How to Control Leaf Spots on Wheat

Among the main fungal diseases that attack the aerial part of wheat crops is the leaf spot complex capable of causing losses of up to 80% of productivity. In addition to harming grain production, its incidence also causes damage to the physiological quality of the seeds. The rational and judicious use of fungicides is one of the tools to face this challenge.

wheat (Triticum aestivum L.) is the winter crop with the greatest economic expression in southern Brazil. Due to adverse climatic conditions, combined with the susceptibility of the cultivars, its yield may be affected by the attack of diseases caused by fungi. Among the main fungal diseases that attack the aerial part of the crop, one can highlight the leaf spot complex, considered the most damaging, and can result in losses ranging from 20% to 80% of productivity. In addition to reducing productivity, this stain complex causes a reduction in the physiological and sanitary quality of the seeds produced. The main fungi involved in the leaf spot complex in the crop are Drechslera tritici-repentis cause of yellow spot, Bipolaris sorokiniana causing the brown spot, and more sporadically Stagonospora nodorum  responsible for glume spot (Figure 1).

Climate conditions and winters with higher average temperatures are among the main factors that contribute to the emergence and evolution of this complex of diseases in crops. In recent harvests, wheat producers in the state of Rio Grande do Sul have been facing climate conditions that are extremely favorable to the evolution of diseases, damaging the profitability of the crop. The 2014 harvest caused great losses, due to excessive rainfall with higher temperatures, where there were high incidence rates with rapid evolution in the severity of stains, resulting in, in addition to a reduction in productivity, a low physiological and sanitary quality of the seeds produced.

The productivity and physiological quality of wheat seeds are related to the green or healthy leaf area during grain filling. The occurrence of diseases in the crop tends to affect the quality of the seeds produced, as the action of parasitic fungi affects the normal development of the seeds, reducing their weight and ability to produce plants with good productive potential. The occurrence of diseases can increase the incidence of necrotrophic fungi associated with the seed, as they use the seeds as a host, making them a source of inoculum for leaf spots for the next harvest.

To reduce leaf spot infestations in crops and reduce the damage they cause, it is essential to adopt integrated management, which begins with the choice of cultivars that have genetic resistance to the main diseases that attack wheat crops. Subsequently, seed treatment must be carried out with fungicides. This measure is a safe way to protect the first stages of the plant, where the productive potential of the crop is defined. Finally, constant monitoring since planting with the aim of detecting the first signs of the disease, so that chemical control with fungicides can be implemented in the aerial part in a rational and economical way.

The most productive cultivars tend to be more susceptible to diseases. Regarding necrotrophs, especially leaf spots, none of the most sown cultivars in Rio Grande do Sul has genetic resistance to these fungi. They just present different rates of evolution of these diseases. Thus, it is essential to adopt other resources, such as the use of chemical control, through the application of fungicides to the aerial part, the application of which must occur after the appearance of the first symptoms, to minimize the damage caused by diseases to productivity and possibly to quality of the seeds produced. To control leaf spots, it is recommended to use pre-made mixtures of fungicides with triazoles + strobilurins at recommended doses. Such formulations must be registered with the Ministry of Agriculture, Livestock and Supply (Mapa) for use in wheat cultivation.

Studies show that the interaction between the mechanisms of action triazoles + strobilurins can contribute to increasing the productivity and quality of the seeds produced. Strobilurins slow down leaf senescence and their fungal action is capable of preventing the germination of fungal spores. Triazoles kill fungi on the plant during the colonization process, working effectively in the period between the beginning of infection and the appearance of symptoms.  

In this context, in the winter harvest of 2014, a study was carried out to evaluate the relationship between the incidence of leaf spots in wheat crops and the physiological quality of the seeds produced. The wheat cultivar used in the study was Jadeíte 11 with three treatments: control, without application of fungicide, with application of Azoxystrobin + Cyproconazole + Propiconazole and another with the application of Azoxystrobin + Difenoconazole. Three applications were carried out, the first being after the appearance of the first symptoms of leaf spots, and the others respecting an interval of 21 days. The evaluation of the severity of stains was carried out seven days after application.

Figure 2 shows the values ​​for the germination, first count (PC) and accelerated aging (EA) tests depending on the treatments used. The application of fungicides maintained germination above 80%, the minimum value for seed batch approval. This result highlights the importance of using fungicides to control aerial part diseases. In the evaluations of the first count and accelerated aging, the result was similar where the use of fungicide resulted in better quality seeds. 

These results can be attributed to the negative correlation of the severity of leaf spots in flag leaf with the quality of seeds produced (Table 1), since how much the greater the severity of leaf spots on the flag leaf, the smaller the results for germination, first count (PC) and accelerated aging tests (AND THE).

Maintaining sanity of the aerial part of wheat, especially the flag leaf, is important for increase the production and physiological quality of the seeds produced. The leaf flag is the last to enter senescence and is also responsible for contribute most of the photoassimilates to grain filling, considering that the severity of diseases has a major impact on photosynthesis as well as other important metabolic processes that occur in plants.

The incidence of leaf spots caused a reduction in the physiological quality of the seeds produced, demonstrating the importance of control not only for grain production but also for the physiological quality of the seeds produced.

Francine Zaiosc Simmi, Lucas Navarini, Marcos Paulo Ludwig, IFRS

Article published in issue 204 of Cultivar Grandes Culturas.