August Technical Notebook Part 1: Focus on the vegetative

Several diseases affect soybeans in Brazil, causing damage to yield and grain quality. Asian rust is the most important, and accounts for more than 80% of the demand for fungicide applications in the crop. The leaf spot complex and anthracnose make up the second group in importance, followed by white mold and powdery mildew. This order of relevance varies depending on the region considered, sowing time and environmental conditions of each harvest.

Leaf spots and anthracnose (Figure 1) are caused by necrotrophic fungi, which, in the off-season, remain on the seeds and crop residues in the crop. For this reason, they are the first diseases present in the crop, right in the vegetative phase, as soon as protection through seed treatment is exhausted. The younger the plants, the greater their susceptibility to these fungi, which can also infect and colonize plants without producing symptoms in all of them. In practical terms, in the vegetative phase of the soybean crop, there are many more infections present in relation to those that quantification by symptoms can determine.

The importance, need, benefit and, consequently, the decision to apply fungicides in the vegetative phase of soybean crops depend on several factors. Continuous cultivation of soybeans in the same areas ensures the presence of leaf spots and anthracnose every year. What varies is its intensity, proportional to the temperature and frequency of rain in the initial phase of the crop. In regions with milder temperatures, there may be less initial pressure from these diseases, but powdery mildew and also rust (asymptomatic) are present, as occurs in southern Brazil, in several locations. Therefore, there is always the presence of diseases to be controlled in the vegetative phase of soybeans. What changes is the prevalence of one over the other and its intensity. Therefore, the benefit of applying fungicides is also variable.

The feasibility of applying fungicides in the vegetative phase of soybeans has been studied in some soybean cultivation scenarios. In our experiments, the objective has been to evaluate the contribution of the treatment at two specific moments: pre-closing the rows (ensuring the deposition of the fungicide in the lower third of the plant) and a few days before, during the herbicide application.

In the last 12 years of research, most of them still as a professor and researcher at the University of Passo Fundo, comparing experiments with and without vegetative application (pre-closing between rows), we observed that the average difference was 360kg/ha . In the 2018-19 soybean harvest, with a greater presence of diseases, including rust, in crops established in November, this difference reached up to 654kg/ha. The lack of pre-closing application reduced leaf spot control from 63,4% to 31,7%, and rust control from 69,6% to 40,8%.

In this same 2018/19 harvest, two experiments were conducted in Passo Fundo, Rio Grande do Sul, with soybean cultivar maturity group 5.9, with a variable number of fungicide applications. The objective was to evaluate the contribution of initial and final applications to the crop's grain yield. The results are shown in Table 1, with some photos in Figure 2. In treatments T2 to T6, different fungicides were used, between T7 and T11 a single product. There is a large variation in productivity with the subtraction of applications in the vegetative phase (30 days and 40 days).

Variations in the contribution of fungicide applications are absolutely normal, which is why they must be evaluated regionally, and in different harvests. In 2017-18, in crops in the first half of the sowing window, treatments carried out at the V5 stage provided differences of 222kg/ha. At the other extreme, in the 2018-19 harvest, when sown in the second half of the planting window, the contribution reached 948kg/ha. Carrying out this application becomes an alternative, for several reasons: it controls newly installed infections and prevents the occurrence of others; if carried out with fungicides compatible with herbicides, it allows their joint use and makes the operation of the next application more flexible.

The choice of fungicides to be applied depends on the diseases to be controlled. In soybean monoculture areas, the main focus is on necrotrophic fungi that cause leaf spots and anthracnose. Depending on the region and sowing time, also powdery mildew and Asian rust. The contribution of benzimidazole fungicides is currently very small, due to the resistance already developed by several fungi. Triazoles such as difenoconazole have been very useful. In an experiment conducted in the 2018-19 harvest, in Passo Fundo, with a super-early soybean cultivar, an application of difenoconazole + propiconazole at 30 days provided differences of 378kg/ha to 576kg/ha.

Later sowings, subject to the presence of Asian rust, require more robust combinations, such as mixtures of triazoles, triazoles + multisites, or triazoles + strobilurins, as exemplified in Table 2. In this experiment, only the first application varied, while the others were carried out with the same treatment, at each application time, throughout the area. There were differences from 78kg/ha to 948kg/ha depending on the fungicide used, with a greater contribution from mixtures of triazoles + strobilurins, or triazoles + multisites, due to better control of Asian rust.

The next important moment in the application is the pre-closing of the lines, which aims to control leaf spots, anthracnose, powdery mildew and rust. This already requires the use of more complete options, including reinforcements, which can be with triazoles or multisites (mancozeb, chlorothalonil or cuprics). The choice of fungicides and reinforcements will depend on the predominant diseases in the crop, depending on the sowing time and environmental conditions. Table 3 presents the average result of two similar experiments, conducted in two locations (Passo Fundo, Rio Grande do Sul, and Cotripal Condor-Rio Grande do Sul) and two cultivars. Application in V7 reduced the severity of end-of-cycle diseases from 13,5% to 4,9% to 7,2%. Regarding rust, the reduction was from 46,7% to 28,4% to 38,2%. The impact on grain productivity ranged from 306kg/ha to 525kg/ha.

As previously mentioned, the contribution of fungicide applications in the vegetative phase of soybeans varies according to the history of each crop, depending on the region, sowing time, environmental condition and disease prevalence. Therefore, management recommendations must be regionalized and adjusted to the conditions of each soybean crop.

a

Carlos A. Forcelini 

Doctor in Phytopathology

Rafael Roehrig

Master in Phytopathology