Alternative control of root-knot nematode in soybean crops

More than 100 species of nematodes are known, involving around 50 genera capable of causing concern regarding productivity in soybean crops, in the genus Meloidogyne, the species M. unexplained e M. javanica (knot nematodes) are the ones that cause the most damage in Brazil. These are extremely small microorganisms present in the soil, almost invisible to human eyes, but with symptoms that are clearly visible to the farmer's eyes, such as the appearance of lumps in crops and a significant drop in productivity. Nematodes of the genus Meloidogyne They are popularly called root-knot nematodes due to the plant's reaction, which, in response to nematode attack, forms galls on its roots, as shown in photo 1. 

Due to the galls, the plant has difficulty passing water and nutrients through the conductive vessels. Additionally, the appearance of yellowed leaves or necrosis between the veins of the leaf, commonly called “carijó” leaf, may occur, a secondary symptom, indicating that there is no normal water circulation, as exemplified in photo 2. From this, we can interpret that the plant may suffer from wilting, defoliation, reduction in plant size, reduction in the number of pods and grains.

Photo 2: On the left we have a soybean leaflet with a secondary symptom of necrosis between the veins. On the right, a healthy soybean leaflet for comparison purposes.

It is important to understand that nematodes have very limited movement (during the year, probably not exceeding 50 cm) and may even take years to occupy large areas on the property. As a result, the presence of these organisms does not always worry farmers due to the low potential for progress in crop infestation, however, if efficient measures are not taken to manage them, they can end up becoming a serious problem, after all they are hidden in the ground. In addition to this, in sandy soils the development of nematodes is favored in relation to clayey soils and the explanation is in relation to the difference in pore size and the ability to move the water film in these two types of soil.

In a 2022 survey, a loss caused by nematodes was estimated at around R$27,7 billion, in soybean crops alone. The use of measures to control these microorganisms ends up being difficult due to the environment in which they are found, and we must bear in mind that it is practically impossible to eradicate all nematodes from crops, but this does not make it impossible to reduce the damage they cause. Knowledge of the damage caused by nematodes and thinking specifically about soybean cultivation with a view to managing Meloidogyne, it is necessary, first of all, to avoid the entry of this nematode into the area as much as possible, since the main methods of movement/infestation of these organisms are through agricultural machinery and through rainwater and irrigation.

In the case of nematodes, unlike other diseases caused by other agents, all control measures must be taken preventively because once the damage to the planted crop is verified, it is no longer possible to act on it. After verifying the problem in the crop, it is recommended, first, to identify the genus and species of the worm present there by collecting roots with symptoms and soil samples weighing around 200 g, then sending the samples to a laboratory of nematology. Based on this identification, new precautions to be taken in the infected area should be planned to control the problem.

The adoption of methods such as biological control has been growing in recent years, showing promising results in the control of various diseases, in addition to presenting differences in relation to chemical control, ranging from non-contamination of the environment to a lower application cost. Furthermore, the great challenge of current agriculture is to produce sustainably, seeking alternative products, aiming to meet the food needs of the general population, producing more and better quality food.

In soybeans, an increasing number of farmers are seeking this alternative form of control. Researchers point out that the use of biological products combined with other forms of combating nematodes can bring significant results in productivity, an example of this was a Brazilian property in which economic losses were reported several years ago associated with the presence of nematodes in soybean crops, and Just with the use of biological control, the quantity went from 28 bags per hectare to 60 bags, with savings of up to 30% in production costs.

The use of biological control aims to reduce the nematode's potential to attack the plant and reproduce, using bacteria, fungi or viruses that will be applied to the plant or the environment in which it lives, to carry out control. For soybean crops, research has revealed promising results in controlling root-knot nematodes using, for example, soil fungus Paecilomyces lilacinus that parasitizes nematode eggs or bacteria Bacillus sp. producer of nematoxic substances.

The application of bionematicides is generally carried out in the soil and depending on the organism present in the product, the microbiological nematicide can act in different ways, such as hindering the penetration of the pathogen, preventing reproduction or even causing the immediate death of the nematode. The Ministry of Agriculture, Livestock and Supply (MAPA) currently presents few biological products registered for the control of nematodes in soybean crops, however, they are considered efficient as long as they are combined with other control strategies.

The list of these registered nematicides is available at www.agricultura.gov.br, link Serviços – Pesticides/Sistema Agrofit. In addition to biological and chemical control, we can mention cultural control, which is carried out by manipulating pre-planting conditions and during soybean development, with the aim of reducing contact between soybeans and plants. Meloidogyne javanica which can be effective with the use of healthy seeds from resistant cultivars.

Embrapa, in partnership with Fundação Cerrados and Fundação Bahia, launched in 2019, a soybean cultivar with high productive potential and resistance to root-knot nematodes (BRS 7481), in addition to this, there are several other soybean cultivars resistant to Meloidogyne. It is worth noting that the resistance levels of these cultivars are not very high and it is recommended that they be used in succession to a crop that does not host these organisms. Likewise, trap plants can be used, in which nematodes infest the roots and are subsequently eliminated, interrupting the nematode cycle.

Furthermore, it is important to eliminate diseased plants from the area, rotate crops using plants of species that the nematode cannot cause damage, directly affecting the survival of the pathogen, use green manure and rotation with plants that inhibit the reproduction of nematodes and They can later be incorporated into the soil, adding biomass and nutrients and also promoting the growth of natural enemies of various diseases. It is also recommended to clean agricultural tools and machinery before carrying out work in areas that are not yet infested.

In the table below adapted from the Aegro website, examples of crops capable of reducing the population of Meloidogyne in the field, being classified into different colors according to the effect on the nematode (increase, decrease or variable performance in the infestation levels of these microorganisms). Using cotton cultivation as an example, it can reduce levels of M. javanica and increase levels of M. unexplained, the importance of detecting the nematode species is evident to act more accurately in controlling this pathogen.