Proper management of nematodes in crops

Nematodes plant parasites, present in practically all agricultural soils, are a source of growing concern among producers. With a large number of species capable of causing reductions in production in crops such as soybeans, corn, cotton and sugar cane, the management of these microorganisms demands succession adequate cultivation, promotion of ideal conditions for good plant development and integration of chemical, biological and genetics on the market.     

The relevance of producing more food, with fewer resources, in an intensive and highly productive agricultural system, allocated in tropical and subtropical scenarios, also contrasts with the conditions extremely favorable for the occurrence and incidence of pests and diseases. Factors that occur during any phase of the crop development cycle and which culminate in the reduction of productivity.

The group of microorganisms that have acquired of concern in recent years are plant parasitic nematodes. Prague soil, biotrophic organisms (depend solely and exclusively on plants to feed and reproduce), have a high degree of polyphagy (capacity of parasitizing a vast number of plants from different botanical families) and is present in practically all types of agricultural soil. There is approximately 20 years ago, the problem involving these microorganisms was practically restricted to two species of economic importance, both belonging to the same gender, a situation that is already outdated today. Considering some crops, such as: sugar cane, corn, soybeans, cotton, beans, among others, the number of species capable of causing reductions in the production of these crops is quite broad (Figure 1).

Symptoms on plant roots infected by phytonematodes, in general, can vary from thickening in the root system (known as galls), decrease in volume and growth (mainly in secondary roots), and partial destruction (accompanied by dark lesions in several spots). This complex of insults compromises the physiological functioning of the plant, directly interfering with the flow of absorption and translocation of water and nutrients, culminating in the reduction of productivity.

In crops, symptoms in the aerial part of plants will occur in patches or patches, formed by plants yellowish, followed by irregular growth and, when in longer periods dry, “summer” conditions, can cause plants to wilt. However, it is not a rule for all crops, as it depends on the phytonematodes involved, its population density present in the area and the factors genetics linked to the susceptibility of the crop to parasitism. The example Furthermore, it is possible to mention the cultivation of sugar cane and corn, which are more tolerant to attacks by these parasites, and may or may not present such symptoms.

A widely used control method used is crop rotation, a practice that consists of exchanging or replacement of a given host crop with one resistant to nematode. In Brazil, considering the main intensive cultivation systems, the adoption of this important tool is almost always not used, due to issues linked to work logistics, crop limitation, among others. In reason Furthermore, most producers adopt the crop succession scheme, however not with the objective of controlling nematodes but rather maximizing land use, which can provide favorable conditions for the increase of these microorganisms in the environment. However, it is possible to use succession appropriately to reduce the increase of this pest in the soil.

Currently, the progressive increase in this group of parasites has been warned by many researchers and research institutions research, pointing out the countless cases of areas where the productive potential has already been is linked to good coexistence with this soil pest. It should be noted that, the eradication or elimination of these species, when established in areas of cultivation at damaging levels is practically impossible, mainly due to remarkable capacity for survival and competition of these microorganisms in the soil, due to the issue of costs and operational time.

If we analyze some of the factors associated with the increase in this soil pest, taking into account its cycle biological, at least three important events are interconnected.

The first linked to the continuous cultivation of susceptible crops over the years, a factor that has contributed significantly to the rapid population growth of these species in soil, as well as its preservation. The second aspect is related to the lack of care with the transit of agricultural machinery and implements from infested areas, to uninfested areas, significantly contributing to its spread. It is worth mentioning here that the mobility capacity of this group of microorganisms on the ground it is very low, close to one meter per year. The third point, no less important is the diagnosis of the problem, when many of the damages caused by phytonematodes are not credited by producers. Whether due to lack of knowledge, confusing them with other events, such as: soil compaction, waterlogging, nutritional deficiency, and other soil pathogens, or even Even, in disbelief with the plague that, in the beginning, causes subtle losses, a situation extremely momentary, as populations evolve quickly on the soil of a harvest to another.

In recent years, the Phytus Institute has been monitoring several crops with serious nematode problems. However, what calls attention is that in most problem areas, the scenarios that are observed are from soils with low fertility, very low levels of organic matter, soils without cover and, in some cases, visibly compacted. By analyzing these points it is possible to understand the how complex the task of any professional is in measuring losses, arising from the parasitism of these microorganisms. In addition to these points, factors climatic conditions, intrinsic characteristics of each species, the characteristic genetics of crops linked to the defense response against attack by different species of phytonematodes, as well as the population density of these microorganisms in the soil, can also determine the degree of loss in productivity.

Producers must be careful with different schemes rotation and/or succession in areas infested by phytonematodes. It is always recommended opt for crops that do not host the species, pests present in your crop, otherwise, favorable conditions will be provided for increasing population on the ground. To achieve this, it is essential to identify and quantification of which species or species are present in your crop, to therefore, opt for a scheme (rotation or succession) that does not provide such conditions.

Collection for analysis can be carried out at the flowering stage of crops or within 50 days - 70 days after planting the crop. For For a more consistent analysis, it is necessary to collect soil and roots in different points of the affected area (5 points per ha – 10 points per ha), removing Final an amount of 1-2 kg of soil and 50 g of root (Figure 2). After this, ensure that the sample reaches a laboratory as quickly as possible for carrying out the analysis and, if possible, placing it in a Styrofoam box, to avoid any sudden change in temperature.         

The effect of rotation or succession with non-host plants can reduce nematode population density in the ground. The exchange of the main crop, almost always a favorable host, for a non-host culture, prevents pathogenic nematode contact with the roots, for a certain period of time. This process directly interferes with the demand for food from this pest in the soil, culminating in death from starvation (lack of food), reducing the population on the ground. In areas where presence of more than one parasite species, care must be taken, as rotation and/or succession options end up being limited.

One of the solutions is rotation with plants antagonists, which have very high efficiency in reducing the population of nematodes in the soil. These plants have characteristics of both immunity to the parasitism of phytonematodes, as well as the release of toxic substances that act directly on nematode mortality and contribute to the rapid population reduction on the ground. Depending on the culture and species of the phytonematode, can reach values ​​ranging from 30% to 70%. The main species of this group of plants are the Crotalarias, Brachiarias, Tagetes, Mucunas in between others.

Considerations finals

The appropriate choice of crop, in the rotation scheme and/or succession, is a key point in the management of this soil pest, however it is necessary look at the problem as a whole, as there is no single, isolated measure able to solve the problem. A combination of practices is necessary to optimize the control. It can be said that the presence of the nematode associated with low productivity is the main point. However, producers must promote ideal conditions for the good development of the plant. Once this is done, you must integrate the chemical, biological and genetic tools on the market.     

Article published in issue 215 of Cultivar Grandes Culturas, April 2017.