Application of fungicides against yellow spot in wheat

Yellow spot is a limiting factor for wheat cultivation, especially in direct planting and monoculture systems, in colder regions, with frequent rains, overcast days and high air humidity, as occurs in southern Brazil. The foliar application of fungicides, especially trizols and strobilurins, associated with other management strategies, has become essential in recent years given the increase in disease intensity and control difficulties.

In the south of Brazil, the climate contributes to the occurrence and increase in the intensity of diseases, as during the development of the crop, excessive rainfall, accumulation of overcast days and high air humidity are frequently observed. In this scenario, yellow spot on wheat leaves gains importance, especially in colder regions where wheat is grown in monoculture and direct planting systems, reaching a damage potential of up to 48%.

The disease is caused by fungi Drechslera tritici-sudden (Died.) Shoemaker and Drechslera siccans (Drechsler) Shoemaker. The first symptoms appear in the initial stages of the plant, generally on leaves close to the ground. Once inside the plant, the fungus secretes toxins that can cause yellowing and necrosis of tissues, causing small oval lesions with a yellowish halo and a necrotic center. These lesions evolve, through a process called lesion expansion, and can coalesce and take up a large proportion of the leaf tissue. This process is less dependent on the environment, which is why leaf spots tend to evolve under conditions adverse to sporulation, dispersion of the inoculum and induction of new infections.

The main ways the pathogen survives from one crop to another is through infected seeds and crop residues. In crop residues, the pathogen forms a survival structure, which releases sexual spores at the beginning of the harvest under favorable conditions, starting the disease cycle in a direct planting system with monoculture. During the development of the crop, under favorable conditions and without the adoption of chemical control, asexual spores are formed on the lesions and allow the disease to expand and become more aggressive. Within this context, integrated measures to reduce the amount of inoculum in seeds and crop residues are important. The intention is to delay the onset of the disease in the field, and thus reduce damage to the crop and losses to the farmer.

In areas with yellow spot problems, crop rotation with non-host species for at least one year, such as oats, forage radish and canola, can be effective in reducing the amount of inoculum in the field, as can the use of good quality seeds. sanitary. Whenever possible, cultivars that are less susceptible to the disease should be chosen, with this information being offered at field days, research institutions and technical information on the desired cultivar. Seed treatment with fungicides is another important tool, and must be carried out whenever the pathogen is present in the area or on the seed. Finally, the foliar application of fungicides together with other disease management measures is essential, due to the increase in disease intensity and difficulty in controlling it in recent years in southern Brazil.  

Considering that the yellow spot is difficult to control, the University of Passo Fundo (UPF), through professors Carlos Forcelini, Carolina Deuner and mentors, is developing work in this direction. Wheat yield responses to foliar fungicide application are highly variable, being influenced by the cultivar's genetic resistance to the disease, amount of disease present in the field, season, dose and number of applications, application technology, potential crop yield, conditions climate during the crop cycle and virulence of the pathogen.

Most foliar fungicides used to control yellow spot have the chemical groups triazole or strobilurin, or a mixture of both. In work carried out in the UPF experimental area, 2013 harvest, the performance of fungicides on yellow spot control was evaluated. The cultivar used was TBio Pioneiro, susceptible to yellow spot. The fungicides used were pyraclostrobin + epoxiconazole (66,5+25 g a.i./ha), azoxystrobin + cyproconazole (60+24), trifloxystrobin + prothioconazole (60+70), propiconazole (100) and epoxiconazole (62,5) at the stages of tillering, elongation and flowering. 

The percentage of yellow spot control varied between treatments (Table 1). The best control efficiency was found for treatment 9, the first application of trifloxystrobin + prothioconazole with the addition of propiconazole and the two subsequent applications of trifloxystrobin + prothioconazole, with an efficiency of 72%. The two treatments that showed the lowest efficiency were treatments 5 and 7 (34%), both with applications of azoxystrobin + cyproconazole, however, treatment 7 had the fungicide epoxiconazole added to this mixture only in the first application. A possible explanation for the result of lower control efficiency presented by the two treatments is the lower amount of triazole in the formulation, 24 g a.i./ha compared, for example, with treatment 9 which has 70 g a.i./ha. Leaf spots are better controlled by triazoles, responding positively to the addition of more triazole to the mixture (triazole + strobilurin). This procedure is essential in scenarios favorable to leaf spots, such as susceptible cultivars, wheat monoculture and favorable environmental conditions. According to the technical indications of wheat research (REUNIÃO, 2006), good control is considered when the fungicide has an efficiency greater than 70%, and regular control when the efficiency is between 50% and 70%. Therefore, the treatment that showed good control, according to the criteria described was treatment 9, and all others, except treatments 5 and 7, which showed regular control, with efficiency varying from 58% to 66%.

The highest yields were observed in the treatments: 2 - three applications of the mixture pyraclostrobin + epoxiconazole (5.034,1 kg/ha); 3 - same mixture with addition of propiconazole in the first application (5.072,5 kg/ha); 9 - applications of trifloxystrobin + prothioconazole, with propiconazole added in the first application (4955,0 kg/ha). 

The control produced 1.147,1 kg/ha less compared to treatment 2, which presented the highest productivity. This reinforces the importance of good crop management and the help of fungicides, which are an important tool for minimizing damage caused by diseases, being an emergency, quick and efficient measure. For wheat cultivation to be sustainable, it is necessary to maintain high levels of grain yield. In this sense, fungicides must be applied according to criteria that ensure economic return, considering the biological cycle of the pathogen, the behavior of cultivars, and climatic conditions.

Research aimed at the integrated management of leaf spots must be intensified and continuous, with the aim of guiding farmers' management to reduce damage in wheat farming. There is a need to develop new fungicides or reposition fungicides on the market to mitigate the problem of controlling yellow spot. However, this must be associated with other management strategies, such as genetic resistance and crop rotation.

Wheat and fungal diseases 

Wheat (Triticum spp.) is the second most produced cereal in the world, with significant importance in human nutrition and in the global agricultural economy. The cereal is grown in the South, Southeast and Central-West regions of the country, with the South being responsible for more than 90% of national production. 

Brazilian wheat production supplies approximately half of national demand. However, it is common knowledge that Brazil has immense potential to produce wheat, without requiring the incorporation of areas not yet cultivated with grains. Therefore, among the components that limit the productive potential of the crop, diseases of fungal origin stand out, as they compromise grain quality and yield.

Victória V. Bertagnolli, Camila Ranzi, Valéria C. Ghissi, Pedro Bertagnolli Neto, Giovani Pastre, Gustavo Visintin, Carolina C. Deuner, Federal University of Passo Fundo

Article published in issue 202 of Cultivar Grandes Culturas.