Integrated management of soybean diseases

Soybean production in Brazil is affected by a wide range of diseases, caused by soil and aerial fungi, favored by local characteristics such as the prevailing climate in the country's growing regions. Managing them is a major challenge that involves integrating genetic, legislative, cultural, physical, biological and chemical methods in search of a balance between the sustainability of the agroecosystem and a satisfactory financial return to the producer.

Soybean cultivation was consolidated in Brazil and became one of the main commodities, representing an important slice of the Brazilian economy. The country is the second largest producer of this oilseed in the world, which occupies extensive areas of cultivation annually.

Soybean production, on a large scale in Brazil, and the lack of care in sustainable management results in the current scenario, in relation to the phytosanitary state of the crop, with an increase in different diseases and many that have started to present greater severity, both in the aerial part , as well as in the root system.

The use of modern techniques for soybean production, essential for large scales such as in Brazil, brought good solutions to meet demand, but also provided some natural imbalances, evidenced in changes in crop pest and pathogen populations.

The soybean cultivars in use are generally composed of populations with the same genetic makeup. If a disease occurs in this cultivar, all plants in the crop can be attacked by the pathogen that causes this disease, making its control more difficult.

The Brazilian climate, which is tropical to subtropical, also favors the survival of pests and pathogens in the field. Countries with cold climates have a natural break, climate stress, which can interrupt the life cycles of pests and pathogens, which contributes to their reduction or even eradication.

In relation to chemical control, the soybean crop already suffers from populations of pathogens that are less sensitive to active ingredients from the triazole and strobilurin groups, which were highly effective when launched on the market and, a few harvests later, no longer show this effectiveness in the soybean pathosystem. x Phakopsora pachyrhizi.

To control diseases in soybeans, as well as other crops, the ideal is to use all possible forms of control, integrated disease management. It is important to constantly monitor crops, from sowing to harvest, and use the control methods indicated at each stage of the crop.

Many diseases can occur in soybean crops and more than 40 have already been described in Brazil, the majority caused by fungi that are transmitted and/or spread by contaminated seeds. Diseases can be caused by pathogens in the aerial part and inhabitants of the soil, which cause root system rot and plant death.

Main diseases of the aerial part of soybean crops

The main aerial part diseases, pathogens, ideal conditions for their development and survival are shown in Table 1.

Stem canker diseases (Diaporthe aspalathi) and frogeye spot are controlled through genetic resistance to pathogens, incorporated into the soybean cultivars in use.

Symptoms of the main diseases of the aerial part of soybean crops

 Diseases caused by soil-dwelling fungi

Table 2 lists the main fungi that cause rot in the root system, inhabitants of the soil, and ideal conditions for their development and survival.

Asian soybean rust

In the current phytosanitary scenario, the disease that causes the most concern for soybean producers is Asian rust.

Sequence of recommended measures against Asian rust

Health Vacuum: most Brazilian states adopt this period, which consists of the absence of a live soybean plant for 60 days to 90 days, as determined by each state. The objectives are to reduce the inoculum of Phakopsora pachyrhizi, delaying the onset of the disease, reducing fungicide applications, reducing application risk and reducing production costs.

Start sowing, if possible, right after the Sanitary Vacuum, in soil analyzed and corrected and with balanced fertilization, giving preference to cultivars with an early cycle, with the aim of escaping rust, or in the event of it, suffering low inoculum pressure.

Give preference to cultivars that are upright and do not become lodging, to provide greater aeration to the crop, which can reduce humidity. If available, use cultivars with resistance to Phakopsora pachyrhizi, but even these cultivars need to be complemented with chemical control, in some situations. 

Monitor the crop, from emergence, paying attention to weather conditions. The fungus Phakopsora pachyrhizi, which causes rust, develops well in rainy periods, high humidity favors it and has a wide temperature range, from 8 to 30 °C, with the range from 18 to 21 °C being optimal for infection. Pay close attention not to miss the exact moment to start chemical control, preventively.     

Control with fungicides formulated in a mixture of different chemical groups has proven to be efficient. It is important to rotate products with different modes of action, to avoid an increase in the population of P.pachyrhizi less sensitive to the active ingredient and provide a longer shelf life to the product. The recommendations of the company that owns the product must be followed. The main groups are triazoles, strobilurins and carboxamides.

On site  http://www.agricultura.gov.br/servicos-e-sistemas/sistemas/agrofit There are fungicides recommended for controlling rust and other diseases. 

Integrated management of soybean diseases

The objective of integrated disease management (IDM), which includes genetic, legislative, cultural, physical, biological and chemical methods, is to obtain the maximum expression of the genetic potential of the cultivar, with high productivity and good quality of the harvested soybeans. These methods must be used together, considering the sustainability of the agroecosystem and the satisfactory financial return to the producer.

In the MID, the soybean crop is 'constructed', from choosing the area and sowing time, observing crop rotation/green manure, to reduce the inoculum potential of pathogens, direct planting (sustainable practice), correction of soil, balanced fertilization, cultivar with genetic resistance to pathogens, good physiological and sanitary quality of seeds, seed treatment, close monitoring of the crop, biological control and chemical control. 

Margarida Fumiko Ito, Agronomic Institute (IAC)

Article published in issue 206 of Cultivar Grandes Culturas.