How to successfully manage diseases in the soybean crop

Soybean cultivation occupies an estimated area of ​​36,9 million hectares in Brazil, with 12 million hectares located in the South Region (Conab, 2020), consolidating the importance of the crop for Brazilian agriculture. This importance increases the need for management practices that minimize production losses due to the occurrence of diseases. However, in the field there are still doubts regarding the diagnosis and identification of the diseases that are present.

Brown spot, caused by the fungus Septoria glycines, is the first disease to be detected in the plant due to its ability to survive in crop residues and to present symptoms already on the unifoliate leaf (Figure 1). Cercospora leaf blight, caused by Cercospora kikuchii, occurs throughout the cycle (Figure 2). This pathogen has a high capacity to survive in crop residues and to be transmitted via seeds, where it causes purple spot (Figure 3).

In addition to leaf spots, another disease that has been causing concern in the field is anthracnose, caused by Colletotrichum truncatum. Symptoms begin in the veins of the leaves (Figure 4) and can progress to the petioles and legs (Figure 5), where the greatest damage to productivity occurs.

Powdery mildew, caused by Microsphaera diffusa, was the disease that had the most pronounced occurrence in Rio Grande do Sul in the 2019/20 harvest, due to the lack of rainfall, a condition that is favorable to this pathogen. This disease can be very aggressive in some soybean cultivars, and can cause early defoliation of the plant when the attack is severe (Figure 6).

Asian rust, caused by the fungus Phakopsora pachyrhizi, is the disease with the greatest potential to damage soybean crops. This pathogen attacks the leaves (Figure 7), causing early defoliation (Figure 8) and impairing grain filling. This fact explains the concern that this disease generates among soybean producers.

Both Asian rust and powdery mildew are caused by biotrophic pathogens that require a living host to survive during the off-season. Therefore, the presence of volunteer soybeans in the field is one of the most effective ways to maintain these pathogens in the cropping system. During the winter period in Rio Grande do Sul, soybean plants normally do not survive due to low temperatures. However, the winter of 2020 has presented periods of above-average temperatures and this condition favors the development of volunteer soybean plants. In Figure 9, the presence of powdery mildew on the leaves of the plants is visually detected and, in subsequent laboratory analysis, active spores of Phakopsora pachyrhizi were observed. This fact generates an alert for the 2020/21 harvest, as these fungi will probably be present from the beginning of the soybean sowing period.

In this scenario of risk of disease occurrence for the 2020/21 harvest, understanding the dynamics of the disease complex is essential for successful management. In this sense, it is worth highlighting that the vast majority of soybean areas in Brazil are in a monoculture system and thus there is an accumulation of inoculum of necrotrophic pathogens in the crop residues. This explains the fact that leaf spots and anthracnose are causing a productive impact, to the point of generating concern for producers. This group of fungi is present in the cultivation system from the initial stages of the crop and depending on the environmental conditions at this stage, symptoms of these diseases can be detected in the first 30 days of the soybean's life.

The results obtained in the 2018/19 and 2019/20 harvests show that the positioning of the fungicide program focused on the late occurrence of Asian rust allows other diseases to become established and cause production losses. The production levels of the 2018/19 harvest were higher than in the 2019/20 harvest due to the severe drought that hit Rio Grande do Sul. Even so, significant responses were observed to the positioning of fungicides.

In the 2018/19 harvest experiment (Figure 10), the objective was to measure the productive impact of bringing forward applications for the vegetative stage and making the intervals between applications more flexible. The results demonstrate that the fungicide programs started in the vegetative stage presented, on average, 6,4 sc/ha more than the programs started in R1. This difference is probably related to the presence of leaf spots in treatments where application occurred in R1, around 45 days after soybean emergence. Furthermore, the data shows that making the interval between applications more flexible from 15 to 21 or 25 days results in greater losses when the first application occurs later.

Therefore, if for some reason the interval between applications becomes more flexible in the field, the presence of an active ingredient in the bass leaves provided by the vegetative application helps protect the tissues, preventing the epidemic from establishing itself aggressively. The deposition of active ingredient in the basswood leaves caused by the application of vegetative agent has an impact on disease epidemics, mainly leaf spots, and on the longevity of these leaves due to its effect on the plant's physiology. Therefore, the sum of these effects impacts soybean productivity.

In the 2019/20 harvest, it was expected that productive responses would not occur due to the severe drought that hit Rio Grande do Sul, but the data shows that even with low disease pressure there was a productive impact due to plant protection. Soybeans sown in October still achieved normal yields (Figure 11A). In the second sowing season, water stress worsened and productivity fell considerably (Figure 11B).

The objective of these trials was to verify the effect of the timing of the start of the fungicide program and different periods of soybean protection, being 75 days with five applications or 60 days with four, three and two applications with different intervals. For the experiment installed on soybeans sown in October (Figure 11A), the highest productivity was obtained with five applications, with 13,4 sc/ha more than the untreated area. It is worth noting that at this time the presence of Asian rust was detected with a final severity of 35%. This may explain this result.

The programs with four and three applications did not differ from each other, however with four applications there were almost three sc/ha more than with three applications. In this case, it is worth noting that this harvest was low pressure and probably in years with normal water availability, this difference would be greater. When only two applications were carried out 30 days apart, productivity did not differ statistically from the control, with a delay in the first application and a very long interval, there was a possibility of infection of the plant by the disease.

In the experiment installed on soybeans sown in November, the highest productivity, 12,7 sc/ha greater than the untreated area, was obtained with four applications and not with five applications. This result can be explained by the lack of rainfall that resulted in lower disease pressure. At this time, the programs with three and two applications did not differ from each other, nor from the untreated portion.

What may explain this result is the fact that the plants were subjected to very prolonged water stress and when the interval between applications was longer than 15 days the effect of the fungicide on the plant's physiology was impaired. However, in many areas in Rio Grande do Sul applications were not carried out due to the drought, and it is possible that this has worsened the loss of soybean production.

Another surprising result in this harvest was the positive response to the adoption of multisites in fungicide programs, which is evident in Figure 12. The productive increases due to the association of multisites ranged from 272,6kgh/ha to 416kg/ha of soybeans, indicating the importance of this group of fungicides in the production system.

It is important to highlight that modern cultivars, with high productive potential, generate plants that are increasingly sensitive to interference. In this sense, systemic and multisite fungicides play a fundamental role in protecting the expression of this productive potential.

* By Monica Paula Debortoli, Phytus Institute