Seed treatment to combat nematodes in soybean crops

Soy is the most important oilseed cultivated in the world. However, to achieve high production rates it is necessary to correctly manage a series of pests and diseases that affect the crop. Among the diseases, nematodes have been considered major villains in reducing soybean productivity in the areas where they are present.

More than 100 species of nematodes involving approximately 50 genera are associated with soybean crops. In Brazil, the nematodes that cause the greatest damage to crops are: the root-knot nematode (Meloidogyne javanica e Meloidogyne incognita), soybean cyst nematode (Heterodera glycines), the root lesion nematode (Pratylenchus brachyurus) and the reniform nematode (Rotylenchulus reniformis).

The symptoms related to the attack of these phytoparasites are very similar. In areas with the presence of nematodes, it is common to find reboleiras, with stunted and chlorotic plants, and the reduction in crop productivity is inevitable, as they parasitize the soybean root system, making it difficult to absorb water and nutrients. In cases of very high populations, plant death may occur.

Depending on the species present in the area, some additional symptoms may be observed. For species of the genus Meloidogyne, the presence of galls on the soybean root system is common. In areas with the presence of cyst nematodes, the root system and nodulation are reduced and it is possible to observe the presence of yellowish females on the roots. Plants parasitized by the root lesion nematode have dark lesions on their roots, and it is common for this nematode to be associated with soil fungi, such as those of the genus Fusarium. In areas with the presence of the reniform nematode, the symptoms differ slightly from those already mentioned, with no reboleiras being present and it being possible to observe a layer of earth adhered to the nematode egg masses, which are produced externally, on the roots.

The first step in managing this disease is the correct identification of the nematode species present in the area. With this information, various management tactics must be employed in order to keep nematode population levels as low as possible. Among the management alternatives, crop rotation/succession, soil management, green manure and genetic resistance have been the most widespread techniques used by farmers.

Each species requires different attention regarding the management techniques used. Rotation/succession with non-host crops should be adopted in the management of all nematode species and for some species (H. glycines, M. unexplained, M. javanica e R. reniformis) there are resistant soybean cultivars. However, the use of genetic resistance should never be used in isolation to avoid the selection of individuals capable of parasitizing resistant cultivars. This fact is very common for soybean cyst nematode which, as it has great genetic variability, the emergence of new races can occur due to poor management of resistant cultivars.

With the aim of increasing the efficiency of nematode control in infested areas and reducing selection pressure, alternative control measures have been sought. The use of nematicides applied to the soil in soybean crops is quite limited due to toxicological problems of environmental contamination and the high cost of treatment.

In the search to reduce the environmental impacts inherent to the application of agrochemicals and the cost of treatment, seed treatment has been used as an additional alternative to nematode management in soybean crops. This reduces costs and environmental impact, as the amount of product used is reduced and seed treatment allows for a safer application of the chemical product.

Seed treatment aims to suppress nematodes during the first weeks after sowing, reducing penetration, protecting seedlings and allowing adequate development of the root system. Plants that are able to have a vigorous initial development of their roots are able to better resist nematode attacks.

Among the active principles used in the management of nematodes, via seed treatment, those based on abamectin and thiodicarb stand out. These products have been used in various dosages and generally associated with treatment with fungicides and/or insecticides (Table 1).

Table 1 - Chemical products registered for the management of nematodes in soybean crops via seed treatment

Active ingredient	Commercial Product	Recommended dose (mL/100kg seeds)	Controlled nematodes
Abamectin		100 to 125	P. brachyurus M. unexplained
Imidacloprid + Thiodicarb		500 to 700	P. brachyurus M. javanica

Active ingredient

Commercial Product

Recommended dose

(mL/100kg seeds)

Controlled nematodes

Abamectin

100 to 125

P. brachyurus

M. unexplained

Imidacloprid + Thiodicarb

500 to 700

P. brachyurus

M. javanica

Source: http://agrofit.agricultura.gov.br/

These chemical products have as their main objective the reduction of nematode penetration into soybean roots and consequently the reduction of parasitism. In research conducted in the field and in a greenhouse, by the Nematology Laboratory of IF Goiano, Urutaí Campus, reductions in the penetration and parasitism of nematodes were observed. M. unexplained, P. brachyurus e H. glycines, when soybean seeds were treated with chemicals based on abamectin and imidacloprid + thiodicarb. These results are most visible within the first 30 days of the culture's life, coinciding with the residual period of the products. As plants develop, differences in parasitism become imperceptible.

The initial protection provided by nematicides, applied via seed treatment, can often be seen in the productivity of the soybean crop. This is due to the fact that the plants have good initial establishment, more vigorous root systems and, consequently, greater tolerance to nematode attack. However, it is important to highlight that as the residual effect of the product is approximately 30 days, the seed treatment should never be used in isolation in the management of nematodes, once this period has been overcome, the population levels of the nematode resume their growth, reaching values ​​higher than those found before planting.

In an attempt to increase efficiency in nematode management, via seed treatment, the association of chemical products with biological control agents has been sought. The fungi of the genera stand out Paecilomyces e Arthrobotrys and some bacteria. The two fungi are quite widespread and act on nematode juveniles and eggs, respectively. The use of bacteria is based on the modification of the rhizosphere, which directly or indirectly affects these parasites. The relevance of these bacteria for nematode control is greater when they have the ability to colonize the interior of plants, being able to act on nematodes before and after penetration of plants.

In Brazil, it is now possible to find products based on the fungus Paecilomyces lilacinus, registered for other crops, and which has been tested, together with products based on bacteria of the genus Pasteuria e Bacillus, for soybean cultivation. It is interesting to highlight that these tests have been carried out with biological control agents alone and in mixture with chemical products already registered for seed treatment in nematode management.

The joint use of chemical products and biological control agents is very interesting as it allows for increased efficiency in controlling nematodes. The chemical products would mainly reduce the penetration of nematodes and the biological control agents would act on the parasitism of eggs, juveniles or even adults. However, it is important to note that as this research is still in its early stages and seed treatment with chemicals alone does not have a long residual period, these forms of management should never be the only option for controlling nematodes in soybean crops.