Triazole fungicide sensitivity among South African Puccinia graminis f. sp. tritici isolates
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Date
2022
Authors
Du Toit, Isabella
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Publisher
University of the Free State
Abstract
Wheat is an important crop and a key component in the diet of people, particularly in developing countries such as South Africa (SA; Pathak and Shrivastav, 2015). The crop is produced worldwide and is considered the most important source of food (Igrejas and Branlard, 2020). Two closely related species Triticum aestivum L. (bread wheat) and T. durum (durum wheat) are widely cultivated with global production dominated by countries including China, India, Russia, Ukraine and the United States of America (USA; OECD-FAO, 2020). During the 2019 to 2021 production seasons, SA produced an average of 1.8 million metric tons of bread wheat (CEC-Sagis, 2021), with nearly 42.2% of the total wheat yield contributed by spring wheat cultivars planted under dry land in the Western Cape province (Galal, 2021). Rust diseases are present in most areas where wheat is cultivated (Kolmer, 2005; Pretorius et al., 2020). Stem rust of wheat and barley (Hordeum vulgare L.), caused by Puccinia graminis f. sp. tritici Eriks. and E. Henn. (Pgt), is considered the most damaging wheat disease in history (Pretorius et al., 2017). Recent reports indicate this pathogen’s re-emergence in areas after decades of absence (Lewis et al., 2018; Tsushima et al., 2022). Kernels harvested from rust infected wheat are nutrient poor and low in test weight, while healthy kernels are plump and nutrient rich (Agrios, 2005). Disease control is important to maintain the production of high yielding wheat and is achieved through different control strategies, including fungicide application, a strategy widely practiced in SA (Roelfs et al., 1992; Soko et al., 2018; Anonymous, 2020b).
Rust pathogens are known for their ability to overcome monogenic sources of resistance, rapid dispersal over thousands of kilometres (Visser et al., 2019) and exponential rate of development into epidemics under favourable conditions. Stem rust mainly affects the stems of wheat during grain fill, as such fungicide applications are considered less effective if not applied preventatively (Wanyera et al., 2009; Tadesse et al., 2010). The effective chemical control of stem rust depends on application timing, adequate downward translocation of the active ingredient (a. i.) or coverage of the stem area (Bayer Crop Science, n.d.). The occurrence of fungicide insensitivity in populations of fungal pathogens limits the efficacy and period during which fungicides remain effective. Loss in fungicide efficiency contributes to increased costs associated with new fungicide development (Ma and Michailides, 2005). Fungicide insensitivity is the acquired and heritable decrease in the sensitivity of isolates to the a. i. present within a fungicide formulation (Brent and Hollomon, 2007). The widespread, repeated, and incorrect fungicide application as well as overreliance on a particular a. i. increases the risk of fungicide insensitivity. However, it may also occur without prior exposure in a phenomenon known as cross-resistance where insensitivity develops for different fungicides having a. i. with similar modes of action (Kang et al., 2019; McGrath, 2001). This insensitivity is usually to the mode of action of the a. i. of the specific fungicide and is a result of selection pressure exerted on the fungal population (Brent and Hollomon, 2007).
Description
Dissertation (M.Sc.(Botany))--University of the Free State, 2022