Associating fungicides and management strategies essential for controlling soybean rust

Brazilian scientists have concluded that using multisite fungicides, active in more than one metabolic process of the fungus, and associating pesticides with a package of strategies reduces the severity of soybean rust.

This was one of the conclusions of the research network, comprised 32 Brazilian scientists, which evaluated the efficiency of fungicides (registered and in the registration phase) in controlling Asian soybean rust. The studies took place during the 2022/2023 harvest, and the results have just been published in Technical Circular 195: Efficiency of fungicides to control Asian soybean rust, Phakopsora pachyrhizi .

″The results obtained and shared annually by the network have been used to help technicians and producers in determining more efficient control programs for Asian rust, the most severe soybean disease,″ highlighted Embrapa Soja researcher Cláudia Godoy, one of the publication authors.

Published by Embrapa, the publication is the result of cooperative trials carried out by 23 institutions from seven Brazilian states (Bahia, Goiás, Mato Grosso, Mato Grosso do Sul, Paraná, Rio Grande do Sul and São Paulo) and the Federal District.

Network brings together 23 institutions in the fight against soybean rust

Discover the 23 public institutions, universities, research support foundations and consultancies that make up the research network: Desafios Agro, Famiva Agricultural Research and Solutions, Agro Carregal Research and Protection of Eireli Plants, Embrapa Soja, 3M Agricultural Experimentation, Coamo, Desafios Agro, Agrodynamics, Chapadão Foundation, Proteplan, G12 Agro Research and Agronomic Consultancy, Ceres Agronomic Consultancy, University of Rio Verde (UniRV / Campos Agricultural Research), Assist Consultancy and Agronomic Experimentation, Phytus/ Staphyt Institute, Rio Verde Foundation, Tagro - Tecnologia Agriculture, Ide Consultoria Experimental Station, MS Foundation, MT Foundation, Copacol Agricultural Research Center and Agrotecno Research.

Godoy said cooperative experiments have been carried out yearly since the 2003/2004 harvest. ″The results make it possible to monitor changes in the efficiency of fungicides over the years due to the adaptation of the fungus. With that, we take the information so that the producer uses the most efficient fungicides, always in rotation of the different products and adapting the control programs to the sowing time,″ he explained.

Types of fungicides for soybeans

The researcher explained that most fungicides used to control rust belong to three distinct groups: demethylation inhibitors (IDM, "triazoles"), external quinone inhibitors (IQe, "strobilurins") and succinate dehydrogenase inhibitors ( ISDH, "carboxamides"). These groups of fungicides are also called site-specific because they act on specific fungus points. Currently, the fungus that causes the disease, P. pachyrhizi, has mutations that make it less sensitive to these three groups. That is, the microorganism has become more resistant to these molecules.

One of the alternatives to mitigate the effect of resistance is to associate it with multisite fungicides, which interfere in more than one metabolic process of the fungus, registered in the soybean crop from 2013/2014. They have been recommended to increase the efficiency of site-specific fungicides (which act on only one fungal process) and delay the increase in pathogen resistance.

Package of management strategies essential in control of Asian rust

According to Godoy, the fungus that causes the disease can adapt control strategies, by losing sensitivity to fungicides or ″breaking down″ the genetic resistance present in some soybean cultivars. ″To delay the resistance selection process and increase the efficiency of fungicides, all management strategies must be adopted, including the adoption of the sanitary void, sowing at the beginning of the recommended season with early cultivars or with resistance gene, the use of fungicides preventively or at the first symptoms, always with rotation of fungicides and the inclusion of multisites in those that do not have it in their formulation,″ the specialist recommended.

The fallow period is at least 90 days without live soybean plants in the field. The strategy reduces the inoculum of the fungus that needs live plants to survive and multiply. This even reduces the presence of the fungus in the field and, associated with sowing at the beginning of the season, composes a strategy for ″escape″ from the disease. More information on the soybean fallow period, sowing timing and disease management strategies are available here. The Fungicide Resistance Action Committee (FRAC) also brings information on the issue.

In addition to the void, the following are recommended in Brazil: adopting cultivars with resistance and early cycle genes, preference for sowing at the beginning of the season and respect for the sowing calendar, established by regulations defined in each state. The Embrapa researcher explained that the definition of these sowing windows helps to reduce the number of fungicide applications throughout the season. ″This helps to delay the selection of populations of the fungus that are resistant or less sensitive to fungicides,″ she clarified.

Crop monitoring

Network researchers recommend soybean farmers monitor the crop from the beginning of development to define the best time for chemical control, avoiding delays in applications since the fungicides on the market have low curative efficiency. Some regions that use early cultivars and have a second crop with corn or cotton, for example, have managed to ″escape″ from the disease or present late incidence, which facilitates control. ″In regions that sow later, cultivars with a resistance gene and fungicides have provided good control, as long as there are no delays in applications and fungicides are used more efficiently, associated with multisites,″ Godoy explained.

The researcher also informed that in recent harvests, changes in the sensitivity of the rust fungus to fungicides from the triazole group had influenced disease control with the active ingredients prothioconazole and tebuconazole (both active ingredients from the same group). ″The presence of new mutations and the variation in the efficiency of actives from the same group, such as prothioconazole and tebuconazole, reinforces the need to rotate active ingredients within the same group in programs to control Asian rust,″ she pointed out.

The search

The test network carried out four protocols for rust in the main producing regions of the legume. The objective was to evaluate the new fungicides in the registration phase, the fungicides registered without and with multisite fungicides, and also to monitor changes in the sensitivity of the fungus P. pachyrhizi to fungicides.

The Embrapa researcher explained that the experiments were installed on soybeans sown later, in November and December, to ensure a greater probability of the disease appearing due to the multiplication of the fungus in the first sowings. ″Sowing at the beginning of the recommended period is one of the rust management strategies to escape the period of a greater amount of fungus inoculum in the environment,″ she pointed out.

In the case of registered fungicides, all treatments showed lower severity than the control plot, in which no fungicide was applied. The percentage of control of registered fungicides varied between 28% and 69%. ″When registered fungicides were mixed with multisite fungicides, control efficiency increased from 7% to 14%, showing the importance of using multisite fungicides in late sowings, with high rust pressure,″ he stated.

Concerning fungicides in the registration phase, all treatments also showed lower severity than crops without pesticides. Control percentages ranged from 60% to 75%. ″These fungicides are new combinations of actives that are already available on the market,″ he said.

In tests to monitor the sensitivity of the fungus to fungicides with a single active ingredient, among the triazoles, the lowest severities were observed for prothioconazole and tebuconazole. ″The efficiency of these two actives varied between regions and within regions, evidencing the presence of populations with differentiated sensitivity to triazoles and the need to rotate fungicides with these two active ingredients for efficient control of Asian rust,″ Godoy stated.

However, according to the researcher, the low average productivity observed in the treatments of this protocol occurred due to the low efficiency of control of the isolated active ingredients. ″This reinforces that the fungicides used to control Asian rust should always be used in commercial mixtures or tank mixtures for greater control efficiency and to delay the resistance process,″ she pointed out.