Wheat Producers, Treat Fungicides With Caution!

South African wheat producers should guard against over-application of fungicide, according to Dr Tarekegn Terefe, senior researcher at ARC-Small Grain, Bethlehem. The unnecessary use of fungicides can harm the environment and result in the emergence of fungicide-resistant strains. Farmers should also rotate between fungicides from different chemical groups, he writes.

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Fungicides are widely used by South African wheat producers against foliar diseases. Common diseases treated with them include leaf rust, stripe rust, stem rust, powdery mildew, Septoria tritici blotch (STB) and Stagonospora nodorum blotch (SNB). Because genetically resistant cultivars may not be available, or because of new pathogen strains that break down resistance, fungicides will remain an important part of local wheat production.

For example, the emergence of new stripe rust races in 1998 and 2005 resulted in the outbreaks of this disease on previously resistant wheat cultivars such as Hugenoot and PAN 3191.

New stem rust races that are virulent on the resistance gene Sr31 were detected for the first time in South Africa in 2009 and 2017. Similarly, several leaf rust races that resulted in the breakdown of resistance genes such as Lr37 were identified locally in recent years.

MINIMISE FUNGICIDE USAGE

When previously resistant cultivars become susceptible due to the emergence of new races, fungicides are used to control these rust diseases.

Despite this, disease control options other than fungicides should be considered first. This includes the use of resistant cultivars. Fungicides should be used as a component of an integrated disease control strategy, not in lieu of it. Resistant cultivars have proved to be highly successful in combating foliar diseases; it is therefore wise to use these cultivars and cut down on fungicide use.

The cultivars commercially available in South Africa present varying levels of resistance to foliar diseases, especially wheat rusts, which can result in significant yield losses. Therefore, using two or more cultivars that have the highest levels of resistance to disease in a specific region will minimise the risk of disease outbreaks.