In recent years, plant-parasitic nematodes have been one of the main groups capable of limiting food production, driving the pace of cultivation in some producing regions of Brazil. The main species associated with the decrease in productivity of soybean crops are root-knot nematodes (Meloidogyne javanica e M. unexplained), the cyst nematode (Heterodera glycines), the lesion nematode (Pratylenchus brachyurus) and the reniform nematode (Rotylenchulus reniformis). In addition to these species, there has been an increase in occurrence, as is the case of Helicotylenchus dihystera, Scutellonema brachyurus, Tubixaba tuxaua e Aphelenchoides besseyi. This is partly due to the highly intensive and productive production system model, linked to conditions conducive to the growth of these soil microorganisms.
In surveys carried out by the Phytus-RS Institute in recent harvests, the occurrence of the main species associated with soybean cultivation was observed. In an analysis of 5000 soil and root samples, the following species were found: Meloidogyne spp. (M. javanica e M. unexplained), P. brachyurus, H. dihystera, R. reniformis e H. glycines. Analyzing these data and considering the occurrence of species at three population levels, 100, 200 and 300 individuals/specimens per 100 cm³ of soil and/or per gram of root, we observed the species with the greatest predominance in the South region, being in the roots Meloidogyne spp (M. javanica e M. unexplained) and on the ground H. dihystera.
In these results the root-knot nematode Meloidogyne spp. (M. javanica e M. unexplained) currently recognized as one of the greatest problems for soybean crops, due to its widespread occurrence and its ability to limit productivity, was the species with the greatest predominance in the roots of the soybean plants analyzed. The increase in the population of this species in crops is related to several factors, the main ones being associated with the agricultural model adopted in recent years, the use of a narrow genetic base, the continuous cultivation of susceptible host plant species and the lack of diagnosis of this species present in productive areas.
The integration of nematode management techniques is essential. It begins with the identification and quantification of nematode species present in the area, the use of cover crops, after the selection of resistant cultivars (when available), and the use of biological nematicides.
Microorganism-based nematicides have grown significantly in recent years. This technology uses beneficial microorganisms to regulate the activities and populations of plant nematodes. The main groups of biological control agents with formulations for use in the management of plant nematodes are composed mostly of fungi and bacteria. Currently, there are more than 30 biological nematicides registered for the management of plant parasitic nematodes.
These products can be applied via seed treatment or seed furrow treatment. However, the mechanisms of action between bacteria and fungi are distinct. Fungal bacteria, under favorable environmental conditions, act by parasitizing or preying on eggs, juveniles and adults present in the soil. Bacteria act by colonizing the root system (biofilm) and altering the composition of root exudates, thereby confusing nematodes about the location of the root and thus contributing to energy expenditure, thus depleting all their reserves.
Another mechanism involved in both control agents (bacteria and fungi) is the production of substances capable of directly interfering with the infection, development and reproduction of phytonematodes. The main toxins/enzymes excreted are proteases, chitinases and lipases. These toxins can act in different phases of the biological cycle of phytonematodes, from the egg to the nematode-plant interaction phase. In addition to these characteristics, nematicides can also aid in the development of the root system, accelerating its growth, leaving the root system exposed to phytonematode infection for less time.
Furthermore, if the control agent is used on cover crops in the off-season, it will provide greater protection for the roots, and may directly interfere with the juveniles and eggs present in the soil, thus causing a greater reduction in the nematode population. Even when biological products are used on cover crops susceptible to the main nematode species, an improvement in the development of the root system of soybean plants can be observed, due to the morphological characteristics of these plants, which play an important role in the physical, chemical and biological aspects of the soil.
However, the benefits of using nematicides on cover crops and the response of soybean cultivars in terms of productivity, grown after this integration, are still unclear. In order to better understand the interaction between cover crops and biological nematicides and their impact on productivity, a field experiment was conducted using biological nematicides based on Bacillus firmus e Trichoderma harzianum associated with three cover plants, Black Oat (Avena strigosa Schreb), White Oats (Avena sativa L.) and Forage Turnip (Raphanus sativus L.) and two soybean cultivars (Pioneer 95R51 and Monsoy 5917 IPRO).
The trial was conducted in a commercial soybean production area naturally infested with M. javanica (root-knot nematode) and P. brachyurus (lesion nematode) located in the municipality of Júlio de Castilhos - RS. After soybean cultivation, cover crops were sown in strips of 24 by 36 meters, treated with the respective nematicides (Figure 1). The infestation level of the area before the installation of the experiment varied from 20 to 30 second-stage juveniles per gram of root for M. javanica (root-knot nematode) and for P. brachyurus (lesion nematode) from 19 to 25 specimens.
After the cultivation of these cover crops, desiccation was carried out and the soybean cultivars Pioneer 95R51 and Monsoy 5917 IPRO were sown. At 60 days after emergence, the population density in each cultivar was evaluated and at the end of the cycle, the response in productivity. Analyzing the effect of the use of biological nematicides in the cover crops and its reflection in the soybean cultivars, it was found that the number of M. javanica e P. brachyurus in the roots of both soybean cultivars at 60 days after emergence, varied between the treatments with cover crops and biological nematicides, and in all plots with application of biological nematicides in the cover crops, reductions were observed in the final numbers of both nematodes associated with the root tissue (Graphs 1, 2, 4 and 5). For the productivity variable, there were numerical increases for both cultivars, which were significant when comparing the treatments with application of biological nematicides in the different cover crops versus the controls without application (Figures 3 and 6).
In general, when comparing the increase in productivity of soybean cultivars associated with cover crops and biological nematicides (Figures 3 and 6), an increase in kg/hectare was observed in both cultivars, reaching values that varied from 207 to 379 kg/ha for cultivar Pioneer 95R51 and from 330 to 397 for Monsoy 5917 IPRO, regardless of the cover and associated biological nematicide.
These results may be associated with the benefits of using these cover crops, as well as the use of biological nematicides. However, the use of nematicides in cover crops is not yet a widely used technique. The current model for using these technologies (nematicides) has contemplated their use only in the main crop. In the case of a group of microorganisms that are very difficult to control (phytonematodes) and impossible to eradicate, perhaps anticipating the use of these technologies could yield good results, given that the effect of both strategies (cover crops and nematicides) could act directly on reducing the nematode population, preparing the soil for the main crop.
Cover crops have the ability to improve the production system as a whole, due to their ability to act on the physical, chemical and biological attributes of the soil. Furthermore, they are essential against erosion processes and nutrient leaching, as well as for the formation of mulch, an increase in organic matter that is essential for its fertility. Regarding biological products that will have the “role” of reducing the nematode population in the off-season, it is worth noting that this group of microorganisms (fungi and bacteria), in addition to these characteristics, has a close relationship with the rhizosphere of plants. The vast majority of them have the ability to associate with the roots, which results in direct benefits in the development of plants and some of the benefits mentioned above.
Therefore, while the problem with plant-parasitic nematodes continues to grow year after year, new ways of using technologies aimed at reducing the population of these microorganisms in the soil certainly continue to be investigated. And cover crops show enormous potential to add to the management employed. The model involving the application of biological nematicides in the soybean off-season still needs to be investigated. However, given the results observed, it could be an alternative for medium- to long-term management, since the success of specific applications of an immediate nature, when dealing with phytonematodes, is directly linked to the varietal component.
By Paulo Santos, Cristiano Bellé e Andrezza Lopes, Phytus Institute
Article published in issue 287 of Cultivar Grandes Culturas Magazine
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