*Editorial por Me Paulo Santos; Ph.D. Cristiano Bellé
O nRoot-knot nematodes (Meloidogyne sp) are considered one of the biggest problems in many crops, such as soybeans, rice, cotton, beans, sugarcane, tobacco, coffee, pasture, vegetable and fruit crops, and they also reproduce on weed species. It is reported that more than 2,000 vegetable species are susceptible to the infection by root-knot nematodes, which may lead to approximately 5% to 10% of crop loss. They are found worldwide, especially in regions with high temperatures, including tropical and subtropical regions.
Among the root-knot nematode species reported in Brazil’s growing areas, Meloidogyne javanica and Meloidogyne incognita are considered the most common in crops, where high population rates can affect productivity. Root-knot nematodes that attack other species have also been reported, such as: Meloidogyne graminicola, Meloidogyne paranaensis, Meloidogyne hispanica, Meloidogyne ethiopica, Meloidogyne luci, Meloidogyne hapla, Meloidogyne arenaria, Meloidogyne morocciensis and Meloidogyne enterolobii, attacking specific crops in different regions of Brazil.
Damage and symptoms
Above-ground symptoms include stunting, wilting, chlorosis and other symptoms related to nutritional deficiency, and reduction in quality and crop yield (Figures 1 and 2). This combination results in shallow, poor and deformed root systems. Other below-ground structures can also have knots. (Figure 3). Cracks occur occasionally, such as in beets, sweet potatoes and carrots. In susceptible carrots, it is common to observe forked vegetables, with serious losses regarding the quality of the final product.
Root-knot nematodes are endoparasites that, after penetrating plant roots, establish a feeding site and form giant cells around it. In parallel, root knots are formed, which are characteristic symptoms of infection by Meloidogyne sp. The light brown egg masses on the root knots can be seen, and inside the roots, pearly white female nematodes can be observed.
Plant stand failures may occur in areas where population levels are high. In addition, severely damaged roots by root-knot nematodes can be infected by fungi and bacteria, which intensify the damages and can cause roots to rot. However, it does not necessarily happen to all crops as it depends on the root-knot nematode species, its population density in the area, and genetic factors linked to the susceptibility of crops to parasitism. We can mention sugarcane and corn, for example, which have small root knots that cannot be seen by the naked eye or may not even develop such symptoms.
Life cycle and favorable conditions
Throughout their life cycle, root-knot nematodes have four juvenile stages before becoming adults. The first ecdysis or cuticle exchange occurs inside the eggs. Then, second-stage juveniles (J2) hatch and go to the soil or penetrate roots right away.
Os J2 são vermiformes e medem entre 0,2 mm e 0,4 mm (Figura 4). Apenas o J2 é a forma infectante do nematoide-das-galhasSecond-stage juveniles (J2) are vermiform and range from 0.2mm and 0.4mm (Figure 4). Only J2 root-knot nematodes are infective forms that move through the soil until they reach the roots of host plants. The juvenile forms penetrate into the growing root tips (root cap) and migrate through the cells until they establish a feeding site. At this point, they become sedentary endoparasites (Figure 5). Secretions produced by their esophageal glands stimulate the formation of several giant cells in the parasitized roots, which provide them with nutrients. They rapidly increase in size, undergo ecdyses, becoming third-stage and fourth-stage juveniles, and finally adults. (Figure 6). Male nematodes are rarely found in M. incognita and M. javanica; when they are found, they migrate out of the roots and do not feed on them.
Throughout their life cycle, females lay from hundreds to up to 2,000 eggs. The eggs are laid in masses on the surface of the root knots, covered in mucilage that protect them from desiccation and other adverse conditions.
O ciclo dos nematoides-das-galhas leva cerca de três a quatro semanas no verão; no inverno, este tempo The life cycle of root-knot nematodes range from three to four weeks in the summer; in the winter, it can be as long as seven weeks. Thus, the length of their life cycle is highly influenced by temperature and is longer as soil temperature decreases.
The survival of root-knot nematodes and their life cycle depend on the successful growth of host plants and environmental conditions. Considering the role of female nematodes, male nematodes participate less. The development of the latter is apparently irrelevant, since most species reproduce by parthenogenesis, that is, without fertilization.
Due to the fact that nematodes move slowly in the soil (throughout the year, they are likely to move up to 50cm), the most important form of dissemination is passive, by the movement of soil, water, contaminated agricultural implements, people and animals in the crop areas, as well as by contaminated seedlings.
Figura 1 – Reboleiras de nematoide-das-galhas (Meloidogyne spp.) em lavoura de soja
Figura 2 - Amarelecimento nas extremidades das folhas de soja, devido ao parasitismo do nematoide-das-galhas
Figura 3 – Sintomas em raízes parasitadas pelo nematoide-das-galhas
Figura 4 – Espécimes e ovos do nematoide-das-galhas
Figura 5 - Fêmeas de Meloidogyne spp., de coloração esbranquiçada no tecido das raízes
Figura 6 - Ciclo do nematoide-das-galhas (Meloidogyne sp.), adaptado de Agrios (2005)
*Paulo Santos e Cristiano Bellé pertencem ao Grupo Phytus/Elevagro, parceiro homologado da Biotrop para a realização de análises laboratoriais de raiz e solo. Artigo escrito com exclusividade para a parceria.