Cultural management against nematodes

Responsible for the greatest damage caused to soybean crops during recent harvests is the root lesion nematode (Pratylenchus brachyurus) has challenged producers. Given the scarcity of technically effective or economically viable measures, the use of cultural practices in the off-season becomes fundamental. Judicious weed control is important. The use of sunn hemp can also help reduce the population of this species, but care must be taken to avoid aggravating other phytosanitary problems such as target spot and white mold.

In recent years, damage levels caused by phytonematodes have increased significantly due to the expansion of soybeans into new agricultural frontiers, the adoption of inadequate management practices and the intensification of monoculture. In Brazil, the main species associated with annual crops are root-knot nematodes (Meloidogyne incognita e M. javanica), the root lesion nematode (Pratylenchus brachyurus e P. zeae), soybean cyst nematode (Heterodera glycines), the reniform nematode (Rotylenchulus reniformis) and, more recently, the Tubixaba tuxaua, which frequently occur in soybean, corn, beans, cotton, sugar cane, sunflower crops, among others. Among these species, the Pratylenchus brachyurus has been responsible for the greatest damage caused to soybean crops in recent harvests, especially in the Central-West region, where its occurrence is more intense, due to the predominance of sandy soils and also the use of corn or cotton in succession to soybeans, crops also susceptible to the nematode. Another problem that affects the population control of this nematode is the unavailability of soybean cultivars with high levels of resistance and the occurrence of mixed populations (more than one species in the same area), which often leads to errors in diagnosis. In this sense, if root-knot nematodes occur in association with root lesions, the symptoms of the former will be easily identified and, in this case, the choice of management strategies may allow for an increase in the nematode population. Pratylenchus. Therefore, laboratory analysis is, without a shadow of a doubt, essential for better management.

The self-dissemination of lesion nematodes is relatively small, as they move short distances, generally forming patches that increase every year, in the same location. Greater spread is related to soil movement through contaminated agricultural implements, movement of workers and animals, water runoff in sloped areas and irrigation water.

The symptoms can be verified through the root system, where the roots show partially or totally dark areas, resulting from the coalescence of the many necrotic lesions caused internally by the nematode. These anomalies caused in the root system limit the absorption and transport of water and nutrients, leading the attacked plant to exhibit different reflex symptoms in the aerial part, in particular stunting, dwarfism, wilting in the hottest hours of the day, chlorosis and other signs of nutritional disorders, such as defoliation, drop in productivity, etc.

There are several management measures that can be considered to solve a particular problem of a nematological nature, however, to Pratylenchus brachyurus, Most of them do not prove to be technically effective or economically feasible, which makes the options quite restricted. The use of varietal control, for example, although it would be ideal, is not within reach at the moment, as there are no soybean cultivars that are highly resistant to P. brachyurus. It is within the scope of this issue that the cultural practices used in the soybean off-season become fundamental for the management of root lesion nematodes, since the population has increased with each harvest.

An experiment was conducted under commercial crop conditions in an area naturally infested by Pratylenchus brachyurus, in the municipality of Montividiu, Goiás. The reboleira was identified during the 2013/14 harvest at the nematology laboratory of Agro Carregal Pesquisa e Protecção de Plantas. An average of 21 nematodes were quantified in 100 cm³ of soil.

In March 2014, an experiment began where the plots were arranged in a randomized block experimental design containing different cultural options for the off-season, namely: Millet ADR 300 (T1); Crotalaria ochroleuca (T2); Corn P30K75 (T3); Crotalaria spectabilis (T4); Revolved area (T5); Constantly weeded area (no weeds) (T6); Area with weeds (no weeding) (T7). After desiccating the treatments, soybean was sown in October 2014. The population of P. brachyurus was determined in each plot, analyzing both the soil and the soybean roots at 45 days and 90 days after sowing (DAS). 

At 45 days after sowing, it was observed that the plots cultivated in the off-season with Crotalaria ochroleuca or Crotalaria spectabilis, the disturbed area and the area without weeds, showed lower populations of Pratylenchus brachyurus in soybeans. Corn and millet treatments occupied an intermediate position. The treatment with weeds resulted in higher populations of this nematode (Table 1).

Sunn hemp works as trap plants, allowing juveniles to penetrate their roots, but preventing their development into adulthood. In the case of the disturbed area, the soil remains clean in order to leave the nematode without shelter and exposed to adverse environments, so that it does not find food to develop, being killed by starvation, desiccation or exposure to the sun (attention must be paid to problems of erosion and soil degradation over time). Even in the absence of a living host, root lesion nematodes are capable of surviving in decomposing soybean roots for a period of at least 30 weeks. In this case, the best form of weeding is chemical, which reduces the time this nematode spends on soybean roots and, in addition, is a viable practice for farmers who grow soybeans in large areas.

At 90 days after sowing, there was intense multiplication of the nematode in the area with weeds and in the area cultivated with corn (Table 1).

Soybean productivity and thousand-grain weight decreased with the increase in the soybean population. P. brachyurus. The highest yields and thousand-grain weight were observed in the following treatments: area cultivated in the off-season with Crotalaria ochroleuca ou Crotalaria spectabilis, disturbed area and area free of weeds. Millet also proved to be a more viable alternative when compared to corn (Table 2). It is worth noting that in the southwest of Goiás, the predominant cultivation system is summer soybeans and off-season corn, which has contributed to the increase in the population of this nematode. Areas where post-harvest invasive management has not been carried out can further increase the population of this nematode.

The farmer must also be alerted that the incorrect use of sunn hemp can generate other phytosanitary problems such as an increase in the incidence of target spot (Corynespora cassiicola) and white mold (Sclerotinia sclerotiorum) in soybeans. Therefore, always consult an agronomist.

Celso Mattes de Oliveira, Luís Henrique Carregal, Agro Carregal Plant Research and Protection – Rio Verde/GO

Article published in issue 211 of Cultivar Grandes Culturas.