Soybean production in Brazil grows, but Asian rust puts harvest at risk

Brazil has remained the world's largest soybean producer over the past four years, with more than 70% of its production destined for export. 

By November 2024, the country had recorded US$42.08 billion in foreign sales, and the 2024-2025 harvest had already reached 90% of the expected area, according to data from the National Supply Company (Conab). However, Asian rust continues to be a critical challenge, putting crop productivity at risk.

Caused by the fungus Phakopsora pachyrhizi, Asian rust is found in all producing regions of Brazil and can reduce productivity by up to 90%. The disease cycle begins with the dispersion of spores produced on host plants during the off-season, which are carried by the wind and deposited on soybean leaves. When the temperature (between 18°C and 26°C) and humidity (leaf wetness for at least six hours) conditions are favorable, the spores germinate, and the fungus penetrates the leaf, colonizing its tissues.

The most common symptoms include small spots that evolve into visible lesions of gray, brown, or brown coloration, mainly on the underside of the leaves. In addition, small protrusions, called uredia, similar to blisters, appear and early defoliation occurs, impairing the development of the grains. Each uredia can generate spores for about 21 days, spreading the infection to other parts of the crop or even to more distant areas. At the end of the cycle, the fungus can survive on soybean plants, in guaxas or tigueras, and even in crops under irrigation.

Strategies for combating Asian rust

Given that the fungus has high genetic variability, developing resistant cultivars is a challenge. Therefore, chemical control is the most effective alternative. ″To ensure efficacy, the producer must choose the appropriate fungicide and apply it at the right time, always following the manufacturer’s instructions to avoid pathogen resistance,″ said Fábio Kagi, Regulatory Affairs Manager at the National Union of Plant Protection Products Industry (Sindiveg).

In addition, constant crop monitoring is essential. Regular inspections help detect the first signs of the disease, allowing a quick response. Alternating soybeans with other crops can also reduce the incidence of the fungus in the soil. Removing volunteer plants and other hosts is another critical measure to prevent the pathogen from taking refuge. Integrated crop management also involves adopting a sanitary gap (with a minimum period of 60 days without soybeans in the field during the off-season), adjusting the area to be sown with the spraying capacity, and maintaining balanced fertilization.

In addition, it is vital to concentrate on sowing at the beginning of the season indicated for each region, as this favors the development of soybeans in conditions less prone to disease. Proper spacing between plants also facilitates the penetration of fungicides into the leaves, increasing their effectiveness.

Roots affected by rot

Another problem affecting soybeans is root rot, which can cause major crop losses. Different pathogens, such as Phytophthora sojae, can cause the disease. Its symptoms include seed rot, seedling death, and primary and secondary roots, which result in wilting and plant death. This type of infection occurs in acidic, cold, and waterlogged soils. Prevention involves the use of resistant varieties, good soil drainage, crop rotation, and the application of specific fungicides.

Root rot can also be caused by Fusarium spp, whose symptoms include plant wilting and root discoloration. This disease develops in compacted soils with excess moisture. The solution to combat it involves improving drainage, avoiding excessive nitrogen use, and performing crop rotation.

″Continuously monitoring the crop and acting quickly when identifying the first signs of disease are essential attitudes. The use of appropriate management strategies, combined with the efficient use of pesticides, guarantees the health of the soybean and its profitability,″ Kagi said in conclusion.