Regrowth of parasitic nematodes

The use of resistant cultivars, antagonistic plants and the application of chemical or biological products, via seed treatment or sowing furrow, form a complex of tools with great potential for use in management.

Soybean cultivation in recent years has become extremely challenging, due to the high diversity of problems phytosanitary measures that affect the crop. In addition to the diseases that can attack plants via the aerial part, the complex of soil-dwelling pathogens has increased significantly. Microorganisms that are difficult to control attack the main organ of the plant (roots) in its most critical period of development, resulting in losses in crop yield. Among the different groups of microorganisms that inhabit the soil and can attack soybean roots, parasitic nematodes plants have a prominent role, with solid prospects for worsening the coming years, largely due to the intensive cultivation system.

Not long ago, phytonematodes presented status concern intermediary in many producing regions of Brazil, frame per hour, already surpassed in current days. These parasites have a wide geographic distribution and are capable of causing severe damage to the crop, potentially impacting more than 50% of the production. Factors that contribute to this process inevitably involve the degree of susceptibility of the cultivar, the population density of the species involved, as well as the climatic conditions during cultivation.

Of the species that operate below the soil, root-knot nematodes (Meloidogyne javanica e M. unexplained), O root lesion nematode (Pratylenchus brachyurus), soybean cyst nematode (Heterodera glycines) and the reniformis nematode (Rotylenchulus reniformis) appear as the main species associated with the root system of plants soy. At the However, recent mapping studies in several regions of Brazil have pointed out the emergence of potential nematode species for crops, as is the case in Helicotylenchus dihystera, Scutellonema brachyurus e Tubixaba tuxaua.

In the search for information about the incident of phytonematodes associated with soybeans in the southern region of the country, the Phytus Institute, Rio Grande do Sul, has been mapping commercial production areas, with the aim of verifying the presence of species harmful to the crop. This one study began in the 2012/13 harvest and its first compilation took place in 2014/15 covering 23 municipalities in the region. In this 2017/18 harvest, this The same study covers 71 municipalities in the region (Table 1).

The results of the analyzes showed the presence of root-knot nematodes Meloidogyne spp., soybean cyst nematode Heterodera glycines, root lesion nematode (Pratylenchus brachyurus), reniformis nematode (Rotylenchulus reniformis) It is the spiral nematode Helicotylenchus dihystera. In the population survey carried out between 2012 and 2014, of the 23 sampled municipalities 21,7% showed the presence of Heterodera glycines, and, in the 2017/2018 survey, this percentage increased to 31,42% of the 71 municipalities evaluated. In this case, even being a nematode that until recently was not reported in the state of Rio Grande do Sul saw an increase of 9,72% in 6 years (Graph 1).

No case of P. brachyurus, at the first survey carried out, 69% of the samples showed the presence of nematode at different population levels. In 2018, there was an increase in approximately 4%, being present in 72,8% of the samples. Already Meloidogyne spp. the nematode turned out to be most common in the state of Rio Grande do Sul, as in the survey started in 2012, the presence was found in 86,9% of the samples. In 2018 this percentage dropped to 72,85%, however, it is still extremely significant.

The nematode R. reniformis although less frequent compared to the others, its incidence practically doubled in the sampled cities. In this case there was increase from 8,69% (2012/2014) to 14,28% (2018). Helicotylenchus dihystera had an attendance rate in 2012/14 of 60,8% and in 2018 there was an incidence of this nematode in practically all samples analyzed, with a percentage of 98,5%. However, there are variations only in the percentage of incidence of nematodes found in the points sampled (considering the surveys carried out), but also in their population density. It should be noted that all these species interact in partially or intimately with the plants, requiring perforation of the system radicular, culminating in injuries that can serve as a gateway for pathogenic fungi present in the soil, capable of increasing damage to plants.

These results will certainly follow presenting variation, as studies progress annually for municipalities not yet covered. Regarding producers' lack of knowledge about the presence of nematodes in their crops, this aspect can cause an increase in problematic, as the spread of nematodes occurs mainly through transit of agricultural implements and vehicles. These machines can carry soil particles containing nematodes adhered to its wheels and bodywork, taking them to areas that may still be exempt, directly implying the introduction of species from other regions of Brazil. An example resides in producers who plant or rent their machinery for sowing or harvesting in other regions and are at great risk of introducing these species into their own areas.

Although there is great variation in its population density (soil and/or roots), this scenario requires some care, as these microorganisms can quickly increase their population density in the soil, due to the high reproduction rate in a short period of time (2 to 4 weeks), especially if the cultivation of susceptible soybean varieties is associated another susceptible host culture, whether preceding or subsequent period, promoting continued feeding and reproduction of these nematodes. It should be noted that over time, failure to take care of the sequencing (Rotation/Succession) or the use of crop monoculture hostesses for a few years will certainly favor the establishment or worsening of problems in certain areas.

Some symptoms that the plant expresses when attacked by these microorganisms, can be easily diagnosed by producers. The first aspect noticed and almost always not credited to the presence of nematodes undergoes a slight decline in production, which evolves considerably in subsequent years. Other events are visualized in the crops, such as decrease in plant size, followed by yellowing in the aerial part, wilting and falling of leaves, usually arranged in patches or patches.

Symptoms displayed below the line of the soil undergo an abrupt reduction in the volume of the root system, presenting a darker color, followed by necrosis caused by rapid colonization of opportunistic agents, or even deformations in the structure of the system root commonly known as galls. However, the The fact of observing these symptoms should not be taken as a sole measure of identification, as they only served as an indication of the problem, requiring species identification. Sending the roots to a nematology laboratory as soon as possible is essential. Identification of the occurrence in the field it must always be accompanied by laboratory analysis.

General management measures

The success of phytonematode management depends firstly on for the identification and quantification of species present in the area. It should opt for combined measures that make it possible to maintain populations close to or below the economic damage threshold. The management and control of phytonematodes is not an easy task, as depending on the level of infestation and the species present in the area, each scenario or situation will require the adoption of a measure different. It should be noted 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 reproduction factor (FR) of the nematode, associated with low temperatures, can reduce nematode population density in the soil, as they prevent pathogenic contact of these organisms with the roots. The use of resistant cultivars, antagonistic plants and the application of chemical or biological products, via seed treatment or sowing furrow, form the tool complex with great potential for use in management. When added together, they can present even more satisfactory results.

Paulo S. Santos, Caroline A. Gulart and Gracieli Rebelatto, Phytus Institute

Article published in issue 230 of Cultivar Grandes Culturas, July 2018.