Fertile soil reduces nematodes

In addition to increasing plant tolerance to damage from the main nematodes that attack soybean roots, correct soil management, focused on the continuous improvement of soil fertility, contributes to the formation of an unfavorable (suppressive) environment for these parasites, reducing their populations over time.

The soil environment can become suppressive to nematodes through changes in attributes related to biology (biotic factors) and/or chemistry and physics (abiotic factors). In the first case, there is conservative biological control, defined as the modification of the environment or management practices adopted in order to protect and favor organisms with an antagonistic effect to reduce the population density of nematodes and, thus, their damage. . Biological control agents favored by improving the environment or adopted management practices can be introduced/applied or present naturally in the area.

The improvement of the soil environment aiming to optimize the conservative biological control of nematodes is based on the principles that govern the direct planting system (SPD), that is, the continuous supply of organic material via straw and roots, the diversity of plant species, the permanent cover and at least revolving. The addition of organic material to the soil, especially via the roots and straw of the plants that make up the production systems, provides energy and nutrients necessary for the soil biota. This contribution must be continuous over time, as the stimulation of soil biota and, consequently, the effects on the suppression of phytonematodes, occur more intensely during the initial phases of decomposition of the fresh organic material added. It is also important to highlight the importance of the presence of living roots that are well distributed in the area for as long as possible, as the continuous release of organic compounds and other nutrients by the root system (exudates) stimulates biological activity in a thin layer of soil around the area. of the roots (rhizosphere).

In addition to the organic compounds and nutrients provided by plant biomass, increased biological activity involves the formation of a physical and chemical environment compatible with soil life. No soil organism, whether native or introduced/applied, can be expected to be effective in suppressing nematodes in a mechanically disturbed, high temperature, low humidity, and excessively acidic soil environment. Therefore, permanent coverage, minimal soil disturbance and acidity correction accordingly, as recommended by research, are basic conditions for the biological suppression of phytonematodes, providing chemical conditions, shelter, humidity and temperature favorable to soil biota.

Finally, the greater the diversity of soil biota, the greater the probability of there being an organism with an antagonistic effect on nematodes. As each plant species favors certain classes and species of soil organisms, mainly in the rhizosphere, the increase in crop diversity in the production system directly reflects on the diversity of soil biota and, thus, on its suppressive effect on phytonematodes.

An example of how adequate soil fertility management favors the conservative biological control of nematodes is shown in Figure 1, drawn from the work of Stirling et al. (2011). This work was carried out in sugarcane areas (harvesting the green plant, with the maintenance of crop residues on the soil surface), carried out on clayey Oxisols in Australia. The most superficial layer of the soil, with the highest content of total organic carbon (TOC) and, above all, the labile fraction (CL, corresponding to the organic fraction most accessible to soil biota), presented greater microbial activities and fungal populations, which , in turn, was reflected in a greater density of non-plant parasitic nematodes, including predators of other nematodes (carnivores). As a result of greater microbial activity, and greater populations of fungi and predatory nematodes, the population density of plant parasitic nematodes was approximately three times lower in the surface layer, with a consequent reduction of 90% in the percentage of parasitized plants and an increase of the root health note.

In summary, the addition of organic material to the soil surface led to changes in the structure of the soil organism community, favoring natural enemies of plant parasitic nematodes, thus forming a suppressive environment for nematodes in the surface layer of the soil.

In Figure 2, the results of a survey of phytonematode populations carried out by Embrapa Soja and Cocamar in commercial soybean crops in the Northern region of Paraná, managed under production systems with different levels of crop diversification, in the 2019/2020 harvest are presented. .

Although the population densities of the four main nematode species diagnosed were generally low, it was observed that the values ​​were 1,8 to 18,7 times lower in areas that adopt improved production systems, with greater diversity of plant species and production of straw and roots, compared to those that adopt the standard system, based on soy/corn or soy/wheat successions.

It is noteworthy that the majority of crops that make up the improved systems are considered susceptible to the diagnosed phytonematodes, showing that the main beneficial effect of these systems was the formation of a biologically suppressive environment.

Another strategy for forming a suppressive environment involves modifying chemical and/or physical characteristics of the soil to values ​​outside the range favorable to nematode survival, reproduction, and damage. This strategy is exemplified in Figure 3, which shows the relationship between the population density of Pratylenchus brachyurus and soil acidity.

For this study, an experiment was conducted in a greenhouse at Embrapa Soja, in Londrina, using soils with contrasting textures from areas under natural forest (without the presence of P. brachyurus) located in the Middle-North region of MT. After correction with different doses of lime, the soil was placed in 1L pots and cultivated with soybean plants, inoculated with an initial population of 500 individuals of P. brachyurus per plant. After 65 days of sowing, the root system of each plant was collected and the nematode population density was determined.

The results showed that the reproduction factor (FR, which corresponds to the final population divided by the initial population of nematodes in each plant) decreased with increasing pH in CaCl2 (Figure 3a) and base saturation - V (Figure 3b), regardless of texture. In this sense, the FR decreased from 14 to eight, with the V value increasing from 40% to 60%. Probably, P. brachyurus is a nematode adapted to acidic soils, with low levels of exchangeable bases, so that correcting acidity generates an environment that is chemically suppressive to the nematode. Furthermore, the increase in pH and calcium levels may have contributed to increasing the resistance of roots to parasitism. P. brachyurus, making its reproduction difficult.

On the other hand, the response of soybean cyst nematode (Heterodera glycines) to soil acidity is different from that presented by P. brachyurus, as shown in Figure 4. While P. brachyurus was favored in the control without liming, the population density of H. glycines was approximately four times lower in the same area. It is noteworthy that the average pH values ​​in CaCl2 and V in the 0cm-20cm layer were, respectively, 4,83% and 35% for the control without liming and 5,17% and 47% with limestone application in September 2014.

Thus, unlike P. brachyurus, H. glycines is best adapted to soils with high pH and high calcium and magnesium contents. It is important to highlight that managing soil acidity more or less, aiming at the formation of an environment that suppresses H. glycines ou P. brachyurus, must take into account the minimum requirements of soybeans, so that the crop's productivity potential is not directly harmed by very high or low pH values.

In view of the above, it is clear that improving the chemical, physical and, above all, biological fertility of the soil, through the adoption of management practices in line with the fundamentals of the SPD, provides a suppressive environment for phytonematodes, keeping their populations in good health. low levels and without causing damage to the soybean crop.

By Henrique Debiasi Julio Cezar Franchini Waldir Pereira Dias Edison Ulisses Ramos Jr. e Alvadi A. Balbinot Jr., Embrapa Soy

Article published in issue 282 of Cultivar Grandes Culturas magazine