Management of nematodes in sugarcane

Nematodes are parasites whose damage varies according to the species present in the sugarcane field, whose productivity reduction can exceed 50% in cases of very high population levels. Due to the absence of resistant commercial varieties, the use of chemical nematicides applied at planting and/or in ratoons stands out.

Nematodes are important productivity reducers in Brazilian sugarcane fields. Analysis of the DMLab laboratory database, referring to samples collected in sugarcane fields in the Center-South Region of Brazil, between 2001 and 2013, revealed that Pratylenchus zeae is, without a doubt, the species most frequently found in populations high enough to cause economic damage, present in 97% of the samples analyzed. The species Meloidogyne javanica e Pratylenchus brachyurus were found in approximately 35% of the samples, while Meloidogyne incognita,  in only around 20% of samples, although it is the most pathogenic species for most sugarcane cultivars (Dinardo-Miranda, 2014).

The damage caused by these parasites varies depending on the species present, the population of each of them, the type of soil, the sugarcane variety and many other factors, but they average between 20% and 40% of productivity reduction. in the first cut, which can reach more than 50%, in cases of very high population levels and very susceptible varieties. In ratoons, productivity is also seriously reduced, compromising the longevity of the crop, which needs to be renewed more frequently in infested areas (Dinardo-Miranda, 2014). The magnitude of damage caused by nematodes in sugarcane and ratoon cane can easily be observed in the field, through the use of nematicide in infested areas, compared to untreated areas: larger plants have lower nematode populations in their roots due to the use of nematicides, when planting or in ratoons.

The first step to reducing nematode problems is knowing the areas where they occur. To achieve this, it is essential to carry out good sampling, which can be done in any crop cycle, with the results used in the adoption of control measures in the following cycle. Two parameters must be considered to carry out good sampling: the season and the number of samples to be collected in the area.

Regarding the sampling time, it must be considered that nematode populations in the roots and soil are influenced by numerous factors, such as temperature and soil humidity. Both exert both a direct influence on nematode populations, affecting their survival and multiplication capacity, and an indirect influence, interfering with root development. Since these nematodes are obligate parasites, root development has a preponderant influence on their development.

In times of lower temperatures and lack of water in the soil, sugarcane loses part of its root system and nematode populations reduce, both due to low humidity and temperature conditions and due to the death of plant roots. plants. Nematode populations only grow again when temperatures rise and water in the soil becomes abundant, conditions that are also favorable for good root development. Therefore, the most suitable period for sampling is the hottest and rainiest of the year, which in the Central-South region of Brazil corresponds to spring and summer. In this region, the peak population of nematodes in sugarcane fields generally occurs between January and March. The ideal is to start sampling around 30 to 40 days after the start of the rains and extend them until 30 to 40 days after the end of the rainy season (Dinardo-Miranda, 2014).

Due to the great spatial variability of nematode populations, it is recommended that the sample that represents a given plot is composed of two subsamples for each hectare of the plot. These are the composite samples that must be sent to the laboratory for analysis. Samples must contain around 100g of roots (Figure 3) and 1L of soil (Dinardo-Miranda, 2014).

Considering the species of nematodes present in the sample, the quantity of each one of them, the sampling period, the crop cycle, the production environment and other factors, based on experimental data, the nematologist interprets the results of the analysis, informing the producer of the population level of nematodes in the area. Medium and high nematode populations are considered above the level of economic damage and in areas with such populations control measures must be adopted.

The use of resistant varieties would undoubtedly be one of the most practical methods to reduce nematode populations (Lordello, 1981). However, there are few varieties currently in cultivation that are resistant to at least one of the economically important nematode species. Furthermore, in the field it is common for two or more species of nematodes to occur in the same area, making the use of a variety resistant to a single species ineffective.

The addition of organic matter to the soil is also frequently cited as effective in reducing the population of certain nematodes, by creating favorable conditions for the multiplication of their natural enemies, mainly fungi, and by releasing, during their decomposition, organic substances with nematicidal action (Lordello, 1981). In sugarcane fields, the most used organic matter is filter cake, but work carried out in the field by Novaretti & Nelli (1985) and Dinardo-Miranda et al. (2003) revealed that filter cake has no significant effect on nematode populations, at least at the doses at which it is used in sugarcane fields. The authors of both studies concluded that the productivity increases observed through the use of filter cake should be attributed to its nutritional effects. When implementing crops in areas infested by nematodes, filter cake has been a valuable tool for obtaining higher productivity, especially when used in conjunction with nematicides, because its beneficial effects (moisture and nutrients) are combined with the benefits of nematicides (nematode population reduction).

Another material widely used in sugarcane fields is vinasse, whose effects on nematode population reduction were verified by Pedrosa et al. (2005), in laboratory and greenhouse studies. Although the authors observed that vinasse in high doses (up to 1500 m³/ha) could harm the hatching of M. javanica e M. unexplained, highlighted the need for field evaluations, mainly because vinasse in those doses harmed plant development.

Castor pie had more significant effects. In a field trial, this product reduced nematode populations and, as a result, promoted increases in productivity (Dinardo-Miranda & Fracasso, 2010a). Despite this, this material has been little studied and is rarely used in sugarcane fields due to its high cost.

Rotation with non-host plants, to reduce damage caused by nematodes, has also been the subject of studies in several crops. In areas traditionally cultivated with sugar cane, sunn hemp are among the most used plants, as they are very productive, which directly reflects on the productivity of the subsequent sugarcane field. Studies conducted in the Northeast of Brazil revealed that sunn hemp can reduce populations of nematodes of the genus Meloidogyne, when cultivated for at least two years in the area (Moura, 1991; Rosa et al., 2003). However, populations of P. zeae (Rosa at al., 2003).

When the crop rotation period is limited to four or five months, the time between the destruction of the old sugarcane field and the implementation of the new crop, as occurs in the Center-South region of Brazil, there is only one cycle of crotalaria cultivation. In this case, there is no significant effect of rotation with sunn hemp on populations of Meloidogyne in the sugarcane plantation subsequently, although there may be an increase in the populations of  Pratylenchus (Dinardo-Miranda & Gil, 2005). Despite this, rotation with sunn hemp generally contributes to increases in productivity in the subsequent sugarcane field, due to its benefits as green manure (Dinardo-Miranda & Gil, 2005), which is why rotation with sunn hemp in areas infested with nematodes has been widely used. in sugarcane, without, however, dispensing with the use of nematicides in the implementation of sugarcane crops.

Due to the lack of commercial varieties resistant to nematodes and the low efficiency of organic matter and crop rotation in reducing nematode populations in sugarcane fields, the management of infested areas is based on the use of chemical nematicides applied at planting and/or in ratoons. , whether or not associated with the use of organic matter and crop rotation. 

When planting, nematicides are applied to the setts, immediately before closing the furrows. By reducing nematode populations during the first two to five months of crop age, they promote better development of the roots and, consequently, of the aerial part of the plants, which results in a 15% to 50% increase in productivity, in relation to the areas without nematicide (Dinardo-Miranda, 2014).

In ratoon crops, nematicides are applied in a cut next to or on the sugarcane rows, contributing to productivity increases of 10% to 25% (Dinardo-Miranda, 2014). The magnitude of the productivity increases is closely linked to the time in which the nematicide is applied to the ratoon crop and this, to the development of the sugarcane root system.

As root death occurs in the dry period of the year and root growth occurs in the rainy period, regardless of the cutting of the aerial part of the plants (Vasconcelos, 2006), the application of nematicides in ratoon crops harvested in the dry period of the year (between June and October, in the Central-South region of Brazil), it has been shown to be more advantageous when done 30 to 90 days after harvest, in order to be close to the subsequent rainy season. On the other hand, for sugarcane fields harvested during the rainy season, applications are more effective and result in greater productivity increases if carried out immediately after cutting (Dinardo-Miranda, 2014).

Leila Luci Dinardo-Miranda, Sugarcane Center, Agronomic Institute, Ribeirão Preto (SP), Brazil

Article published in issue 213 of Cultivar Grandes Culturas.