Crop rotation is one of the indications for controlling phytoparasitic nematodes

“Phytoparasitic nematodes in soybeans” was the theme of the lecture by researcher Guilherme Lafourcade Asmus, from Embrapa Agropecuária Oeste, on July 8th. The theme was part of the second day of the event “Integrated Management of Soybean and Corn Pests”, held by Embrapa Agropecuária Oeste, Embrapa Milho e Sorgo and Jotabasso.

According to Asmus, nematodes can be bacteriophages, with the oral cavity adapted to ingest bacteria; mycophages, which feed on fungi, and there are omnivores and predators that feed on various small organisms in the soil.

The most important are phytoparasitic nematodes, which feed on plant roots. Phytoparasitic nematodes are microscopic worms that inhabit soil water, with few cases of aerial nematodes. All phytoparasitic nematodes have their oral cavity transformed into styliitis. It is with this that the phytoparasite feeds on the roots of plants.

“All phytoparasitic nematodes are obligate parasites, which need host plants to multiply,” declared the researcher. For damage to occur, three factors need to work together: presence of nematodes, susceptible plants, hosts and favorable environment.

Main soybean species: Meloydogine javanica e M. unexplained, both encourage the formation of galls on the roots. Heterodera glycines, commonly called cyst nematode, since, when they die, the females give rise to cysts, full of eggs. Rotylenchulus reniform (reniforme) and Pratylenchus brachyurus (causes dark lesions on the roots resulting in root rot) – it is the only one that remains inside the roots, which is why it necrotizes them.

Under normal farming conditions, in the presence of a susceptible crop, the population density of the nematode increases, decreasing with the end of the crop cycle and the off-season. However, with the continued, sequential planting of susceptible crops, the population density of nematodes continues to grow and can cause damage to plants. “It is important to know that tolerant plants may not express symptoms or damage, but they allow the nematode to multiply”, explains Asmus.

Due to a demand from Jotabasso technicians, researcher Asmus detailed two nematodes: cyst nematode and root lesion nematode.

Cyst nematode – Heterodera glycines

The plants become yellow and stunted. It is common for weeds to grow due to soil exposure to sunlight. The swings are very big. The disease caused is called soybean yellow dwarfism.

Small bright white or yellowish dots can be seen on the roots, which are the female nematodes. At the end of the nematode's life span, the females transform into very resistant cysts, filled with eggs. In cold climate regions, they can survive for up to 11 years in the soil.

Asmus said that, when it appeared in Brazil, the cyst nematode caused many problems, especially in Central Brazil, “but Science found solutions”. However, resurgences are currently occurring and this nematode is stealing soybean productivity. in places like MT and Northeast MS. “Why did he come back? One of the reasons is that corn culture has migrated from a summer crop to an autumn crop. As soon as it appeared, the recommendation was to rotate soybeans with corn, which is a resistant plant. Populations have declined on the ground. But, over time, corn began to be planted in succession to soybeans; rotation was lost.

To show how important rotation is, research was carried out on crop rotation with corn, sugarcane, brachiaria and cotton. After 1 year of rotation, on average, there was a reduction of around 70% in the number of viable cysts in the soil.

Cysts degrade due to fungal attack in the soil. And the presence of organic matter in the soil increases the amount of fungi.

Another cause of high population density is the low rotation of resistant varieties.

The prevalent breeds throughout the world are 1 and 3. But in Brazil we have a huge number of breeds. Soon after the occurrence of cyst nematode, research was mobilized. The vast majority of varieties released were resistant to race 3. As a result, selection ended up over time on other races, which began to progressively increase. 2,4 and 14 are the most frequent. We have 25% of soybean varieties resistant to cyst nematodes, but the vast majority are only resistant to races 1 and 3. Few varieties on the market are resistant to 2, 4 and 14.

To reduce selection pressure on breeds, it is necessary to return susceptible soybeans every two years and maintain the rotation so that the nematode population remains low.

Inadequate soil management: High soil pH increases the multiplication of cyst nematodes, in addition to reducing the availability of micronutrients, accentuating symptoms. Additionally, in conditions of high pH values ​​there will be less activity of soil fungi, which are the main natural enemies of the cyst nematode. Base saturation values ​​around 50% are the most appropriate to minimize problems with the nematode.

The presence of the cyst, between 1 and 3 per 100 cm3 of soil, may be enough to cause damage. Even with this apparently low population level, the nematode will already be “stealing” soybean productivity.

Root lesion nematode – Pratylenchus brachyurus

Slower initial multiplication rate. Symptoms in part area occur most commonly after flowering. The roots become necrotic. When the infestation is very high, the root system becomes extremely impoverished.

It penetrates and remains at the root throughout your life stage.

Until 2003, it was of secondary importance. Until in 2003, there was the first record of losses, with high populations, in MT. And today he once again attracted attention on the field.

It became a problem in the soybean – off-season corn succession. Corn is highly susceptible and soybeans are as well. Already the crotalaria spectabilis it is highly resistant.

Lack of resistant soybean varieties. We have more tolerant ones, which can handle it, but still allow nematodes to multiply.

Inadequate soil management: Sandy soils are more susceptible to soil compaction, water deficit, nutritional deficiency, excess or lack of limestone.

More common in the Cerrado, medium-sandy soils, many hosts, high populations in soybean roots.

Brachiarias multiply nematodes, some more and others less, in different quantities, depending on the species. But its benefit is great, it contributes organic matter, increases macroporosity, among other aspects that must be taken into consideration.

Millet is a strategy for controlling Pratylenchus in spring, before soybean planting. “If the population is very high, sunn hemp comes in during the summer, in rotation. In autumn, when the population is low”, says the researcher.

Biological control: There is a need to be careful when using biological fungicides. If applied in unsuitable environments (high sunlight, bare soil, for example), the probability of success is low. If it is associated with a quantity of organic matter, straw, the chances of success are good.