Doctoral Degrees (Plant Pathology)
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Browsing Doctoral Degrees (Plant Pathology) by browse.metadata.advisor "Mazzola, M."
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- ItemEcology and characterization of Streptomyces species associated with common scab disease conducive and biofumigated soils in South Africa(Stellenbosch : Stellenbosch University, 2013-03) Gouws, Reinette; McLeod, Adele; Mazzola, M.; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.ENGLISH ABSTRACT: Common scab of potato is a serious cosmetic disease in South Africa as well as internationally. The disease affects the appearance and quality of potatoes resulting in major annual losses. Potato producers in South Africa, in the commercial, emerging and processing potato industries, struggle to manage the incidence of common scab, especially soilborne inoculum. Existing products and management programs against common scab are often insufficient. The two main aims of the study were to i) characterize and determine the pathogenic Streptomyces spp. occurring in potato production regions in South Africa and ii) investigate the mechanisms through which Brassica soil amendments can reduce common scab and ways in which it can be included in a sustainable management program. In South Africa, Streptomyces scabiei is still regarded as the main causal agent of common scab. However, world-wide, the disease is caused by a complex of Streptomyces species, with the dominant species varying in different regions. Therefore, a total of 132 Streptomyces isolates collected from six South African potato production regions were characterized. Potato pot trials showed that 53 % of the isolates were pathogenic. Analyses using species specific primers and phylogenetic analyses (16S rRNA phylogeny and multilocus phylogeny) showed that S. scabiei was the most prominent species in South Africa comprising 51.4 % of the pathogenic isolates, followed by S. europascabiei (30 %), S. cavisabies (5.7 %), and S. stelliscabies (1.45 %). The remaining 11.45 % of the pathogenic isolates comprised three taxa, which are related and fit within phylogenetic clades that do not contain common scab isolates from any country other than South Africa. The taxa are named here Streptomyces strains RSA1 (5.7 %), RSA2 (4.3 %) and RSA3 (1.45 %). Streptomyces strain RSA1, which occurred in two production regions, is of special concern since these isolates produce fissure scab symptoms that result in severe cosmetic tuber damage. Fissure scab has not been reported from any other region of the world and is of concern in South Africa since it occurs on the cultivar Mondial that is tolerant to typical common scab. PCR analyses targeting three marker pathogenicity island (PAI) genes (txtAB, nec1, tomA) showed that among the pathogenic isolates nec1 occurred in 89 % of the isolates, tomA in 81 % and txtAB in 89 % of the isolates. The isolates (11 %) that did not contain the txtAB gene and also did not produce thaxtomin, belonged to S. caviscabies and Streptomyces strains RSA2 and RSA3. The incorporation of Brassica tissue into soil has recently shown some potential for reducing common scab disease incidence. Brassica crop residues contain glucosinolates (GLN) that upon cell disruption are hydrolysed by the enzyme myrosinase to yield a diversity of biologically-active hydrolysis products that are toxic to soil microbes. This control mechanism is known as biofumigation. The current study showed that common scab was significantly reduced under field conditions through incorporation of fresh or air-dried residues of Brassica oleracea var. capitata (cabbage) in two consecutive potato plantings. The in-vitro effect of volatile emissions from various Brassica species towards Streptomyces was evaluated using two bioassay methods. An in-vitro agar plate bioassay showed that, in general, volatile emissions from water activated freeze-dried tissue of a B. juncea/S. alba mix and B. napus were superior to those from B. oleracea var italica and B. oleracea var capitata for suppression of growth and sporulation of Streptomyces. In a gas chamber bioassay that used freshly macerated Brassica tissue, B. oleracea var capitata and a B. juncea/S. alba mix suppressed sporulation but not hyphal growth of Streptomyces. The gas chamber bioassay showed that the biofumigation effect was bacteriostatic, i.e. isolates recovered after volatile exposure. Both bioassays showed that significant components of both the pathogenic (50 %) and non-pathogenic (20 %) Streptomyces population examined were unaffected by the Brassica tissue derived volatiles. Mechanisms of disease reduction through Brassica amendments are not limited to biofumigation, but changes in the structure of microbial communities involved in systemic induced resistance and/or general microbial suppression may also contribute to disease suppression. In the current study a potato split-root experiment that spatially separated the progeny tubers and roots of Brassica juncea/ Sinapus alba (mustard mix) and Brassica oleracea var oleracea (cabbage) amended soil sub-units from non-amended soil sub-units, showed that induced resistance induced in plants was involved in common scab suppression. The role of toxic GLN hydrolysis products was ruled out in the induced resistance mediated disease suppression, since volatiles were released from Brassica amended soil prior to initiating the experiment. Increased microbial activity in the Brassica amended units was evidenced by significant increases in ß-glucosidase and urease activities. Principal component analyses revealed some trends in the overall soil, tuber and root associated microbial genera (Trichoderma, Pseudomonas, Streptomyces, total bacteria and Fusarium) in the Brassica amended and non-amended units. The mustard amended treatment, and to a lesser extend the cabbage amended units, showed trends towards increases in soil Fusarium and Trichoderma and root Trichoderma populations, and decreases in total bacterial and Streptomyces populations in soil and tubers, and Streptomyces in roots. This study has contributed towards our knowledge of the Streptomyces species causing potato common scab in South Africa, and mechanisms through which Brassica soil amendments can reduce common scab. Several Streptomyces species, including novel pathogenic taxa, are involved in causing common scab and their differential virulence, and responses to being suppressed by Brassica amendments will require the implementation of an integrated management program. The planting of cabbage as a cash crop, with the subsequent incorporation of residues into soil shows promise as a management strategy for subsistence farmers. The mechanisms involved in common scab suppression through Brassica amendments were shown to involve systemic induced resistance in plants and general microbial suppression. Altogether, knowledge obtained in this study can be used to i) optimize management strategies for sustainable potato production, ii) further elucidate the mechanisms involved in disease suppression and iii) develop molecular techniques, such as quantitative real-time PCR for rapid identification and quantification of common scab-causing species in South Africa.
- ItemSpectrum 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.