Soybean rust, an old acquaintance that cannot be forgotten

Asian soybean rust, caused by the fungus Phakopsora pachyrhizi, continues to be one of, if not the main concern of producers, especially in the southern region of Brazil. The damage caused by the disease varies between cultivars, regions and crops, but can reach up to 90%. On average over the last 10 harvests, for every 1% increase in rust severity, between 20 and 50 kg ha-1 were lost. Although the disease is somewhat forgotten in the South region, due to three subsequent harvests under the influence of the La Niña phenomenon, the rust returns in the 23/24 harvest with full force.

This is because El Niño, a predominant phenomenon since September 2023, results in a greater distribution of rainfall in the South region and less in the Central-West region. The first soybean harvests in Mato Grosso record low productivity due to one of the biggest droughts ever observed. In the South, high precipitation, associated with the absence of frost and the presence of voluntary soybeans in the field, favored the maintenance of rust inoculum in the off-season. Due to excessive rainfall in Rio Grande do Sul, the wheat harvest was delayed and consequently, soybean sowing, which was concentrated in a short window, practically in December 2023, coinciding with the first cases of rust in commercial crops in Rio Grande do Sul. LOL. The disease triangle for the 23/24 harvest is complete, resulting in greater favorability for the occurrence of epidemics in the South region, not only of Asian rust, but of other important diseases for soybeans, such as leaf spots.

A similar scenario was observed in the 18/19 harvest, which had a significant occurrence of rust in several Brazilian states. In RS, which had above-average levels of rainfall in the spring of 2018, rust was also identified in December, which led RS to be the state with the record for cases of rust reported on the Antirust Consortium website for that harvest ( RS 127 vs. 58 of PR and 54 of MS).

The graph below shows the occurrences of soybean rust reported on the Consortium website between the months of October and January, starting from the 18/19 harvest. The analysis of the first reported cases corroborates the expectation of great rust pressure in RS in the 23/24 harvest, as the number of cases reported now in December was similar to the number of cases reported in December 18/19 (27 vs. 25 ) (Figure 1). Soybean at the beginning of the development cycle, the presence of rust inoculum and high temperatures combined with frequent rains make the scenario challenging for the producer, since he still has in his memory recent harvests in which rust was not the main disease. 

The spores of P.pachyrhizi They spread easily over long distances through wind currents. Under favorable conditions, around seven days separate the contact of the spores with the leaves from the first sporulating lesions, which makes evolution rapid and the disease extremely destructive. Therefore, rust control must be, whenever possible, preventive, even in the vegetative stage and before closing lines. Delays in applications can lead to difficulties in managing the disease and certain losses in production.

The fungicides used to control rust fall into four distinct chemical groups, three of which are site-specific fungicides (which act on specific metabolic processes of the fungus) and one of which are multisite fungicides (which act on more than one metabolic process). Site-specific ones include fungicides that act on the biosynthesis of sterols in the membrane, such as triazoles (Demethylation Inhibitors - IDM) and morpholines, which have mainly curative action; strobilurins (External Quinone Inhibitors - IQe) and carboxamides (Succinate Dehydrogenase Inhibitors - ISDH) that act on breathing and have preventive action. Multisite fungicides include inorganic fungicides, isophthalonitriles and dithiocarbamates, also with preventive action.

The combination of different chemical groups, which act at different phases of the fungus' development cycle, contributes to better disease control, in addition to helping to manage resistance. This is extremely relevant, as controlling soybean rust is increasingly difficult due to the fungus P.pachyrhizi present less sensitivity to the main site-specific fungicides used in management (triazoles, strobilurins and carboxamides).

In recent work, Stilgenbauer et al., (2023) reported new mutations (V130A, I145V and F154Y) to IDM fungicides, which include the most effective active ingredients within this group, prothioconazole and tebuconazole. Now, a total of nine mutations have been identified in the cyp51 gene (required for the ergosterol synthesis pathway in fungi) of P.pachyrhizi (F120L, V130A, Y131F/H, K142R, I145V/F, F154Y, I475T), present in different combinations. The recently identified mutation combinations F120L + V130A + Y131F, F120L + Y131H + I145V and F120L + Y131H + F154Y are associated with different sensitivities to IDMs. The variation in efficiency of triazole fungicides between different regions reinforces the need to rotate these two active ingredients in rust control programs.

In view of these and other mutations that confer less sensitivity to fungicides from the IQe (F129 L) and ISDH (C-I86F) groups, the FRAC-BR recommendation goes beyond rotating different mechanisms of action or chemical groups, it includes the importance of rotating the active ingredients within the same mechanism of action or chemical group (rotation between different triazoles, strobilurins and carboxamides).

Despite reports of lower sensitivity, all chemical groups are useful in rust management. When used in combination there is synergism, greater residual and complementation of modes of action, with an effect on different phases of the fungus' infectious process (Figure 2). 

The use of multisite fungicides (with protective effect) in association with site-specific fungicides in all applications is essential, not only for contributing to resistance management, but for reinforcing disease control, acting directly on the main component of rust progression, the formation and germination of spores, contributing to the reduction of the infection rate of P. pachyrhizi. The rotation of different multisites is also interesting, since there are differences in the control spectrum between the fungicides most used in soybeans: mancozeb, chlorothalonil and copper oxychloride. Rust is not the only disease affecting soybeans and the spectrum of action of the products must be taken into consideration when choosing the best partners for each phase of the crop cycle. Currently, the existence of products already containing multisites in the formulation facilitates routine applications.

In addition to the measures already mentioned, the sanitary vacuum, the use of early cycle cultivars, sown at the beginning of the recommended planting period, the use of cultivars with resistance genes, disease monitoring, the use of fungicides at the recommended doses and intervals by manufacturers and the adoption of appropriate application technology contribute to more efficient management of soybean rust (Godoy et al., 2023).

Every year adjustments are necessary. It is clear that this is not a vintage to look in the rearview mirror. Late sowing and high rust inoculum pressure require early, robust and assertive management in order to maintain good levels of productivity in regions where there will be no lack of rain. 

*Per Caroline Wesp Guterres, Adjunct Professor in the Department of Plant Health at the Faculty of Agronomy at the Federal University of Rio Grande do Sul (UFRGS)