Integrated management of phytonematodes in soybeans and cotton

Identifying the nematode species present in the cultivation area and determining their population density is the first step before defining control measures. Once this stage has been overcome, the integrated adoption of compatible chemicals and biologicals is the best strategy to combat phytonematodes in soybean and cotton crops.

Until the early 1990s, the only nematodes that caused concern to soybean farmers in Brazil were root-knot nematodes (Meloidogyne javanica e M. unknown). From the 1991/92 harvest onwards, soybean cyst nematode (Heterodera glycines) began to appear as a pathogen of great importance. At the beginning of the 11st century, that is, 2002 years later, in the 03/XNUMX harvest, another species began to attract the attention of soybean farmers: the lesion nematode (Pratylenchus brachyurus). In the same decade, occasional records of losses caused by the reniform nematode (Rotylenchulus reniformis) in soybean cultivation.

The species mentioned today rarely occur in isolation, which has made management difficult and increased losses, often causing the producer to be unable to pay production costs and abandon the area, or transfer the problem to others. In the state of Mato Grosso, the most common species is the lesion nematode and the one with the lowest incidence is the kidney nematode, both in soybean and cotton crops. The latter is a potentially serious problem, especially in areas of soybean-cotton succession. The soybean cyst nematode is the species best known to soybean farmers. Even though it was reported in Brazil for the first time relatively recently (1991/92), it currently occurs on more than five million hectares. This species presents great genetic diversity and, when under selection pressure (continuous use of one or a few resistant soybean cultivars), new races can be selected.

A key point to consider, before taking any action in relation to specific management for nematodes, is to know for sure which species of nematodes are present in the area and also their population density. To this end, a specialized laboratory must be hired to provide information for the collection and adequate packaging of soil samples where these nematodes occur; and subsequent sending of these samples for correct identification and determination of population density (number of nematodes/cm³ soil). It is recommended that the chosen laboratory is as close to the property as possible. With the results of nematological infestations in the cultivation areas, the responsible technician must plan the management activities to be established.

What should not be forgotten is that once the presence of the nematode is detected, management practices must be planned in the medium or long term so that populations can gradually reduce until they no longer cause damage to the crop.

What is recommended in phytonematode management is that practices have a logical sequence including cultural management, use of resistant cultivars, and use of chemical and/or biological products. The biological control strategy makes it possible to reduce the population density of phytonematodes in the area and promotes balance of soil microfauna, making the pathogen suppressive. The available biocontrol agents (fungi or bacteria) have some advantages over chemical control, but they require adequate conditions for their viability.

Around 75% of the identified antagonistic microorganisms are fungi that normally inhabit the soil and can be parasites of eggs, predators of juveniles, adults or cysts, or even produce metabolites that are toxic to nematodes. Among the various nematophagous fungi, ovicidal or opportunistic fungi are among the most promising for controlling root-knot nematodes, as the egg mass of these nematodes is compacted into a gelatinous matrix in each female, facilitating colonization. O Trichoderma sp is represented by necrotrophic species, with great capacity to control soil fungi. However, due to its ability to degrade chitin, it is used in the management of nematodes. The fungus Paecilomyces lilacinus is known for its parasitic action on eggs and females of Meloidogyne sp.

When the plant is infected by a pathogen, rhizobacteria can act as biological control agents, through the production of bacterial metabolites such as antibiotics and enzymes that degrade the pathogen's cell wall, directly affecting it. The metabolic products of these bacteria can act directly on the mobility and/or survival of embryos inside eggs and at different developmental stages of nematodes.

The impact of bacteria on nematodes may be due to parasitism, the production of antibiotics, toxins and enzymes, interference in the plant-host recognition process, induction of resistance and/or providing healthy plant development.

The main bacterial genera associated with nematode biocontrol are Pseudomonas spp. And Bacillus spp. The bacteria Bacillus subtillis affects nematode orientation, induces systemic resistance, acts as a toxic or repellent substance, and feeds on nematode eggs. Another studied and promising example is the bacteria Pasteuria penetrans, considered the most potent biological agent against root-knot nematodes.

However, much preliminary research is needed to commercialize these antagonists, given that their performance in the field can be quite inconsistent and variable. When it comes to nematodes, and the conditions that involve their control, no isolated tool is a guarantee of success, especially biological control agents, which, as they are living microorganisms, environmental conditions, linked to the physics and chemistry of soils, directly affect their effectiveness on nematodes. In the state of Mato Grosso, in the last three years the demand for organic products has increased significantly, consequently the supply. This led companies to invest more in formulations, which are easier to apply and have less possibility of losing the viability of the microorganisms to be applied.

Biocontrol microorganisms have shown good effects when used together with cover crops, especially those that are not resistant to all nematode species. A good example of this are the brachiaria, efficient in managing root-knot and cyst nematodes, but hosts Pratylenchus brachyurus. In this case, it is recommended to use Brachiaria ruzizizensis, because among the brachiarias, this is one of the ones with the lowest multiplication factor for this nematode. In this way, a cumulative effect occurs, from the non-host plant to the gall and cyst and from the microorganisms protecting the roots from the lesion nematode. This effect can be seen in figure 1.

Furthermore, if the microorganism is used on cover crops in the off-season, the rainy season has already been established and there is sufficient humidity for them to develop, promote greater root protection and attack the juveniles and eggs in the soil, causing a greater reduction in the population. of the nematode. Even when biological products are used on cover crops resistant to the main nematode species, such as Crotalaria spectabilis, in succession to soybean, an improvement in the development of the root system of soybean plants can be observed (Figure 2), especially if the nematode population is high, as the off-season period is short and there is still a survival rate, for which the microorganism will collaborate.

To reduce the nematode population in soybean crops and also improve soil coverage for subsequent cultivation, several producers have adopted plant intercropping. One example is corn (resistant to Meloidogyne javanica) consortium with Crotalaria spectabilis, and after the consortium the use of microorganisms in soybean plants. As a result, better conditions for plant development have been observed (Figure 3).

In addition to better rooting, a greater increase in productivity has been observed in areas where the biological product or biological fertilizer is used as the only management tool. However, if the nematode infestation is high, it is difficult to restore productivity using this practice alone. However, in tests where these products have been used, increases of up to four bags have been observed in relation to the untreated control (Figure 4). 

However, when a producer uses a product and increases it by three to four bags, in an area where productivity went from 40 bags to 43 or 44, no value has been attributed to the product. The same does not occur if productivity increases from 52 to 55 or 56 bags. The way of visualizing the product is different, when in fact the increment is the same. The difference lies in the severity of the problem that exists in the area, which is why the products have been recommended as a management tool and not in isolation in areas where nematodes are above the level of economic damage.

With regard to chemicals, it is possible to consider the same proposal mentioned for biologicals. It is just another tool, but it must be used in conjunction with other techniques, as it alone will not bring satisfactory results in controlling and reducing nematodes over time. Nematicides do not have selective action, therefore, the intensive use of chemical control alone can cause imbalances over time. The residual of chemical seed treatments is a maximum of 30 to 40 days. After this, the tendency is for the nematode population to grow again, that is, at the end of the crop cycle it may be even higher in the treated plots, as the root volume will be greater in relation to the control plots, which will mean more food for nematodes. In figure 5, the initial protection effect for the roots can be seen. 

This protection allows for better plant establishment in the presence of the nematode. Furthermore, it has been observed that when chemical management is added to cultural management the effect is much better, as a reduction in the population has already occurred and, combined with this, the nematodes that remain will have greater difficulty in penetrating the roots, allowing better establishment and greater rooting. (Figure 6) and consequently greater plant development and increases in productivity (Figure 7).

Some producers who suffer more from nematode problems have combined the two management practices (biological and chemical) in some areas. However, you need to pay attention to the compatibility of the products. Most nematicides are not incompatible with microorganisms, however, care must be taken with the fungicide that accompanies the treatment, especially the industrial one. Before using an organic product, you must contact a product representative and find out about its compatibility with those present in the purchased seeds. If the producer does not use industrial treatment, he must select products compatible with the microorganisms he intends to adopt, so as not to make them unviable or slow down their growth. When working with microorganisms, it is always necessary to remember that they are alive and must remain that way, in order to colonize the soil, roots and/or nematodes present, depending on the product used.

Rosangela Silva, MT Foundation; Lennis Rodrigues, MT Foundation

Article published in issue 201 of Cultivar Grandes Culturas.