Managing nematodes in cotton

With an underground habit and difficult to see, nematodes that cause economic damage are tiny, silent beings, with the potential to cause gigantic losses. The fastest and most efficient management of these soil-dwelling organisms lies in the adoption of joint techniques, such as cultivation of resistant material, chemical seed treatment, adoption of crops with a low reproduction factor in the off-season and integration with biological control.

During the phenological development of cotton crops, there are several factors that can negatively affect it. Phytonematodes that cause economic damage stand out, as they impair the absorption of water and nutrients, causing a decrease in productivity. The main nematode species causing damage to cotton crops are Meloidogyne incognita, Pratylenchus brachyurus e Rotylenchulus reniformis.

According to a survey carried out annually by the AgroLab Laboratory in Primavera do Leste/Mato Grosso, there is the following distribution in the state (Graph 1).

The values ​​observed in Graph 1 correspond to the percentage in which each gender is found in the samples. Of course, these samples are not only from the cotton crop, but in most cases the most cultivated crops are soybeans, corn and cotton.

Wound nematode (Pratylenchus brachyurus)

The high dissemination of Pratylenchus brachyurus is what draws the most attention, as it is present in 99% of the samples. It prefers sandy soil, and attacked plants do not develop normally. The affected areas are more difficult to identify, as there is no formation of galls and galls, typical of nematodes. Sometimes, Pratylenchus brachyurus causes yellowing in the aerial part of the plant, but the most important fact is the reduction of the root system.

Their populations vary greatly, and reach astronomical levels in suitable areas (60.000 juveniles in 10 grams of root).

Root-knot nematode (Meloidogyne incognita)

In cotton crops, races 3 and 4 are the most common, with this nematode being the main cause of damage to the crop. It occurs in a more localized manner, with its predominance in reboleiras and in more specific areas. It causes symptoms in the aerial part of the cotton (carijó leaf) and in the roots (galls), which facilitates its identification. It also causes damage to soybean crops, and even infects some covers used by cotton farmers in the off-season.

The reniform nematode (Rotylenchulus reniformis)

The reniform nematode (Rotylenchulus reniformis Linford & Oliveira 1940)  is an important root pathogen of several crops of economic interest in tropical and subtropical regions, being reported associated with melon, passion fruit, tomato, soybean and cotton crops in Brazil. For a long time, R. reniformis was considered a nematode of little importance, probably due to the absence of characteristic symptoms in parasitized plants, unlike what occurs with root-knot nematodes, more common in cotton.

It is still the least frequent, except in some regions that already have high populations and aggressive damage. It prefers clay soils and perhaps that is why its frequency is lower in the state of Mato Grosso. It appears in only 3% of samples. 

Nematological analysis

To apply nematode management strategies, it is necessary to carry out prior analysis in the laboratory, collecting soil and root samples and sending them to a nematology laboratory that offers this service. In some cases it is not possible to make the correct diagnosis solely based on visual observation of symptoms in the field. Laboratory analysis, therefore, is essential for nematode management. The results of the analysis must contain information on the identification and quantification of the species present at the site. Based on this information, the responsible professional will be able to establish an action plan for appropriate management (or control), with the aim of reducing damage and losses.

The results of nematological analyzes must express, in the most reliable way possible, the real situation in the field, in relation to nematodes occurring in a given location. In this sense, it is very important that sample collection in the field is done correctly.

NEMATOID MANAGEMENT

Currently, there are several management alternatives that can be used to reduce nematode populations. The first and most important is to prevent the spread of nematodes. All dissemination occurs through the transport of soil particles, whether through erosion, transport of machinery or equipment or by another vehicle carrying soil.

Resistant materials

For cotton cultivation, there are few materials resistant to the three main nematodes. Currently resistance occurs to Meloidogyne incognita e Rotylenchulus reniformis. There is still no material resistant to the lesion nematode. What happens are materials with different reproduction factors, some higher and others lower.

physical control

Turning the soil is sometimes an option, but at the same time it can spread nematodes. However, it can also contribute to reducing the population of those exposed to sunlight. This option is widely discussed, but it should always be carefully evaluated taking into account common sense to verify this technique.

Alternative cultures

There are currently several crops being used as alternatives to reduce nematode populations. Studies carried out by AgroLab have demonstrated great efficiency in the use of crotalaria, millet, castor beans, sorghum, and corn as nematode reducers. These crops are being inserted into Mato Grosso's management system little by little, and more studies are still needed.

Biological control

Biological products consisting of fungi or bacteria are being widely used to control nematodes. Most are based on Paecilomyces lilacinus, Bacillus subtilis and Pasteuria spp. These products are highly efficient if used correctly. The biggest problem encountered lies at the time of application, as they are living organisms and cannot be handled at any time of the day. The restrictions are operational and transport/storage.

Chemical Control

Most chemicals are used via seed treatment, and in some cases via planting furrows. They are efficient products, with faster action than others, but with some restrictions regarding toxicity and the period in which they act in the soil. It is an excellent option, but very dangerous in isolation and may not be as efficient as several combined options. 

THE MOST IMPORTANT THING FOR NEMATOID MANAGEMENT

Once present in the area, populations will never be reduced to zero again. Coexistence will be inevitable. The best, fastest and most efficient management is the adoption of combined techniques, that is, resistant material together with chemical seed treatment, then a crop with a low reproduction factor in the off-season. After that, the adoption of biological control, and so on.
Year after year populations are increasing rapidly and in all regions. The problem may go unnoticed in the first few years, but damage will certainly occur in subsequent harvests. Failure to adopt management measures can cause major losses or even make production unfeasible in areas with high populations.

Important reminders 

1) Survey of where the problem is, what the nematodes are and their quantities;
2) Organize a planting/spraying/harvesting plan in the areas, separately, so as not to spread;
3) Adopt a management system for nematodes, with several joint options;
4) Routinely evaluate the options and results of each practice;
If these factors are satisfied, the nematode will certainly not reduce production and will not be a problem.

Tatiane Cheila Zambiasi, Emanoelli Tonetti, Juliana do Nascimento Azevedo, Agrolab

Article published in issue 214 of Cultivar Grandes Culturas.