Reaction of soybean cultivars to root-knot nematodes

Root-knot nematodes, Meloidogyne javanica, are among the main phytonematodes present in Brazil, capable of attacking soybean roots and causing severe damage to the crop. The continuous cultivation of varieties with a high pest reproduction factor has worsened the losses. Hence the importance of constantly studying the behavior of cultivars in relation to this species.

Soybean cultivation has become challenging due to the high diversity of phytosanitary problems that affect the crop. If we only analyzed the disease scenario, it would be scary. In this group, problems that occur in the seed, roots and aerial part of plants can be grouped. Regarding organisms that can attack soybean roots, phytonematodes stand out, distributed throughout all producing regions in Brazil and capable of causing significant damage. Annually, the incidence of nematodes has been increasing, increasing problems of soil compaction or even the incidence of other soil pathogens. As the symptoms can be varied, depending on the complexity of the interaction between phytonematode species and the different soybean cultivars, several problems observed in the crop are, erroneously, attributed to other factors, minimizing a problem that actually has worrying dimensions.

This soil pest feeds on the root system, causing interference with the plant's physiological processes, compromising the absorption and translocation of nutrients, reducing productivity. The feeding of these microorganisms, when established in the initial stages of development of seedlings of susceptible soybean varieties and depending on the population density present in the soil, can favor the entry of soil fungi that infect the root cells, compromising the development of these seedlings and the initial stand. .

In surveys carried out by the Phytus Institute, Rio Grande do Sul, from 2012/13 onwards, the presence of the root-knot nematode has been observed (Meloidogyne javanica) in more than 70% of the sampled areas (Figure 1), followed by soybean cyst nematode (Heterodera glycines) and root lesion nematode (Pratylenchus brachyurus).

One of the possible explanations for the high occurrence of m javanica, almost always causing high levels of damage, is the continuous use of soybean varieties with a high reproduction factor. As a result, the Phytus Institute develops studies to determine the reaction of 20 (twenty) soybean varieties to the root-knot nematode M. javanica, in a greenhouse, year 2014/15 (Table 1). Studies have been conducted in pots with a capacity of 2,8 liters filled with soil and sand (2:1). The soybean varieties were sown and, after emergence, a total of 5000 eggs and juveniles were inoculated. Meloidogyne javanica. After 60 days of inoculation, all soybean varieties were evaluated, using the reproduction factor (FR) determined by the division between the final population and the initial population (5000), according to the methodology described by Oostenbrink (1966), considering resistant cultivars with FR 1,00. (Graphic 1,00).

The Reproduction and/or Multiplication Factor refers to the nematode's ability to reproduce M. javanica on a given host plant, in this case, soybean cultivars. It is important to highlight that these results were obtained under controlled conditions (nematode population 5.000/plant for each variety), using the reproduction factor methodology. Only reproductive capacity was considered, as there are other assessment methodologies that analyze other aspects, such as visual damage caused to the host. Although this is a limited method, it provides good indications about the behavior of the varieties.

The results obtained showed different levels of multiplication between cultivars, which indicates a scarcity of materials with a low reproduction factor. M. javanica. With each harvest, numerous soybean cultivars emerge, requiring continuous study of their reaction to species of phytonematodes important for each region.

If an annual management plan is created, in addition to the varietal reaction, the contribution of different winter crops to the population density of M. javanica becomes of great importance for reducing the population on the ground. In this case, in addition to reducing the impact of phytonematodes on soybean productivity, possible selection pressure exerted by different populations on varieties that present genetic mechanisms linked to resistance would also be reduced. As a rule, resistant soybean varieties must be adjusted in a management program, using rotation/succession with non-host crops.

Advances in genetic improvement for the launch of resistant cultivars have been evolving. The resistance mechanisms that act in the nematode/soybean pathosystem involve factors present at two moments: before penetration of the organism, through the release of root exudates that have chemical repellent characteristics, nematicidal action. And after nematode penetration, through the action of elicitors that trigger biochemical events, preventing the establishment of nematodes.

In general, the producer must alternate the rotation or succession of non-host crops, in order to reduce the initial population from one harvest to another, allowing coexistence with this organism and preserving sources of resistance.

Paulo Santos, Andrezza Lopez, Gracieli Rebelato and Maiquel Frigo, Instituto Phytus; Pablo Tuzi Serafini and Jacson Zuhl, Federal University of Santa Maria

Article published in issue 203 of Cultivar Grandes Culturas.