Department of Agronomy

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Browsing Department of Agronomy by browse.metadata.advisor "Cairns, A. L. P."

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    Paraquat and glyphosate resistance in Conyza bonariensis in the Western Cape in the Republic of South Africa
    (Stellenbosch : Stellenbosch University, 2005-12) De Wet, Hestia; Cairns, A. L. P.; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
    ENGLISH ABSTRACT: Conyza bonariensis (flaxleaf fleabane) was reported for the first time, as a weed in California in 1893-1896. The first report of the occurrence of this weed in South Africa was made in May 1895 in Franschoek, South Africa. Paraquat resistance in C. bonariensis was first reported in the 1970s and early 1980s when resistance was observed from vineyards and citrus plantations in Egypt. More recently a report of herbicide resistance in South Africa was made in January 2003 when resistance occurred in C. bonariensis in the Breede Valley, South Africa. The resistance was to glyphosate, but recently reports of resistance to glyphosate and paraquat were received. C. bonariensis seeds were found to be positively photoblastic and germinated only under unfiltered white light and red light whilst no germination occurred under far-red light and in the dark. The optimum temperature range for C. bonariensis seed was found to be between 15 and 30°C, with no germination occurring at 0-5°C and at 35- 40°C. Optimum germination occurred at the soil surface. No germination occurred at depths of 2 cm and deeper. Although the optimum temperature range was found to be the same for the different biotypes tested. However, germination was highest in the susceptible biotype. Since farmers first reported paraquat and glyphosate resistance in C. bonariensis in the Breede Valley, South Africa, reports of resistance increase every year. Seed collected from populations suspected of being resistant to paraquat and glyphosate were obtained from the Breede Valley and screened for resistance. To determine the easiest, quickest, and most effective method to screen for paraquat and glyphosate resistance, two tests were evaluated viz. the petri dish assay method and the whoIeplant dose-response method. Both screening methods identified paraquat and glyphosate resistant biotypes. The petri dish assay method was found to be a more rapid method of screening for resistance in C. bonariensis. During this study it was found that both paraquat and glyphosate resistance does occur in the Breede Valley. The effect of growth stage on the level of herbicide resistance in C. bonariensis was tested. Herbicides other than paraquat and glyphosate were tested to determine if they could be used to control resistant C. bonariensis seedlings. The alternative herbicides tested included MCPA and Sorgomil Gold 600. The four herbicides were sprayed at different leaf stages. During the study it was found that growth stage does play an important role in the level of herbicide resistance. It was found that the control of different herbicides decreased with an increase in growth stage. The different herbicides showed varying levels of control depending on growth stage and resistant profile. Overall MCPA gave the best control at all leaf stages tested. What is gratifying was the finding that every biotype tested could be controlled by at least one of the treatments applied. This means that the producer using the most appropriate herbicide applied at the optimum application stage will be able to control most if not all the resistant populations of C. bonariensis that occur in the Western Cape.
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    Resistance of ryegrass (Lolium spp.) to paraquat and glyphosate in the Western Cape
    (Stellenbosch : Stellenbosch University, 2007-12) Eksteen, Frederik Heinrich; Cairns, A. L. P.; Pieterse, P. J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
    ENGLISH ABSTRACT: The origin of this study dates back to 2001 when ryegrass (Lolium spp.) from a vineyard near Tulbagh was found by the author to be resistant to glyphosate. Several other populations of glyphosate resistant ryegrass were found on nearby farms. In attempts to eliminate what was initially thought to be a very localised resistance problem, paraquat was applied at several of these sites. It soon became apparent that significant numbers of these ryegrass populations were also resistant to paraquat. Populations that were susceptible to glyphosate but resistant to paraquat were also found. This was the first recorded incidence of paraquat resistance in ryegrass. Further screening of Western Cape ryegrass resulted in the identification of populations that showed multiple resistance to glyphosate, paraquat and ACCase inhibitors. In laboratory trials this study established that the mechanism which confers resistance to paraquat is based on the failure of the herbicide to reach its site of action in the plant. This lack of trans location was ascribed to sequestration of the active ingredient in a non-physiologically active site in the plant, such as the vacuole. This form of resistance was overcome by applying glufosinate ammonium, either prior to, or together with the paraquat application. The same degree of synergy was observed using photosystem II inhibitors with paraquat. The resistance mechanism is overcome by these products by either preventing paraquat from being 'sequestrated and/or freeing up of herbicide which had already been bound. Mixtures containing paraquat, glufosinate ammonium and a photosystem II inhibitor such as terbuthylazine gave the best control of resi~tant ryegrass populations. Independent confirmation, of the findings in the laboratory perfonned trials, was done in field trials, conducted during the 2007 season by Syngenta SA on paraquat resistant ryegrass populations in the Western Cape.