctoral Degrees (Plant Pathology) by Author "Nyoni, Makomborero"

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    Spectrum of in-vitro activity and efficacy of phosphonates for management of apple replant disease and Oomycete root rot pathogens in South Africa
    (Stellenbosch : Stellenbosch University, 2018-03) Nyoni, Makomborero; McLeod, Adele; Mazzola, M.; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: In South Africa, apple replant disease (ARD) and Phytophthora root rot are two soilborne diseases that are important in apple production. ARD occurs when old apple orchards are replanted, causing a reduction in tree growth. Several biotic agents are involved. In South Africa, a few Pythium spp. and Phytophthora cactorum were previously shown to be prominent in the development of ARD, with Pratylenchus spp. occasionally being involved. Phytophthora root rot, caused by P. cactorum, most often becomes problematic in the 2nd or 3rd year post-plant causing tree death and reduced tree growth. The pathogen is most likely introduced through nursery trees, irrigation water and residual soil populations (unfumigated inter-row strips). Management of ARD mainly consists of preplant fumigation of tree rows with chloropicrin/1,3-dichloropropene. Phytophthora root rot can be controlled using phosphonate fungicides, but these are not registered for apples in South Africa. Phosphonates, which breaks down to phosphite in plants, are highly mobile in plants and can reduce disease through a direct toxic effect towards pathogens or the induction of host plant defences. Three orchard trials were conducted to determine whether ARD can be managed using semi-selective chemicals and different chloropicrin formulations. In all three trials, tree growth (trunk diameter and shoot growth) was improved significantly relative to the control by preplant fumigation with either of two formulations of chloropicrin/1,3-dichloropropene (formulations containing chloropicrin at 60.8% or 57.0%), or with a postplant semi-selective treatment programme that included applications of fenamiphos, phosphite, imidacloprid and metalaxyl. Yield increases did not always accompany the tree growth increases. In one orchard, yield was only increased significantly by combining semi-selectives with a fumigation treatment, whereas in the other two orchards all fumigation treatments significantly increased yield. Phytophthora cactorum and Pratylenchus spp. likely interacted synergistically and were important ARD pathogens. In a second set of two orchard trials, the temporal nature of root phosphite concentrations in asymptomatic apple trees [trees where oomycete pathogens were present in roots, but no foliar symptoms were evident] was examined following different methods of application of phosphonates (foliar sprays, stem sprays, soil drenching and trunk paints) applied in summer and fall. A trunk paint application, was the best application method based on root phosphite concentrations. Foliar sprays, which were only applied in summer, also showed potential based on root phosphite concentrations. Phytophthora cactorum and Pythium irregulare DNA quantities in the roots of trees receiving phosphonate treatments were significantly lower than the quantities in the control treatment. In vitro studies showed that medium type (liquid or solid) and phosphate concentration significantly influenced the percentage mycelial growth inhibition of P. cactorum and P. irregulare by phosphite. This made it problematic to assess the relative effect of root phosphite concentrations as a determinant of pathogen suppression in orchard tree roots. A third set of trials were conducted, aimed at evaluating the curative efficacy of phosphonates in three apple orchards with Phytophthora root symptoms. Different phosphonate application methods (foliar sprays, trunk sprays and trunk paints), yielded similar levels of shoot growth in trees, which was significantly better than the control in two trials in the Grabouw region after 11-months, but not in the Koue Bokkeveld trial. Yield data could only be obtained in the latter trial, which was also not significantly increased by phosphonate applications. In the two Grabouw trials, all application methods yielded relative high root phosphite concentrations for fall phosphonate applications 13-weeks postapplication, but not in the Koue Bokkeveld trial. In all three trials, P. cactorum root quantities were not reduced by any of the phosphonate treatments. The study showed that phosphonates have potential for managing Phytophthora root rot in apple orchards. Phosphonates combined with other semi-selective chemicals (fenamiphos, imidacloprid and metalaxyl), can also be used to manage ARD. The relationship between phosphite concentrations required in tree roots for suppression of P. cactorum and P. irregulare, and phosphite concentrations required for pathogen suppression in vitro is unclear due to (i) various factors influencing the in vitro sensitivity of isolates and (ii) the seasonal fluctuation of root phosphite concentration in apple trees. Future work should focus on determining whether root phosphite concentrations are important for direct pathogen suppression by co-quantification of root phosphite and pathogens in time course studies in orchard trials. Furthermore, the effect of root phosphite concentrations on host plant defence induction must be investigated.