Department of Plant Pathology

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Browsing Department of Plant Pathology by Subject "Alternaria diseases -- Control"

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    Evaluation 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.