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.
Should foliar disease be detected on moderately resistant cultivars, a single fungicide application would probably be sufficient to control it. This reduces application costs and the risk of environmental contamination.
Stem rust infects mainly the stems and leaves of wheat, but can also occur in the heads and awns of susceptible cultivars. This disease is found regularly in the winter rainfall wheat-growing regions, sometimes reaching epidemic levels and causing yield losses of more than 35%.
Leaf rust mostly occurs in the winter rainfall regions of the Western Cape and on irrigated wheat. It can result in a yield loss of up to 50%.
Stripe rust was first detected in South Africa in 1996, and went on to become a major problem in the cool-temperature wheat-growing regions of the eastern Free State.
Powdery mildew poses a threat in highly humid areas, especially during the growing season. It also poses a threat to irrigated wheat with dense plant populations and high nitrogen applications. The disease can cause yield losses of more than 60% in susceptible cultivars.
STB and SNB commonly occur together in most wheat-growing countries. STB occurs more frequently early on in the season, whereas SNB typically occurs later in the season. The former can cause yield losses of up to 50%.
Low levels of STB and SNB are usually observed in the Western Cape wheat-growing areas. The severity of an outbreak depends on weather conditions.
Monitoring and timing
Accurate disease identification is critical to ensure that the correct fungicides are used to combat the disease. It is therefore important for farmers to regularly monitor their wheatlands.
They should familiarise themselves with the symptoms and signs of wheat diseases in their particular production regions to ensure accurate diagnosis and effective management.
Monitoring should commence at around the seven-leaf stage (seven fully unfolded leaves on the main stem).
However, as most leaf diseases flourish in moist and warm conditions, potential epidemics depend not only on cultivar susceptibility, but also on weather conditions. These should thus be monitored.
Seven-leaf and flag leaf stages
Long-term research conducted at the Agricultural Research Council-Small Grain Institute indicates that the seven-leaf and flag leaf (the last top leaf) growth stages can be used as guidelines for monitoring disease and fungicide application.
Most leaf diseases of wheat favour moist and warm conditions, as this allows fungal spores to germinate and penetrate wheat leaves, causing infection and disease development.
When signs of disease, coupled with continuous wet conditions, occur during the seven-leaf growth stage, a farmer should consider applying fungicide.
The flag leaf stage is very important, as it responsible for more than 50% of the grain yield. A disease outbreak at this stage will have a serious impact on yield. Proper monitoring during the flag leaf stage and protecting the crop against infection are therefore vital.
Fungicides can be used if trace levels of a specific foliar disease during ongoing wet weather are discovered. If the weather is dry and hot after the emergence of the flag leaf, application can be delayed until the boot or head emergence stages.
Different fungicides should be used during different growth stages. For example, Soprano, with the active ingredient epoxiconazole, should not be applied after complete ear emergence, but Prosper Trio, with the active ingredients tebuconazole, triadimenol and spiroxamine, can be applied until the early milk kernel development stage.
Many fungal pathogens are highly variable and can adapt to repeated fungicide applications. This can cause the emergence of strains that are less sensitive to previously effective fungicides.
The occurrence of fungicide resistance may lead to serious yield loss.
Fungicide resistance commonly occurs when products within the same chemical group, having the same modes of action, are used repeatedly.
A farmer should therefore rotate between fungicides from different chemical groups to prevent or delay the development of fungicide-resistant strains, and to increase the lifespan of existing fungicides.
Most of the fungicides currently used against foliar diseases of wheat belong to demethylation inhibitors (DMI) and quinone outside inhibitor (QoI) fungicides.
Amongst fungicides commonly used on wheat, Capitan, Folicur, Soprano and Prosaro are DMI fungicides (fungicide group 3), while Acanto belongs to QoI fungicides (group 11).
Abacus consists of active ingredients from both the DMI and QoI group, whereas Prosper Trio is a mixture of DMI (group 3) and the group 5 compound amines (morpholines).
Members of group 3 and group 11 fungicides have very specific modes of actions and hence possess a high risk of fungicide resistance.
A farmer should therefore avoid repeated applications of DMI or QoI fungicides alone in the same season. Fungicides belonging to these groups should be interchanged frequently or used in mixtures to reduce the risk of fungicide resistance.
Phone Dr Tarekegn Terefe on 058 307 3440, or email him at [email protected]