Management of foliar diseases in wheat

Leaf diseases such as powdery mildew and leaf spot are capable of limiting productivity and quality of wheat crops. In the search for efficient and sustainable strategies for control the use of silicon-based enhancers has good potential to help the producer in combating these diseases.

Belonging to the family of semi-metals, silicon is present in the crust terrestrial. Its content in the soil can vary from less than 1% to 45% of the mass dry and represents 0,00001 mg/kg to 0,001 mg/kg (0,1%-10%) of the dry mass of the plant. In soil, silicon is present as silicic acid (H4SiO4) and when absorbed by the plant, it polymerizes into amorphous silica, depositing in cell walls, associating with polysaccharides, lignin and proteins.

Plants from the Poaceae family, such as wheat and rice, accumulate silicon in the order of 4% or more, being considered plants that accumulate this chemical element. In addition to having a structural function, the Silicon has important activity in the defense of plants against pathogens. Two biological activities related to the defense process against stress biotics are described: (1) silica represents a physical barrier, limiting the entry of pathogens such as fungi or bacteria and also (2) activate the natural plant defense by stimulating the production of PR proteins such as chitinases and peroxidases, polyphenols, oxidases, phytoalexins, among others. You benefits of using silicon are proven not only in known plants as accumulators, as is the case with wheat and rice, for example, but also, in non-accumulators of silicon such as tomatoes and cerrado plants. Second researchers at the Federal University of Uberlândia, silicon should be included in the fertilization of certain crops, ensuring the sustainability of agricultural production and can potentially increase productivity. In the case of sugarcane, when applying silicon-based fertilizers, gains of up to 30%.

In addition to the benefits related to resistance to pathogens, tolerance to salinity, drought, temperature and metal toxicity have also been reported and associated with the application of silicon. The silica layer on family members Poaceae is located just below the cuticle, around 2-5µm below. In wheat, older leaves can contain up to 11,8% silicon. The presence This layer keeps the leaves more upright and rigid, increasing interception of light and, consequently, the photosynthetic rate. This layer also reduces the loss of water to the environment, i.e. the reduction of water through transpiration being related to indirect quality and productivity factors.

Silicon absorption is dependent on the water supply in the soil and the presence of green leaves, which produce transpiration and, consequently, movement of this element from the soil to the leaves, where it is deposited. Despite not being considered an essential element, the International Institute of Nutrition of Plants (IPNI) recognizes that this element is beneficial and helps in crops in the rice, corn and wheat mainly, but other crops such as cotton and soybeans They also respond to the application of silicon. Application doses depend on the plant species and the plant's stage of development.

This work aimed to evaluate the influence of the use of silicon-based enhancer in the control of foliar diseases in wheat.

The experiment was conducted at municipality of Ibirapuitã, in the field, with completely randomized blocks of 10 x 16 m. The wheat cultivar used was Amethyst, sown on July 04, 2016 with a spacing of 17 cm between rows and a density of 300 plants/m². They were two applications of urea of ​​120 kg/ha and 100 kg/ha were carried out on the 15th of August 2016 and August 29, 2016, respectively. The experiment counted with four treatments and five replications. In the different treatments, three chemical product applications were carried out, as described in Table 1, treatment 1 (T1) being the one where the plots received only applications of fungicides; treatment 2 (T2) received the same chemical treatment with fungicides than T1, but with the addition of a silicon-based enhancer; O treatment 3 (T3) received only the enhancer, without application of fungicides; and treatment 4 (T4) is the control treatment, in which there was no application of neither fungicides nor enhancers. The equipment used To carry out the applications, a 16-meter-long tractorized sprayer was used. bar using TT110 015 VP nozzles with a spacing of 50cm using 110L of syrup/hectare in all applications.

The severity of foliar diseases, powdery mildew and leaf spot were evaluated using diagrammatic scales where the percentage of affected leaf area was determined every 14 days from the appearance of the first symptoms, totaling four assessments. Ten plants per plot and repetition were evaluated, totaling 50 plants per treatment. Severity averages were calculated and the data obtained were subjected to analysis of variance (p≤0,05) using the SISVAR statistical program.

Among the treatments, it was not possible to verify a significant difference for the severity of powdery mildew, indicating that silicon, despite not presenting superior control to fungicides showed similar results. In the case of leaf spots, treatments with only enhancer (T3) and enhancer plus fungicides (T2) were those that showed better control of the disease. Treatment with fungicides alone (T1) showed control lower than T3 and T2 (with silicon), and the control treatment was the one that presented greater severity (Table 2).

Powdery mildew is a biotrophic fungus that establishes itself and affects only epidermal cells occurring symptoms mainly on the upper surface of the leaves. Leaf spots, in turn, are necrotrophic pathogens that colonize the inter and intracellular space, with the involvement of toxins and removal of nutrients from the host. The penetration of these fungi occurs through emission of germ tubes that penetrate through the cuticle. Therefore, the The presence of amorphous silica just below the cuticle prevents or restricts the penetration and spread of the fungus through the tissue (Figure 1), which explains the control or reduction of severity in plots with application of silicon.

The application of silicon in these cases, increasing disease control leaves in wheat, confirms the involvement and importance of this element in culture defense system, demonstrating the potential of this element in cost reduction with applications of fungicidal chemical products.

Article published in issue 228 of Cultivar Grandes Culturas, May, 2018.