Importance of genetic resistance against plant nematodes

In order to produce more soybeans under an intensive and highly productive agricultural system, located in areas with tropical and subtropical climates, an environment with favorable conditions for the occurrence of pests and diseases is observed. Among the various groups of agents that cause injuries to soybean crops, plant-parasitic nematodes have stood out season after season. Distributed throughout all producing regions of Brazil, these microorganisms have caused billions in losses to soybean crops, where currently more than 10 species have the capacity to cause productivity losses for this commodity.

The main species are root lesion nematodes (Pratylenchus brachyurus), soybean cyst nematode (Heterodera glycines), root-knot nematodes (Meloidogyne javanica e Meloidogyne incognita) and the reniform nematode (Rotylenchulus reniformis). In recent years, some species have been reported in some regions, such as Helicotylenchus dihystera (Spiral nematode), Scutellonema brachyurus, Tubixaba tuxaua (giant nematode) and Aphelenchoides besseyi (green stem nematode).

Despite this number of species emerging, the first nematode species reported in soybean crops in Brazil still pose risks in several regions. In studies carried out by Staphyt, located in Formosa, in the central-northern region since 2019/20, the presence of the root lesion nematode (Pratylenchus brachyurus) in more than 75,9% of the soybean samples analyzed in the nematology laboratory, followed by root-knot nematodes (Meloidogyne javanica), with 26,4%, the soybean cyst nematode (Heterodera glycines), 17,0%, and the reniform nematode (Rotylenchulus reniformis), 5,7%. Another increasing species in this study has been the spiral nematode (Helicotylenchus dihystera) with 76,3% presence in the samples analyzed.

These soil microorganisms feed on the root system of plants, causing interference in the physiological processes of the plant, compromising the absorption and translocation of nutrients, thus reducing productivity. The feeding of these microorganisms, when established in the initial stages of seedling development, can also favor the entry of other soil microorganisms such as fungi, further compromising plant development.

Numerous technologies have been developed in recent years to help manage coexistence with these microorganisms. As research on the subject advances, it is shown that, in an integrated manner, it is possible to achieve success in managing and controlling these microorganisms. The main tools currently used are preventive, cultural (crop rotation with non-host plants, cultivation of antagonist plants), genetic (resistant soybean varieties) and protective practices through the use of nematicides (chemical and biological).

One of the strategies used in management that plays an important role is the use of resistant or moderately resistant soybean varieties, since, in addition to combining practicality and environmental safety, they present efficient control responses, reducing losses in the field, as well as the population density of the species in the soil. As the understanding of the problem advances, it becomes more evident that the management of phytonematodes converges towards coexistence and not eradication, and in this regard it is essential to include genetic control in the structuring of management.

Choosing a resistant soybean variety associated with the correct identification of the species or race present in the area will provide greater security for the integration of protective tools (nematicides), increasing the spectrum of control. Currently, some soybean varieties are resistant or moderately resistant to root-knot nematodes (Meloidogyne spp.), the soybean cyst nematode (Heterodera glycines, some races) and the nematode-reniformis (Rotylenchulus reniformis) which are of fundamental importance in the management of these species.

Resistance mechanisms to phytonematodes are divided into passive (pre-infection) or active (post-infection) resistance. The first involves the presence of toxic or repellent substances to phytonematodes; this mechanism can be represented in most non-host plants. The second consists of the plant's ability to react to parasitism through defense mechanisms. The phenotypic expression of plant resistance to nematodes is generally characterized by the hypersensitivity reaction (HR), which consists of the programmed death of cells at the feeding site, thus limiting their development and reproduction.

Within the MIN (Integrated Nematode Management) structure, well-positioned soybean cultivars change the appearance of the crop, indicating a possible path to assertiveness in varietal choice. The difficulty of varietal choice currently contrasts with the speed of new materials being released, which sometimes do not provide information related to the reaction of these species of phytonematodes.

The impacts caused by these microorganisms on the roots are reflected in the aerial part of the plants, and are highly variable, depending on factors related to population density, climate factors, as well as those related to the production environment. A priori, this information in the structuring of the MIN involves the possibility of better positioning of the materials in the areas, however, when this tool is positioned incorrectly, on a certain species, the responses end up being disastrous with significant losses.  

Therefore, one of the concerns with this management tool (resistant cultivars) is its use in isolation. Adjusting these varieties with protection tools (chemical or biological nematicides), linked to a rotation system, is practically vital for the success of a medium to long-term management program. In order to be effective, the management of phytonematodes needs to be thought of as part of the production system and not as an isolated crop, thus bringing greater stability to the system as a whole, keeping the phytonematode population at low levels, allowing for good production. 

By Paul S. Santos, Phytus Staphyte

Article published in issue 298 of Cultivar Grandes Culturas Magazine