Reaction of cover crops to Meloidogyne enterolobii originating from cotton

Cover crops have been expanding in Brazilian agricultural systems due to improved soil quality. Among the benefits, the following stand out: increased organic matter, improved physical structure, nutrient cycling, and assistance in managing weeds and phytopathogens. Their use in crop succession/rotation contributes to reducing exclusive dependence on chemical inputs and increasing the sustainability of production systems.

Despite this, plant-parasitic nematodes remain important limiting factors for productivity. Species of the genus Meloidogyne (root-knot nematodes) stand out, being widely distributed and capable of parasitizing various crops, causing alterations in the root system, impaired water and nutrient absorption, and yield losses.

Among these species, Meloidogyne enterolobii has gained prominence as an emerging and highly aggressive species, characterized by its high virulence and wide host range, being able to infect and overcome plant resistance to M. incognita, M. javanica, and M. arenaria.

In Brazil, concern intensified from 2019 onwards, with reports in cotton (Gossypium hirsutum L.) in Minas Gerais, Bahia and Mato Grosso, including infections in previously resistant cultivars, evidencing a "breakdown" in resistance.

Studies conducted in Brazil indicate the presence of physiologically distinct groups of M. enterolobii, with differentiated behavior towards host plants, based on tests with differential hosts. In general, these groups have been referred to as "race 1," associated with guava and vegetables, and "race 2," associated with populations originating from cotton. The latter, characterized by its high reproductive capacity in cotton, has shown potential to overcome previously effective sources of resistance.

This scenario is particularly concerning in intensive systems, such as soybean-cotton rotation, where the nematode can multiply and persist throughout the growing seasons, reducing the efficiency of practices such as crop rotation and requiring adjustments in management.

In this context, the selection of cover crops becomes strategic for reducing nematode populations and improving soil. However, information on the reaction of these species, especially to aggressive populations associated with cotton ("race 2"), is still scarce, which hinders recommendations in integrated management programs.

Given this, the present study aimed to evaluate the reaction of different cover crop species to M. enterolobii, in order to identify resistant or poor host materials, so as to support the selection of more suitable species in rotation and succession systems and contribute to the reduction of nematode populations in the soil.

Material and methods

The experiment was conducted in a greenhouse at the Staphyt Brasil Experimental Station (Conchal - SP), under controlled temperature (25°C - 30°C) and regular irrigation, for 90 days after inoculation (DAI). The design was completely randomized (CRD), with 19 treatments and five replications, each consisting of one pot with one plant.

The treatments included tomato (Solanum lycopersicum, Santa Clara) as a susceptibility standard; 17 species of cover crops: Avena sativa (white oat), Fagopyrum esculentum (buckwheat, IPR 92 Altar), Crotalaria ochroleuca, C. spectabilis, C. juncea, Triticum aestivum (wheat, cultivars TBIO Ponteiro and TBIO Duque), Secale cereale (rye, BRS Serrano), Arachis hypogaea (peanut, IAC OL3), Cenchrus americanus (millet, BRS 1501; formerly Pennisetum glaucum), Canavalia ensiformis (jack bean), Sorghum bicolor (sorghum, genotypes B1G211 and B233), Mucuna pruriens (velvet bean), Megathyrsus maximus (zuri grass, BRS Zuri; formerly Panicum maximum), Crotalaria breviflora and Urochloa ruziziensis (= Brachiaria ruziziensis). Additionally, cotton (Gossypium hirsutum, BRS 416) was used as a complementary treatment to verify the viability of the inoculum.

The inoculum was obtained from cotton plants originating from northern Minas Gerais. The population was previously characterized by isoenzyme electrophoresis as M. enterolobii, adapted to cotton and, therefore, designated in this study as "race 2". Subsequently, it was multiplied for three months in cotton (Gossypium hirsutum, cv. IMA 5801B2RF), in a greenhouse, ensuring the viability and infective capacity of the nematode to this crop.

The plants were grown in styrofoam pots (1 L) containing a mixture of soil and sand (2:1, v/v), previously sterilized in an autoclave at 121°C for two hours.

Twenty days after sowing, inoculation was carried out with a suspension of eggs and second-stage juveniles (J2) of M. enterolobii, with the species identity again confirmed by isoenzyme electrophoresis. An initial population (Pi) of 2.500 eggs/J2 per plant was established. The suspension was applied to two holes (~2 cm deep) around the root zone.

During the experiment, reproductive structures (flowers, pods, fruits, and inflorescences) were manually removed to prolong the vegetative period, maintain vigor, and prevent premature senescence, ensuring uniformity and greater availability of root tissue for evaluating the multiplication of M. enterolobii.

At 90 DAI, the roots were removed, washed in running water, dried on paper towels, and fragmented (< 0,5 cm). Egg and juvenile extraction followed the method of Hussey & Barker (1973), modified by Boneti & Ferraz (1981), with grinding in a 1% NaOCl solution for 45 s, sieving (20, 100, and 500 mesh), and collection of the material retained on the 500 mesh sieve.

Quantification was performed in a Peters chamber under an optical microscope, determining the final population (Pf) of eggs/J2 per plant. Based on these data, the reproduction factor (RF = Pf/Pi) was calculated, according to Oostenbrink (1966), considering Pi = 2.500 eggs/J2 per plant, to evaluate nematode multiplication in cover crop species.

Plants with a FR ≥ 1 were considered susceptible, as they increased the nematode population, while those with FR < 1 were classified as resistant and indicated for management. The data were subjected to statistical analysis, with mean grouping using the Scott-Knott test at a 5% probability level.

Results obtained

The highest reproduction factors (RF) occurred in Santa Clara tomato (25,70) and BRS 416 cotton (6,47), confirming high susceptibility and viability of the inoculum. High RF values ​​indicate that the nematode found favorable conditions to multiply in the root system of the host plant. Under field conditions, this multiplication can result in a significant increase in the pathogen population during the off-season.

Among the cover crops, C. spectabilis (0,48), BRS Zuri Zuri grass (0,42) and Urochloa ruziziensis (0,36) showed FR < 1 (resistant). C. ochroleuca (0,84) and C. breviflora (0,92) were also classified as resistant, although with less efficiency in reducing the nematode population.

Most species were susceptible (FR ≥ 1), including white oats (AF1355), buckwheat (IPR 92 Altar), wheat (TBIO Ponteiro and TBIO Duque), rye (BRS Serrano), millet (BRS 1501), jack bean, sorghum (B1G211 and B233), and peanut (IAC OL3). BRS 1501 millet (6,83) and BRS Serrano rye (5,40) stand out, with FRs similar to or higher than cotton. Under these conditions, the use of these species during the off-season can result in a significant increase in the pathogen population in the soil and should be avoided in infested areas.

Species such as C. juncea (FR = 1,12) and velvet bean (FR = 1,12) showed FR close to 1, being classified as susceptible, but with low multiplication. Despite this, their use should be judicious, as these species tend to maintain the nematode population close to the initial level at the end of the crop cycle, contributing to the persistence of the pathogen in the soil over time.

The use of C. spectabilis, Zuri grass, and U. ruziziensis in rotation or intercropping systems presents a high potential for reducing the population density of M. enterolobii. This effect is maximized when the plants have good establishment, with a uniform stand and high root density, promoting ample soil coverage. Under these conditions, there is greater contact between the nematode and the root system of the plants, favoring a reduction in the population at the end of the cycle.

Appropriate choices

In intensive agricultural systems, especially those based on soybean-cotton rotation, the choice of cover crops plays an important role in nematode management. Susceptible species can intensely multiply the pathogen during the off-season, increasing the population in the soil before the planting of the next crop and increasing the risk of yield losses.

Classification of species

From a practical standpoint, the results allow for the classification of species according to their risk of use in infested areas. Species such as C. spectabilis, zuri grass, and U. ruziziensis can be prioritized, as they show potential to reduce the nematode population throughout its life cycle.

On the other hand, BRS 1501 millet, BRS Serrano rye, wheat (TBIO Ponteiro and TBIO Duque) and jack bean should be avoided in these areas due to the high multiplication capacity of M. enterolobii, “race 2”.

Species with a FR close to 1, such as C. juncea and velvet bean, should be used with caution. Despite the low multiplication rate of the nematode, these species tend to maintain the population close to the initial level when managed until the end of the cycle. In these cases, early management of the cover crop is recommended, aiming to avoid maintaining the pathogen in the soil and promoting population reduction.

The closing thought

Cover crops influence the dynamics of M. enterolobii during the off-season: susceptible species (FR ≥ 1) increase the population in systems such as soybeans and cotton, raising the risk to the following crop, while poor host species (FR < 1) reduce the nematode. Thus, their selection is essential in integrated pest management, aiming for lower inoculum pressure and greater sustainability of the production system.

* By Anderson Vieira Modro, Beatriz Martins, Mireli do Carmo Vieira da Silva (Unasp); Daniela Santana Rodrigues Verssiani (Esalq); Jorge Bleno da Silva Verssiani (Staphyt Brasil)

Article published in issue 321 of Cultivar Grandes Culturas Magazine