Chemical management of phytonematodes in sugarcane

Favorable, due to its own characteristics, to the establishment and development of phytonematode populations, the sugarcane cultivation system demands attention and careful monitoring. Combating these tiny organisms involves chemical control, systemic nematicides and the integrated use of multiple management techniques.   

Nematodes are recognized in all technologically developed sugar and alcohol producing countries as important phytosanitary problems in sugarcane fields. These countries, therefore, systematically monitor the populations of these parasites and carry out control, when necessary.

In the Northeast, as occurs in other sugar and alcohol producing regions in Brazil, two parasitic groups of nematodes are recognized as inducing significant reductions in sugarcane agricultural productivity. These are sedentary endoparasites, represented, firstly, by the species Meloidogyne javanica and M. incognita, equally widespread in the region. These nematodes are responsible for the disease called meloidoginosis, which affects sugarcane varieties cultivated in Brazil without distinction. The primary symptoms of the disease are galls and thickening of the roots, tumor-like formations. These galls can occur isolated or associated in clusters. Examining these hypertrophies at a higher magnification, necrosis can also be seen, produced by opportunistic microorganisms, which are associated with nematodes in the syndromes, increasing the severity of the disease. Areas in the field where meloidoginosis occurs can be seen, reboleiras, that is, a group of plants of the same age, but with different heights. However, in soils of adequate fertility and with the presence of good climatic conditions, parasitized plants present primary symptoms without the formation of reboleiras. Other species of the genus Meloidogyne have been identified associated with sugar cane in Brazil. However, such reports have been sporadic and the virulence of these species (capacity to cause damage to plants) has not been recognized. Root-knot nematode parasitism can cause losses that can reach values ​​exceeding 80% of field productivity (tons of stem per hectare), especially when climatic and environmental conditions are unfavorable to the host, such as nutritional deficiencies and long droughts. The second parasitic group of phytonematodes is that of root lesions, consisting of the species Pratylenchus zeae and P. brachyurus, the first being the prevalent and most virulent. These nematodes cause the disease known as pratylencosis and, like meloidoginosis, it also affects all cultivated varieties of sugarcane. Its primary symptoms are necrotic root lesions, small in size (a few millimeters), but which can converge, forming extensive regions of brown or black color, made up of dead tissue. Under these circumstances, the root system is significantly reduced in volume and the development of the shoot and stem diameter is reduced. In an inattentive field analysis, this syndrome can be confused with that of ratoon rickets, a well-known bacterial disease, very common in the Northeast. Due to these similarities, the diagnosis must be made under laboratory conditions, avoiding field ocular diagnosis. In pratylencoses, generally, no reboleiras are observed, but rather extensive areas with stunted plants that carry other symptoms of the disease.

Nematodes versus damage

The sugarcane cultivation system is very favorable to the establishment and development of phytonematode populations in the soil. Firstly, due to the type of planting, carried out on physically and biologically unstructured soils, caused by soil mechanization in pre-planting. This process disperses micro fauna and flora, preventing suppression and maintenance of population balance between the components of soil communities. This allows phytonematodes to multiply exponentially in the first months of crop development, due to the high food supply and low biological competitiveness. On the other hand, sugar cane is a crop that remains in the field for a long time, in the same place, year after year. These conditions cause nematodes to multiply on a logarithmic scale in the rhizosphere. Therefore, the average population densities (AMDs) of these organisms in the soil reach maximum values ​​and remain at damaging levels throughout successive harvests. When renewing the sugarcane field, ten rhizosphere soil samples and ten root samples must be analyzed, collected from the same tussocks, randomly distributed, for field diagnosis. Sample volumes and aliquots used in laboratory analyzes are standardized. The arithmetic mean found with the results and which characterizes the MPD of the hectare of the plot, is classified into levels called low, medium and high. These levels were established in Brazil by W.R.T. Novaretti, in 1997 (TABLE 1), in the state of São Paulo. 

Later, this system was changed by Moura, in 2005, trying to adjust it to the conditions of the Northeast. Although the numerical values ​​of these levels may vary with climate, variety, nematode population levels and soil fertility conditions, they have helped in epidemiological studies, diagnoses and decision-making in sugarcane nematology. In the Northeast, these assessments are conducted in the months of June, July or August, when DPMs reach their highest annual levels, due to favorable weather conditions, especially rain and milder soil temperatures. In parallel with determining the DPM, assessments of the productivity of the cultivation units must be carried out, using the sampling procedure, standardized for sugar cane. Once you have this data, that is, DPM and productivity, cross-checks are carried out to establish a diagnosis of the field, according to TABLE 2, for subsequent decision making. If a phytonematosis is diagnosed in the production unit, population control of the causal agent must be programmed

Chemical control: systemic nematicides

The first option presented for controlling nematodes in sugarcane fields is the chemical method, which is based on the use of a systemic nematicide. It is a technique especially sought after by farmers as it brings objective results, in the short term (increased productivity, in the same year, in the sugarcane harvest). Nematicides can be granulated or liquid, carbamates or organophosphates, and the selection of the product must be consistent with forecasts of the local climate conditions at the time of application, that is, whether it is the rainy or dry season. General soil conditions are also considered; whether more sandy or clayey. The application of the chemical method is more indicated in cases of populations at medium levels, to be reduced to low levels. For populations at high levels, the application of a systemic nematicide becomes a risky decision, with a low probability of success. To choose the nematicide, how to store it and for safety recommendations on application, handling and disposal of empty packaging, a specialist technician must be consulted.

Chemical control of the two groups of sugarcane phytonematodes has advantages and disadvantages. The first advantage is the practicality of applying the product, which is done using equipment attached to the tractor. These applicators deposit the nematicide at the bottom of the furrow, at the time of planting, with cost control. As for the expected results, research and commercial use have demonstrated significant increases in sprouting, tillering, development and production of stalks in sugarcane harvesting, generating increased productivity. This increase can be of the order of up to 50%, comparing treated and untreated plants. However, the use of nematicides on sugarcane presents risks and epidemiological limitations, which may compromise the success of applications. The first is related to the disappearance of the residual nematicidal effect, which occurs, on average, 90 days after application. This fact allows the nematodes to return to a healthy root system without necrosis. Also, after 90 days, natural enemies are not yet effectively present in the rhizosphere, a fact that provides a high reproduction rate of the target nematode. Consequently, it is common to find higher nematode DPMs in treated plants than in untreated ones, at the time of harvesting sugarcane, despite its greater productivity. This can be seen in Moura and collaborators in “Effect of the application of carbofuram on sugarcane variety CB 45-3” (Fitopatologia Brasileira, p.503, 1998). This fact compromises the longevity of the clogs. To overcome this difficulty, some companies have developed equipment for applying systemic nematicides to ratoons, after the previous harvest. However, due to the costs of the new application and other implications, especially environmental, and also due to the damage caused to the clogs by the equipment, this practice has been avoided. A second problem that has direct implications for the success of using chemical control is the occurrence of rain during the 40 days after application. The absence of rain during this period makes it impossible or reduces the rate of release of the nematicidal molecule for absorption by the roots, while torrential rains cause leaching, with product losses and negative environmental consequences. In the Northeast, it is common for these two climatic phenomena to occur during the sugarcane planting season. Added to these difficulties is the cost-benefit ratio of the method, as the prices of systemic nematicides are associated with exchange rate variations, and the harm to human health and the environment, caused by the toxic effects of nematicidal residues. All systemic nematicides are of toxicological class 1, that is, highly toxic. Currently, a minimum increase of 25 t/h is accepted as a reference value in evaluating the success of an application of systemic nematicide in sugarcane. This relationship may vary with variations in the price of the commercial product and the ton of sugarcane. Under ideal conditions for the application, this value is easily obtained.

Integrated control: multiple associated techniques

The integrated control of sugarcane nematodes was developed by Moura and Oliveira in 2009 (Brazilian Nematologia 67-73) and aims to recover unproductive sugarcane fields destined for renovation. This category includes production units or plots with productivity equal to or less than 40 t/h, in soils with high levels of nematode populations. In the northeast region of Brazil, these diagnoses must be made by the month of September, when milling begins. After being diagnosed, such sugarcane fields must be harvested, preferably at the end of the crushing period, that is, January-February. The integrated control method begins at the beginning of the rainy season (March-April). Areas identified as unproductive, according to the field diagnosis, must be cleaned, with the removal of ratchets, followed by destruction with fire. Then, the soil must be prepared for planting Crotalaria juncea (generally at the beginning of April), incorporating the crop in the first fortnight of July, always at the time of flowering. Thirty days later, after the mineralization of most of the incorporated organic material, black mucuna (Mucuna aterrima) is planted (first half of August), with incorporation in November, also at the time of flowering. During the period from November until the start of the rains of the following year (March-April), the area must be maintained in chemical fallow (herbicides) or mechanical (shallow plowing and incorporation of weeds). As is customary, commercial sugarcane planting in the Northeast takes place at the beginning of the rains. At this time, a plant nutrition specialist should be consulted, especially to verify the need for liming. The benefits with integrated control are many. Initially, regardless of the presence of nematodes in the soil, the simple cultivation of the two legumes before planting sugar cane can increase the productivity of this grass by up to 20%, as evidenced by many works by the Legumes Section of the Campinas Agronomic Institute ( IAC), such as “Residual effect of legumes on the physical and economic performance of sugarcane plants” by A.A. Mascarenhas et al., (1994) (Scientific Bulletin nº 32 and also the work by Ambrosano et al., 1997 , in “Leguminous plants for green manure, appropriate use for crop rotation”, among others. The aforementioned authors found that these two legumes outperformed all other experimental treatments with other legumes in terms of increases in sugarcane productivity. On the other hand, the two legumes, cultivated in sequence, act by differentially suppressing populations of Meloidogyne spp. and Pratylenchus spp. The control action of these legumes is widely known, as pioneeringly demonstrated in Brazil Silva, G.S. in 1988 (doctoral thesis/UFV, MG.), followed by many other pertinent publications.  

To operationalize the integrated control system for sugarcane nematodes, it is essential that isolated plants and distilleries carry out permanent monitoring of phytonematode populations, using their own laboratories and a well-trained nematologist. This monitoring must be carried out in the months most favorable to the parasite's biology, giving preference to areas destined for renewal. Under no circumstances should unproductive plots be left fallow, as this practice can maintain active nematode populations, due to the botanical diversity of invasive herbs, which increases the probability of the occurrence of host plants in the field.

To date, there are no nematode-resistant sugarcane varieties. However, field observations have revealed tolerance reactions in certain varieties. Tolerance in Nematology is understood as the ability of a given genome to provide the host with resistance to the parasitism of a phytonematode, protecting it from significant damage, but providing its reproduction. From an epidemiological point of view, the use of varietal tolerance in Nematology should not be encouraged for routine control practice, as it allows the parasite's population to increase in the soil. Through this uncontrolled population increase, the tolerance character can be affected, as well as the rates of root necrosis, rendering the former ineffective.

Like the chemical method, integrated control of sugarcane nematodes has advantages and disadvantages. The great advantage is that it is a non-polluting method that enriches the soil through the use of green manures. The disadvantages lie in the time required (one year) and the toxicity of sunn hemp to domestic animals. The incorporation of legumes, previously difficult due to lack of adequate equipment, is now done relatively easily.

Although other techniques can be applied to control sugarcane nematodes, those discussed seem to be the most appropriate.

Guidelines 

Romero Marinho de Moura, Federal University of Pernambuco

Article published in issue 202 of Cultivar Grandes Culturas.