Management of fungi and nematodes in soybeans

The emergence and establishment of soybean seedlings are constantly challenged by pathogenic agents, transmitted via seeds or present in the soil, capable of compromising health in the initial phase of cultivation. It is the case that Rhizoctonia solani e Meloidogyne javanica. The association of chemical seed treatment with nematicide of natural origin can help in managing this type of problem. 

Brazil is the second largest producer of soybeans in the world, with an estimate for the 2016 harvest of more than 99 million tons of the grain. The Southern Region of Brazil alone is responsible for the production of more than 30 million tons of soybeans. Despite the very optimistic scenario for the crop, one of the main difficulties faced has been problems with seedling emergence and establishment. Several pathogenic agents interfere with the health of new seedlings, mainly fungi, which in addition to being present in crop remains, can also be transmitted via seeds. Reboleira death, caused by Rhizoctonia solani It has low transmission via seeds, however, it is common in the soil and in crop residues, due to its polyphagous characteristic. The fungus causes damping-off and post-emergence death. Rhizoctonia solani It is one of the most important, as it causes, in addition to tipping over, soybean blight or late blight and watery and black rot at the base of the stem, at different phenological stages of crop development. In addition to the damage caused by soil fungi, the soybean crop is also attacked by nematodes, which, in case of concomitant occurrence with some soil pathogens, can enhance the negative effect of both, further harming plant development.

It is common in some areas, the association of soil fungi such as Rhizoctonia solani e Fusarium spp. with some nematodes, especially Meloidogyne javanica e M. unexplained, which are two species capable of causing huge losses, especially in the southern region of Brazil. It is known that phytoparasitic nematodes can predispose plants, even if resistant, to the action of phytopathogenic fungi that inhabit the soil.

Controlling soil pathogens, in general, is a difficult task to accomplish. As a result, there is a need to revive the use of natural, biologically active substances against pests and pathogens, as well as the use of biological control. Another factor of great importance that contributed to the interest in natural substances was the advancement of the organic production system. As a result, there was a need to seek agricultural practices with low environmental impact, which could add control, reducing the use of chemical control. Therefore, plants, with their antagonistic properties with antifungal and helminthic action, become an important tool in the area of ​​plant protection and biological control. 

The main advantage related to the use of plant extracts in plant protection, when compared to synthetic products, is due to the fact that compounds, such as saponins, tannins, phenols and polyphenols, are not easily inactivated by these pathogens, having a broad mode of action about pathogens. Therefore, this work sought to evaluate the effect of the fungicides Fluxapyroxad (0,06 L/100 kg of seeds), Pyraclostrobin + Methyl Thiophanate + Fipronil (0,2 L/100 kg of seeds) in seed treatment in combination with QL-agri 35 (40 L/ha divided into two applications). In addition to them, Abamectin 500 FS was used as nematicidal standard. The test was carried out on soil infested by Rhizoctonia solani e Meloidogyne javanica in soybean cultivation.

The incidence and severity of Rhizoctonia solani, as well as the number of nematodes in soybean roots, was reduced by the use of the treatments used (Figures 1 and 2).  

In the evaluations carried out at 14, 21 and 28 days, it was possible to observe that, in the control where fungus and nematodes were inoculated, a greater number of juveniles of M. javanica penetrated into the roots (Figure 2). Treatments containing Fluxapyroxad +QL Agri (0,06 L/100 Kg + 40 L/ha) and Pyraclostrobin +Methyl Thiophanate + Fipronil (0,2 L/100 Kg+ 40 L/ha) provided control greater than or close to 80% up to the 21 DAS of soybeans, therefore showing an increase in the residual of the products, even in a situation of high soil infestation with the pathogen in question (Figure 1). There was a reduction in the nematode population caused by the combination of seed treatments, with QL Agri 35. In this case, at 28 DAS, the reduction in nematodes caused by the combinations of Fluxapyroxad +QL Agri and Pyraclostrobin +Methyl thiophanate + Fipronil were statistically superior to that provided by Abamectin. This fact is related to the synergy between the fungicides and the nematicide QL agri 35, which is a product based on Quillay extract (Quillaja saponaria). The action of QL Agri 35 occurs through contact or ingestion, acting mainly on nematode lipids, thus altering the permeability of cell membranes. 

When considering the severity assessments of Rhizoctonia solani in soybean roots, Table 1 shows that the number of juveniles M. javanica It is highly related to the severity of the disease and the area under the disease progress curve. This correlation is related to the fact that nematodes penetrate the roots, opening a “door” that facilitates the entry of soil fungi. As an example of what was observed regarding the interaction between nematodes and soil fungi, the pathogenicity of Pratylenchus spp. it can be influenced by interaction with other pathogens, mainly soil-dwelling fungi. The most frequently reported interactions are with fungi that cause wilting, of the genera Fusarium sp. It is Verticillium sp.. These interactions between the nematode and the fungus are considered synergistic, that is, the association between the two pathogens results in damage greater than the sum of the damage caused by each isolated pathogen. Because of this, it is important to combine fungicides with higher control residuals.

In this sense, it is important to always think in the context of integrated disease management, seeking to combine techniques that together reduce the negative impact of microorganisms on the agricultural system, preferably, with the lowest possible environmental impact. By using products based on plant extracts, it is possible to add residuals to existing products, improving disease control.

Caroline Gulart, Paulo Sergio Santos, Phytus Institute; Ricardo Balardin, Jacson Zuhl, UFSM

Article published in issue 204 of Cultivar Grandes Culturas.