Root-knot nematode compromises corn roots.

Root-knot nematodes are among the phytosanitary problems that can compromise the development and productivity of crops. cornCaused primarily by *Meloidogyne incognita* and *Meloidogyne javanica*, the problem causes alterations in the root system and can result in uneven plants, smaller size, gaps in the stand, and malformation of ears.

The problem has been increasing in importance in corn-producing regions of Bahia, Goiás, Minas Gerais, Mato Grosso do Sul, Mato Grosso, Piauí, Paraná, and Rio Grande do Sul. Among the contributing factors are the intensification of agriculture, crop rotations between corn and soybeans, and the repeated use of susceptible cultivars.

Nematodes are microscopic worms that live in the soil and parasitize roots. In corn, the young forms penetrate newly formed roots and migrate internally to regions responsible for conducting water and nutrients. From there, they induce changes in plant tissues that give rise to so-called galls, characteristic swellings on the roots.

The process compromises functional root volume. With fewer roots able to explore the soil, the plant loses efficiency in absorbing water and nutrients. Therefore, even crops planted in fertile soils and adequately fertilized can exhibit symptoms similar to nutritional deficiencies and increased sensitivity to water stress.

The cycle begins with the hatching of second-stage larvae, called J2, from eggs present in the soil or associated with root debris. After penetration and establishment of the feeding site, galls form. Females produce hundreds of eggs, and some of them may remain in the soil awaiting favorable conditions or the presence of a new host crop.

The relatively short life cycle allows for multiple generations to occur during the same growing season. The wide range of host plants also makes control difficult through simple crop rotations alone. The material also points to the possibility of nematodes associating with fungi that cause root rot, increasing damage to corn.

In the roots, the main signs include gall formation, reduced root length and volume, decreased absorbent roots, and, in severe infestations, necrosis. These changes limit the plant's ability to explore the soil profile and can impair both fertilizer utilization and water absorption at greater depths.

In the aerial part of the plant, unevenness between plants, gaps in stand, reduced size, thinner stems, and less foliage may occur. Delayed flowering and the formation of small, poorly filled ears or ears with missing grains are also reported. In areas with a history of the problem, patches of lower productivity may persist throughout the growing seasons.

The impact on productivity varies according to the nematode population, soil and climate conditions, and the management practices used. The material reports that studies involving annual crops parasitized by *Meloidogyne* frequently indicate productivity reductions of approximately 10% to more than 50% in situations of high infestation, mainly in sandy soils and in areas successively cultivated with host species.

These percentages, however, do not represent an automatic prediction for any crop. The assessment itself emphasizes that the exact loss depends on the conditions of each area.

One suggested method for estimating the economic impact is to delineate the affected areas and compare the productivity of these areas with that of apparently healthy points in the same plot or nearby locations with no history of nematodes. The difference in productivity can then be multiplied by the selling price of corn to obtain an estimate of loss per hectare.

The establishment and vegetative growth period deserves attention. October and November are identified as critical months in different producing regions because they can coincide with higher soil temperatures and the formation of the corn root system. Under these conditions, nematode activity can be favored precisely when the plant has a high demand for water and nutrients.

To confirm the presence of the problem, visual assessment of the crop is not sufficient. The diagnosis must combine field observations and laboratory analysis. In suspected areas, patches with smaller or yellowed plants, uneven development, and root galls may be observed.

The material recommends the separate collection of soil and root samples and their submission to a nematology laboratory, where egg and larvae counts and identification of the species present can be performed. Interpretation should also consider the area's history, soil type, crop rotation, and technology employed.

Factors used to estimate risk include the initial nematode population, soil type, history of host crops, sowing time, and soil organic matter and structure conditions.

In addition to the ongoing corn crop, the presence of nematodes can maintain high populations for subsequent crops, increase the occurrence of associated root diseases, and reduce nutrient uptake efficiency. In intensive corn and soybean systems, these effects can accumulate over several growing seasons.

Accurate quantification of losses, therefore, requires a specific assessment of each crop. Laboratory diagnosis, combined with the area's history and field observation, is presented as a basis for assessing risk and guiding management decisions.