Nematodes: importance and management

An important constraint to sustainable agricultural production is the impact of soil-borne pests and diseases. Nematodes are among the main agricultural pests today.

It is estimated that plant parasitic nematodes consume approximately 10% of global agricultural production, leading to annual economic losses estimated, cautiously, at more than US$125 billion. In Brazil, nematoses are among the most important plant health issues in field crops or in protected systems.  

The genres that occur most frequently are Meloidogyne (knot nematodes), Pratylenchus (lesion nematode), Heterodera (cyst nematodes),Rotylenchulus (reniform nematodes) and Radopholus (cave nematode). However, among all plant parasitic nematodes, there is a consensus among phytosanitary experts that the root-knot nematode is the most important. This pathogen is dispersed in various environments around the world, causing losses to the main agricultural crops.

In Brazil, several species have been reported in association with the main cultivated plants, but Meloidogyne unknown, M. javanica e M. enterolobii They are, admittedly, the most important species due to the damage caused and wide geographic distribution.

 

Regardless of the gender and symptom, the development of the nematode in the plant affects the functioning of the roots, impairing the absorption of water and nutrients and resulting in reduced plant development, wilting, chlorosis and lower production. Furthermore, they can be the cause of other root diseases, as the injuries caused to the roots facilitate infection by fungi and bacteria present in the soil. An example of this is the banana tree, where the association between the presence of the cave nematode and Panama disease, caused by the fungus, is common. Fusarium oxysporum f.sp. cubense.

The dispersion of nematodes in cultivation areas is mainly through propagative material, traffic of agricultural implements, animals and humans, in addition to rainwater runoff or irrigation. Therefore, the use of certified seedlings is crucial to avoid introduction and dissemination, as highlighted in the manuscript published by Professor Dr. Ailton R. Monteiro (ESALQ-USP), in 1981: “NNematodes should not be planted”. In fact, due to the clarity of this publication and the teachings contained therein, it should be mandatory reading for all plant health professionals, and is available at: http://docentes.esalq.usp.br/sbn/nbonline/ol%2005u/13-20%20pb.pdf.

In planting, the foundations of integrated nematode management have traditionally been based on the selection of resistant cultivars and application of nematicides and, in the off-season, crop succession and rotation. However, as demand for food production has increased, there are situations where rotation practices have declined to the point that many crops are produced in monocultures or with significantly reduced rotation periods, resulting in increased nematode populations.

Resistance to a certain plant parasitic nematode has been successfully introduced into some crops, but this process is slow and inter- and intraspecific diversity in the field compromises the efficiency of the method. Consequently, there has been dependence on the application of nematicides to contribute to the management of plant parasitic nematodes, sometimes as part of an integrated management strategy. However, there are increasing environmental and social limitations, leading to the development of alternative methods for nematode control.

Considering that the eradication of phytonematodes is practically impossible, integrated management uses techniques that aim to keep them below the population level of economic damage. Integrated nematode management is defined as the integration of different control measures, with the objective of maximizing the action of the agents, taking into account the ecological and economic characteristics of the crops.

To ensure that the most appropriate regime for nematode management is adopted by producers, the correct identification of species of economic importance is therefore crucial. Therefore, it is initially necessary to promote the taxonomic identification of the phytonematodes involved in the culture, as well as their importance, biological aspects, habits and hosts.

In integrated management, it is recommended to use resistant varieties, registered nematicides, biological control agents, succession and rotation of crops with non-host plants, including green manures and antagonist plants, and land fallow for at least 6 months, eliminating all volunteer plants and wild plants in the area, since the maintenance of the nematode population in the cultivation area can occur through their survival in weeds and volunteer plants of the crop.

The addition of organic matter to the soil in the form of seed cake, plant biomass, agro-industrial and animal waste and even urban waste, such as waste and sewage treatment waste, has been used. The addition of organic material improves the physical-chemical properties of the soil, favoring plant growth and making them more tolerant to nematode attacks. It also promotes the growth of populations of natural enemies of nematodes. Furthermore, the decomposition of organic matter releases compounds that are highly toxic to phytonematodes, such as ammonia and fatty acids, which are formed during its decomposition, in addition to the incorporation of more nutrients that will be used by the crop.

Regarding the choice of the best plant option in rotation or succession aimed at managing nematodes, the Crotalaria spectabilis e C. breviflora are the most recommended from a nematological point of view, due to the wide spectrum of nematodes that are controlled. Peanuts can be effective in controlling M. unexplained e R. reniformis, but causes an increase in the density of P. brachyurus, with negative impacts on the production of peanuts, cotton and soybeans. The brachiariae (Brachiaria decumbens, B. brizantha e B. ruziziensis) to Panicum maximum are effective in controlling M. unexplained e R. reniformis, but, like peanuts, they cause an increase in the population density of P. brachyurus.

Regarding castor beans, in general, there is a lack of more detailed investigations into their interaction  with  nematodes. Most research involves evaluating the efficiency of cakes and other products derived from castor beans for controlling nematodes, but few studies evaluate their resistance to nematode species and the possibility of using castor beans in a crop rotation or succession program. However, it is known that castor bean is resistant to cyst nematode and that the cultivars Íris, Coti, Guarani, Pernambucana, Sangue de Boi, Savana and IAC-80 are resistant to three species of root-knot nematodes (M. javanica, M. incognita  e M. paranaensis), but there are cultivars susceptible to Rotylenchulus reniformis and with an intermediate reaction to P. brachyurus.