Doctoral Degrees (Plant Pathology)
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Browsing Doctoral Degrees (Plant Pathology) by browse.metadata.advisor "Fourie, Paul H."
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- ItemCharacterisation of Cylindrocarpon spp. associated with black foot disease of grapevine(Stellenbosch : Stellenbosch University, 2005-12) Halleen, Francois; Crous, Pedro W.; Fourie, Paul H.; Stellenbosch University. Faculty of Agrisciences. Dept. of Plant Pathology.ENGLISH ABSTRACT: During the past few years a drastic reduction has been noted in the survival rate of grafted grapevines in nurseries, as well as in young vineyards in the Western Cape Province of South Africa. Circumstantial evidence suggested that Cylindrocarpon spp., which cause black foot disease of grapevine, were associated with this decline. Black foot disease of grapevine is a relatively new, and as yet poorly known disease affecting vines in various countries where grapevines are cultivated. Primary aims of this research have been (1) to conduct nursery surveys in order to determine which fungi are involved in the decline phenomenon, with special reference to the involvement of Cylindrocarpon spp., (2) to identify and characterise the organisms believed to be the causal organisms of black foot disease, and (3) the development of control and/or management strategies to prevent or eradicate Cylindrocarpon infections. Nursery grapevines were sampled at different stages from three commercial nurseries in the Wellington area of the Western Cape Province and were investigated during the 19992000 season by means of destructive sampling. The first samples were taken in September from callused cuttings prior to planting in nurseries. After planting, asymptomatic rooted cuttings were selected from nurseries after 3, 6 and 9 months. Isolation studies clearly demonstrated that different “Cylindrocarpon spp.” infected cuttings from nursery soils. These species rarely occurred in rootstock propagation material prior to planting. At the time of planting, the susceptible basal ends (especially the pith area) of most of the nursery cuttings are partly or even fully exposed. Callus roots also break during the planting process, resulting in small wounds susceptible to infection by soilborne pathogens. The isolation studies revealed that the first infections occurred in the roots, followed by infections of the rootstocks. These infections increased progressively during the course of the growing season. Substantial variation in cultural and morphological characters was observed among the Cylindrocarpon isolates obtained from the nursery survey, as well as from isolations that were made from diseased grapevines. Morphological and phylogenetic studies were conducted to identify these “Cylindrocarpon spp.” and to establish their association with black foot disease. Sequences of the partial nuclear large subunit ribosomal DNA (LSU rDNA), internal transcribed spacers 1 and 2 of the rDNA including the 5.8S rDNA gene (ITS), and partial β-tubulin gene introns and exons were used for phylogenetic inference. Phylogenetic analyses confirmed the diversity observed among the isolates and four Cylindrocarpon-like species were identified. One of these species was initially identified as Cylindrocarpon destructans. However, further research revealed C. destructans to represent a species complex. Grapevine isolates of “C. destructans” proved to be identical to the ex-type strain of Cylindrocarpon liriodendri, which also produced a teleomorph, Neonectria liriodendri in culture. A second species was newly described in this study as Cylindrocarpon macrodidymum (Neonectria macrodidyma). The two remaining Cylindrocarpon-like species were placed in a new genus, Campylocarpon. The two species were named Campylocarpon fasciculare and Campylocarpon pseudofasciculare. Pathogenicity studies confirmed that all four species were able to reduce root and shoot mass significantly. Knowledge obtained pertaining to the disease cycle of black foot disease suggest that suitable management strategies should focus on prevention of primary infection in nurseries. However, at present, no fungicides are registered for control of this disease in South African vineyards or nurseries. Thirteen fungicides were screened in vitro for mycelial inhibition of these pathogens. Prochloraz manganese chloride, benomyl, flusilazole and imazalil were the most effective fungicides tested, and were subsequently included in semi-commercial field trials. Basal ends of grafted cuttings were dipped (1 min) in various chemical and biological treatments prior to planting in open-rooted nurseries. Black foot pathogens were not isolated from grafted cuttings prior to planting in nurseries. Additional treatments involved soil amendments with Trichoderma formulations and hot water treatment (50°C for 30 min) of dormant nursery grapevines. Field trials were evaluated after a growing season of eight months. The incidence of black foot pathogens was not significantly and/or consistently reduced by the majority of chemical or biological treatments. However, these pathogens were not isolated from uprooted plants that were subjected to hot water treatment. It is therefore recommended that hot water treatment of dormant nursery plants be included in an integrated strategy for the proactive management of black foot disease in grapevine nurseries.
- ItemEvaluation of adjuvants in fungicide spray application for the control of alternaria brown spot in South African citrus orchards(Stellenbosch : Stellenbosch University, 2019-03) Van Zyl, Johannes Gideon; Fourie, Paul H.; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.ENGLISH ABSTRACT: Citrus fruit and foliar diseases are mainly controlled through pre-harvest application of fungicides. Fungicides are only as effective as the application process and for effective disease control deposition of a uniformly distributed quantity of active ingredient(s) is required on the intended target(s). Adjuvants have the potential to improve fungicide deposition on a target surface. The influence of adjuvants on the deposition of fungicides, especially at the high spray volumes used in South African citrus production is unknown and was therefore investigated. A previously developed deposition assessment protocol, using a yellow fluorescent pigment as tracer for copper oxychloride (CuOCl) deposition, was improved through photomacrography and digital image analyses which proved accurate in determining the quantity and quality of deposition on citrus leaves. Spray deposition benchmarks indicative of the biologically efficacy of CuOCl against Alternaria alternata [causal agent of Alternaria brown spot (ABS) of mandarins] was developed. The deposition assessment protocol and deposition benchmarks was used to evaluate two organosilicone adjuvants (Break-Thru S240 and Break-Thru Union) at reduced spray volumes in dense and less dense citrus canopies in two separate orchard spray trials. Deposition quantity generally increased with increasing spray volume, but normalised values showed better spray efficiency at lower volumes. In pruned and less dense canopies, a beneficial effect of adjuvants was observed in terms of deposition quantity, efficiency and uniformity, especially at reduced volume applications. Some improvement in deposition quality was generally observed with the use of adjuvants. These benefits were not as evident in very dense canopies, illustrating the importance of canopy management when spraying at reduced volumes. Commercially available adjuvants [Break-Thru, Nu-Film-17, Citrole100, Villa51, Wetcit, Entrée and Exit] were evaluated in three orchard spray trials on different citrus types, cultivars and spray volumes. In trial one, adjuvants improved deposition quantity and canopy penetration. In trial 2 and 3, deposition quantity was generally higher at higher spray volumes, but spray efficiency was significantly better at lower spray volumes. Adjuvants generally improved deposition uniformity and deposition quality, but these benefits were significantly influenced by spray volume and the specific adjuvant treatment. Poor performance by adjuvants was ascribed to high spray volumes and/or too high adjuvant concentration used, which led to increased levels of run-off and poor deposition parameters. The effects of adjuvants on deposition quantity, quality and biological efficacy of CuOCl against ABS on mandarin leaves were determined in laboratory trials. Adjuvant treatments varied significantly in deposition quantity and quality and disease control achieved. Higher deposition quantity, beter quality and higher Cu residues was realized at pre- vs. post-run-off volumes. Adjuvants did not improve deposition parameters compared with the control treatment at both spray volumes. Leaf infection analysis indicated that CuOCl with adjuvant sprays (post-run-off volume) realized similar and in some cases slightly better control (although not significant) than copper oxychloride alone, but that deposition and Cu residue loading in some of these adjuvant treatments were markedly lower. This anomaly could be ascribed to direct or indirect effects of the adjuvant and was investigated further. In vivo and in vitro studies were done to identify possible direct adjuvant effects on pathogen development and potential synergistic effects between the adjuvants and CuOCl. Adjuvants alone did not influence conidial adhesion, appressorium formation, germ tube length and percent viable conidia. Adjuvant sprays together with CuOCl reduced conidial adhesion, germ tube length and percent viable conidia numerically; however, not significantly compared with CuOCl alone. Adjuvants also caused conidium/germ tube stress similar to CuOCl, but did not inhibit germination or growth. In the in vitro microtiter assay, adjuvants together with CuOCl improved germination or growth inhibition compared with the CuOCl treatment alone, although not at significant levels. The findings in Chapter 6 did not fully explain the anomalous findings in Chapter 5, and future studies should focus on developing methodology to support histopathology studies on sensitive leaf surfaces, as well as development of a more sensitive method of measuring deposition quality, especially on a microscopic scale.