Crop rotation and succession as strategies for managing reniform nematode

Reniform nematode is the common name for a species of microscopic larva that lives in the soil, attacks plant roots, and is known by the scientific name of Rotylenchulus reniformis. It is a very important species in tropical and subtropical regions, as it causes damage to several crops of economic importance. In Brazil, this species has been assuming, year after year, a prominent role in cotton and soybean crops.

Unlike several other species of phytoparasitic nematodes, the reniform nematode does not cause noticeable symptoms on the roots and its occurrence, multiplication and damage are associated with soils of good fertility and high clay content.

In soybean cultivation, the reniform nematode causes underdevelopment of plants and a consequent reduction in productivity. In cotton, in addition to underdevelopment, the occurrence of mottled leaves or interveinal chlorosis (“carijó” leaves) is common. The nematode population density in the soil at the time of planting annual crops is one of the most important factors for the occurrence of damage. Research data showed that, in Mato Grosso do Sul, productivity losses can reach 32% in soybeans and 60% in cotton, under conditions of high nematode population density in the soil. Even so, there are few resistant soybean and cotton cultivars available on the market. Therefore, management practices aimed at reducing the nematode population are highly recommended.

The rotation or succession of soybeans and cotton with non-host crops are cultural practices that greatly contribute to reducing the reniform nematode population in the soil. During the period in which the infested area is cultivated with non-host or resistant species, there is a reduction in the number of nematodes in the soil.

The principle involved in crop rotation and succession is that the infective, vermiform forms of the nematode do not find roots of susceptible plants available for parasitism and, consequently, die from starvation. It is worth noting that, based on this principle, the soybean-cotton succession (two susceptible crops), a system used in around 80% of cotton production in the Cerrado, represents a high risk for the sustainability of the activity.

The rotation of soybeans or cotton with corn, or even the succession with sorghum, forage radish and forage grasses (brachiaria or panicum) have been shown to be efficient in reducing the reniform nematode population in the soil. In some more serious situations, species with a high potential for reducing the nematode population, such as sunn hemp, can be inserted into the system, in intercropping with corn or forage grasses.

An interesting aspect of the succession of soybeans or cotton with forage grasses lies in the fact that there is a greater reduction in the nematode population than in soil left fallow or fallow during the off-season. The reniform nematode has the ability to enter a state of dormancy (anhydrobiosis) in low humidity conditions, which gives it resistance for long periods, until favorable conditions are reestablished. In the case of soil cover, mainly by forage grasses, humidity remains higher for a longer period after soybean harvest, allowing the infective forms to “swim” in the soil solution in search of host plant roots, and, in the absence of these, they consume bodily energy, resulting in death.

As a general rule, the longer the period without the host crop (soybeans or cotton, for example), the better the effects in reducing the nematode population. This implies that in heavily affected areas there may be a need for long periods of rotation or succession. Experimental results show that, under high populations, better results are obtained when forage grasses remain in the system for longer periods (two or more years), which is possible in crop-livestock integration systems.

Guilherme Lafourcade Asmus, Researcher at Embrapa Agropecuária Oeste