Crops in Rio Grande do Sul show early incidence of Asian soybean rust in the 2023-24 harvest

In January, the Antirust Consortium website recorded 51 reports of Asian rust in Rio Grande do Sul, 96% of which were before the pod filling phase (development stage R5). In other regions of the country, reports of the disease occurred predominantly during the pod filling phase (R5 development stages), which shows an early incidence of the disease in Rio Grande do Sul, warns researcher Cláudia Godoy, from Embrapa Soja. Today, the State has 88 cases of the disease: 37 occurred in December and 51 in January.

Claudia explains that excessive rainfall at the beginning of the harvest in the southern region of the country, mainly in Rio Grande do Sul, delayed the sowing of crops. Consequently, in January, the crops were at different stages of development, from the vegetative stage to pod filling. “In crops sown later, Asian rust may occur earlier, due to the increase in fungus inoculum from crops sown earlier”, she highlights.

The researcher has observed that, on social media, producers and technicians have shared content related to crops with high rust severity, even when fungicides are applied. “This shows that there were control failures”, she assesses. In this sense, Godoy advises producers to carefully observe the choice of fungicides to control the disease. According to her, the most used fungicides belong to three distinct groups (site specific) that act on specific points of the fungus. P.pachyrhizi, which causes the disease: demethylation inhibitors (IDM, "triazoles"), external quinone inhibitors (IQe, "strobilurins") and succinate dehydrogenase inhibitors (ISDH, "carboxamides").

“Controlling Asian rust is increasingly difficult due to the fact that the fungus is less sensitive to these three groups of fungicides”, he warns. “In addition, in recent years a new mutation in the fungus has been reported, influencing the efficiency of fungicides that have the active ingredients prothioconazole and tebuconazole (IDM) in their composition”, he says.

Given this scenario, Embrapa's guidance is for fungicides with these active ingredients to be used in rotation and always with the addition of multisite fungicides - which act at various points of the fungus - to increase control efficiency. “Another warning is that chemical control must be carried out at the beginning of the appearance of symptoms, even if observed during the vegetative period, respecting the application interval of 14 days between applications”, reinforces Godoy. 

Embrapa participates in testing networks with several research institutions where experiments are carried out annually to monitor the efficiency of fungicides to control Asian rust. The most up-to-date information for the 2023/2024 harvest is available in the publication “Efficiency of fungicides for the control of Asian soybean rust, Phakopsora pachyrhizi, in the 2022/2023 harvest: Summary results of cooperative trials (Technical Circular 195), available for free download.  

About Asian soybean rust

Asian soybean rust was first identified in Brazil in 2001, and since then it has been monitored and researched by several public and private centers. According to the Antirust Consortium, this disease, considered the most severe in the crop, can cause losses of up to 90% in productivity if not controlled. The disease management strategies are: the absence of soybean sowing and the elimination of volunteer plants in the off-season through sanitary vacuum to reduce the fungus inoculum, the use of early cycle cultivars and sowing at the beginning of the season recommended as a strategy disease escape and the use of fungicides.