Efficacy of carboxamides in controlling Asian rust in soybeans

Soybean production needs to be properly cultivated, its planting must have favorable climatic conditions, the ideal temperature for good development is around 30 ºC. When they are above 40 ºC, they cause flower abortion and low pod retention capacity. Temperatures below or close to 10°C greatly decrease growth rates. Temperatures below 20 °C can harm seed germination. Soybeans will only flower when the temperature is above 13°C. The total water requirement is around 450 and 800 mm/cycle, but depends on climatic conditions, crop management and variety cycle. Water demands are highest during flowering and grain filling, when crops require 7 to 8 mm of water per day.

The use of tolerant cultivars is essential for soybeans to achieve high productivity, as well as reducing production costs. Crop management has been changing in recent decades due to important technological advances. Cultivation spread from the south of the country to the southeast. It is currently cultivated in all regions of Brazil.

With soybean cultivation in large areas, the appearance of pests, diseases and weeds is inevitable. In the 2000/01 harvest, Asian rust Phakopsora pachyrhizi it was found in commercial crops initially in Paraguay and then in the State of Paraná. After two years it spread throughout Brazil, causing significant damage to soybean farmers.

Therefore, it is necessary to understand that fungicides belong to the class of fundamental pesticides that effectively control plant diseases. Most cultivated plants are subject to attack by pathogens that cause a significant drop in yield, resulting in less food and lost profits for producers. Most research focuses on the impact of fungicides in agriculture and their effectiveness in controlling plant pathogens.

The use of fungicides can trigger a variety of physiological and biochemical processes, resulting in reduced growth, impaired development of reproductive organs, modification of carbon and nitrogen metabolism, and anticipation of the aging process. The ability of a fungicide to cause temporary or permanent stress to a plant is called phytotoxicity. Among the main vital processes affected, those that depend on oxygen, such as aerobic respiration, photosynthesis and photorespiration, can significantly promote the formation of reactive oxygen species (ROS) and induce widespread oxidative stress in plants.

Symptoms can manifest themselves in different ways. The most common phenomenon is observed in the leaves, the organ most exposed to fungicides, where chlorosis can occur due to partial or total destruction of chloroplasts as shown in the figure below. In advanced cases, the most severely affected tissues are necrotic and even the leaves are deformed and thickened. Some of these symptoms, when severe, can accelerate the aging process and result in leaf fall.

The effectiveness of carboxamides

Asian rust is the main disease of soybeans, being its pathogen Phakopsora pachyrhiza, as mentioned previously. Considering the cost of controlling the disease and the economic loss of reduced productivity, the disease costs Brazil US$2 billion per harvest.

The basic proportions of disease management are preventive and must be implemented together. Among them, it is important to respect the sanitary void, crop rotation and use of fungicides. Crop monitoring is essential. The main strategies are the use of resistant cultivars and the planting time. In any case, fungicides must be applied for control to be effective.

The importance of searching for an effective fungicide is a necessity for the producer, which is why carboxamides are the fungicides chosen to explain how their interaction with cultivars works.

According to Godoy, in Brazil, some strategies are recommended for managing the disease: not sowing soybeans and eliminating volunteers through sanitary gaps during the off-season to reduce fungal inoculation; use early cycle cultivars and sow at the beginning of the recommended season as a disease escape strategy; use of varieties with resistance genes; monitoring crops from the beginning of growth to determine the ideal time for chemical control; preventive use of fungicides. Or reduce the number of fungicide applications throughout the season when defining the onset of symptoms and during planting windows, thus trying to delay the selection of resistant or less resistant fungal populations sensitive to fungicides

Carboxamides correspond to both chemical groups of old and new fungicides. Its first representatives (carboxylic fungicides) date back to the 1960s and were introduced to control diseases through seed treatments, first for wheat and then for several other crops including soybeans. Carboxamides inhibit respiratory enzymes (succinate dehydrogenase) in fungi, preventing them from fully utilizing oxygen and producing energy. In the case of a fungal disease in a plant, the process that demands the most energy is that of penetration into the plant, where the fungus must cross a protective physical barrier typified by leaf tissue.

With carboxamides present in the plant, the main action takes place, causing the fungus to collapse. Therefore, the action of carboxamide is preventive, preventing the fungus from entering the plant.

When applying carboxamide when the infection already exists, the action of this fungicide will only occur in the next cycle of the fungus, which occurs between 8 and 9 days. Therefore, preventing carboxamides during management is extremely important.

The application of efficient fungicides is increasingly necessary, causing farmers to invest in strategies for managing Asian rust. To control it, there are several existing means. There are different product action mechanisms and application intervals. These practices do not eliminate rust, but they can reduce its effects and aid in the effectiveness of other control measures.

By Flávio André Vendramin Bertoti, Federal Institute Goiano - Urutai Campus

Article published in issue 294 of Cultivar Grandes Culturas Magazine