Soybean cultivation is affected by several species of phytonematodes, specialized in feeding on the root system of plants, damaging their vegetative and reproductive development. In the South of Brazil this reality is no different, with attacks from Meloidogyne javanica, Heterodera glycines, Pratylenchus brachyurus and the recent detection of Rotylenchulus reniformis in Rio Grande do Sul
The soybean crop has suffered frequent losses resulting from attack by phytonematodes. These microorganisms are specialized in feeding on the root system, directly interfering with the plant's physiological processes, compromising vegetative and consequently reproductive development. The main species associated with decreased productivity in soybean crops are root-knot nematodes (Meloidogyne javanica e M. incognita), the soybean cyst nematode (Heterodera glycines), the root lesion nematode (Pratylenchus brachyurus) and the reniformis nematode (Rotylenchulus reniformis).
Currently, these species are widely distributed in several Brazilian states, mainly in areas with soybean cultivation. In the central region of Brazil, the occurrence and distribution of these phytonematodes are well mapped, through surveys carried out by researchers in the Region. Although it is a difficult task to measure, reports indicate losses caused by the species in the order of 10% to 50%, which may vary due to biotic and abiotic factors.
Symptoms in plants infected by phytonematodes can range from the formation of thickening in the root system, known as galls, reduction in volume and growth and in some cases destruction. This complex of injuries compromises the physiological functioning of the plant. In farming, symptoms occur in patches or patches, formed by yellowing plants followed by irregular growth.
The eradication or elimination of these species, when established in cultivation areas, becomes practically unfeasible mainly due to cost. Combined measures must be chosen that make it possible to maintain populations close to or below the threshold of economic damage. One of the major factors that have contributed to the increase in problems with phytonematodes, generally involves lack of knowledge on the part of producers or the fact that symptoms of nematode action on the root system end up being attributed to any other biotic or abiotic injury, thus compromising its production in a short space of time.
Thinking about obtaining information on the occurrence of phytonematodes associated with soybean cultivation in the southern region of the country, the Phytus Institute (located in Rio Grande do Sul), in the agricultural years 2012/13 to 2013/14, carried out collections in some municipalities in the region , with the aim of verifying the occurrence of the main species causing damage to soybeans.
The results obtained showed the presence of four species capable of causing damage to soybean crops. The root-knot nematode (Meloidogyne javanica), the soybean cyst nematode Heterodera glycines, the root lesion nematode (Pratylenchus brachyurus) and the reniformis nematode (Rotylenchulus reniformis). The first two species are highlighted, as they appeared more frequently at high levels in soil and root samples.
In the municipalities sampled, the root lesion nematode Pratylenchus brachyurus it was present in almost all, however, at low and medium levels. This nematode is widely distributed in regions with tropical and subtropical climates. This is due to its high degree of polyphagy (ability to parasitize and reproduce on a high number of plants, mainly of economic importance, whether annual or perennial). In the Mato Grosso region, the incidence of this nematode in a survey carried out by (Ribeiro et al, 2010) showed a presence of around 96,4%, causing a reduction of 30%-50% in infested areas.
In one of the sampled municipalities, the presence of the nematode was observed Rotylenchulus reniformis, in values above seven thousand (eggs + J2 + adults) in 200cm³ of soil, in an area with soybean cultivation in a direct planting system, with grasses in winter. However, this is the first record of the occurrence of this nematode in the southern region of the country. It is worth highlighting the ability to adapt to different types of soils, and the ability to enter into anhydrobiosis (a physiological mechanism that induces the nematode to drastically reduce its metabolism, remaining in a state of almost ametabolism), in environmental conditions that are unfavorable to its survival. Although R. reniformis is frequently reported causing losses, normally in areas with a long history of cotton cultivation (its main crop), in recent years several studies have shown frequent damage due to the parasitism of this nematode in soybean crops.
Although they occur most of the time in low and medium population density in the soil, this scenario requires some care, as these microorganisms are specialized in feeding directly on the roots, and can quickly increase their population density in the soil in a short period of time, especially if the cultivation of susceptible soybean varieties is also preceded by another crop that allows the feeding and reproduction of these nematodes.
Over time, failure to take care with sequencing (rotation/succession) or the use of monoculture of nematode host crops will certainly favor the establishment or worsening of problems in certain areas. In some cases, the main and first aspect noticed by the producer, and almost always not attributed to the presence of nematodes, is a slight decline in productivity, year after year. The other important aspect is when a series of events occur in cultivation areas, such as a decrease in plant size, followed by yellowing of the aerial part, with wilting and leaf fall, normally arranged in patches or circles (circular). These symptoms are, in many cases, attributed to soil compaction, nutritional deficiency, limestone stain, waterlogging or even another complex of pathogens (fungi, bacteria, viruses, among others).
Producers' lack of knowledge about the presence of nematodes in crops helps their spread, mainly through routine practices with agricultural implements and vehicles. In a way, these machines move the soil, thereby carrying particles containing nematodes stuck to the wheels and bodywork, taking them to other areas. This also implies the introduction of species from other regions of Brazil. For example, producers who plant or rent their machinery for sowing or harvesting in other regions run the great risk of introducing these species into their own areas.
For diagnosis, the classic symptoms expressed in the aerial part are generally the result of a severe attack on the root system, which normally compromises the absorption and translocation processes of water and nutrients by the plant. Irregular growth, yellowing of the leaves, wilting in hot and dry periods of the day and premature leaf fall are characterized as the guiding principles for diagnosis, which may vary according to the species.
This nematode was detected in Brazil in the 1991/92 harvest and is currently present in several states (Minas Gerais, Mato Grosso, Mato Grosso do Sul, Goiás, São Paulo, Paraná, Rio Grande do Sul, Bahia, Tocantins and Maranhão). In Brazil, 11 NCS breeds have already been found (1, 2, 3, 4, 4+, 5, 6, 9, 10, 14 and 14+). Soybean cyst nematode Heterodera glycines is responsible for losses of around 10% to 30%. However, depending on the type of soil and population density, they can reach up to 70%.
In Rio Grande do Sul, breeds 3, 5 and 6 have already been reported in three municipalities in the region by Dias et al (2009). However, new studies on the occurrence of other breeds of H. glycines must be carried out, because, due to lack of care or knowledge on the part of producers about the potential for dissemination by the transit of vehicles and agricultural implements, from one region to another, there is a great possibility of introducing new races of this nematode into the region.
Typically plants under attack by soybean cyst nematode H. glycines They have a reduced size, followed by yellowing in the aerial part. In cases of high population density on the ground, these symptoms may be more severe.
After observing these symptoms in the aerial part, the producer must remove 3-7 plants within these spots (reboleiras) and observe the root system, preferably with the crop between 25-45 days after sowing. In infested areas, females of H. glycines on the surface of soybean roots in their development stage with a whitish color.
Direct observation of the root system, after a period of 25-45 days after sowing, must be much more careful, as over time the nematode completes its life cycle and its tone (color) ends up becoming darker (brown). ). At this stage or period, the producer will practically not be able to observe these symptoms in the root system, as the cyst detaches from the root when the plants are pulled out. Each cyst can store around 100 to 300 eggs, ensuring the nematode's survival for approximately eight years in the soil without the host crop, followed by adverse periods of the year such as high and low temperatures, including desiccation.
the elimination of H. glycines in infested areas it is very difficult. The producer or technician responsible, when observing these symptoms in his crop (aerial part and roots), must collect samples and send them to a nematology laboratory as quickly as possible, to obtain quantification of cysts per 100cm³ of soil and of eggs and forms juveniles of H. glycines.
There are several species of the genus Meloidogyne capable of causing damage to soybean crops. The most frequent are Meloidogyne javanica, M. incognita and M. arenaria. the species Meloidogyne javanica It has a wide range of host plants, most of which are of economic importance, such as soybeans, corn, beans, potatoes, carrots, tomatoes, sugar cane, tobacco, among others. The damage resulting from the parasitism of these nematodes depends on the population level present in the area, associated with the susceptibility of the crop and favorable climatic conditions. Losses are normally between 10%-30% of production and depending on the crop, they can reach 90%.
The symptom of attack by this nematode normally occurs in rebolas, which often go unnoticed, as the crops slowly decrease in vigor and productivity. Some symptoms that the plant expresses under the attack of this microorganism tend to be identified by the producer, who can normally observe a slight underdevelopment (unequal size of plants), followed by yellowing of the aerial part, easily confused with nutritional deficiency. The leaves may present chlorotic spots and necrosis between the veins, known as “carijó” leaves and in some cases there is defoliation (leaf fall), and in hotter periods of the day, wilting in the aerial part.
Symptoms on the root system are known as galls, almost always present on roots infected by Meloidogyne spp, giving a tumoral appearance, with abnormal, clearly visible thickenings. If the cut is made in the gall region, transversely, white nematode females can be observed. Each female produces around 200 eggs – 400 eggs per complete crop cycle. Part of the eggs are deposited in a mucilageous mass that guarantees survival for approximately six months to a year in the soil. It is important to emphasize that in some plants such as corn, rice, sugar cane, wheat, among others, these symptoms may be absent or very reduced, making it difficult to visualize. In general, parasitism of Meloidogyne It can cause nutrient depletion during feeding, a reduction in the efficiency of water and nutrient absorption by the plant, changes in root growth and even tissue destruction.
The observation of these symptoms should not be taken as a sole measure of identification, as they only serve as an indication of the problem and guidance towards the gender. Meloidogyne, It is necessary to send the roots to a nematology laboratory as soon as possible for confirmation. The identification of the occurrence at the field level must always be accompanied by analysis in the laboratory.
The success of phytonematode management primarily involves identifying and quantifying the species present in the area. The management and control of phytonematodes are not easy tasks, as depending on the level of infestation and the species present in the area, each scenario or situation will require the adoption of a different measure. It is worth noting that the difficulty increases mainly in areas with more than one species. In these cases the efforts are even greater.
In general, rotation or succession with non-host plants, or the use of unfavorable hosts with low FR (nematode reproduction factor), associated with temperatures below 12°C, can reduce the nematode population density in the soil, as prevents pathogenic contact with the roots. The use of resistant cultivars, antagonistic plants and the application of chemical and biological products, via seed treatment or sowing furrows, form a complex of tools with great potential for use in management. When added together, they can present even more satisfactory results.
Paulo Santos, Gracieli Rebelato Instituto Phytus; Diego Dalla Favera, Renan Dal Sotto, Ricardo Balardin, Federal University of Santa Maria; Marcelo Gripa Madalosso, Phytus Institute/Univ. Integral Reg. - Santiago
Article published in December 2014, in issue 187 of Cultivar Grandes Culturas.
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