Reaction of weed species to the nematode Meloidogyne incognita

Throughout the development of agriculture and livestock, plants that directly infested areas of human occupation and were of no importance to humans or animals were considered undesirable and were called weeds (Braz et al., 2016).

The presence of weeds in planting areas always causes concern to farmers, because in addition to competing with the main crop, reducing productivity, weeds can serve as intermediate hosts for various pests and diseases during the harvest and off-season periods. In agricultural environments, weed species often have a competitive advantage over cultivated plants. This occurs because weeds have characteristics such as high growth rate, great reproductive ability and high capacity to exploit soil nutrients, ensuring survival in a wide variety of environments (Pitelli, 1985).

The problem becomes more serious when it is found that root-knot nematodes (genus Meloidogyne), extremely harmful to various cultivated species, are frequently found infesting weed roots (Ferraz et al., 1978), limiting the efficiency of certain measures to control these pathogens, such as crop rotation. Plant parasitic nematodes cause considerable reductions in agricultural productivity and the damage caused by them depends on the susceptibility of the crop, environmental conditions, the presence of other pathogens with which they interact and their population density (Lopes and Vieira 2011).

The gender Meloidogyne it is also the group of greatest economic importance in agriculture (Moens et al., 2009). Among them, M. unexplained It has a wide geographic distribution, high severity of damage caused to different crops and great difficulty in control, being responsible for increasing losses. These nematodes incite root galls and cause a reduction in water and nutrient absorption, culminating in plant stunting (Tihohod 2000). These characteristics, associated with the high number of alternative hosts, allow its presence in the most diverse agricultural areas. Among the species that are opportunely parasitized by nematodes, weeds are of high importance, especially during the off-season of agricultural crops (Bellé et al., 2017). 

 In this context, different weed species have been recognized as hosts of Meloidogyne spp., throughout the world (Roese and Oliveira, 2004; Rich et al., 2008; Mônaco et al., 2009), which contributes to the increase in nematode populations in the soil, making their control difficult and aggravating damage to the development of agricultural crops.

From the high polyphagic potential of M. unexplained It is important to know the reaction of invasive plants to root-knot nematodes, allowing for adequate management and efficient control. Therefore, the aim of this study was to evaluate the reaction of some weed species to the root-knot nematode. M. unexplained. 

The experiment was installed and conducted in a greenhouse at the experimental station of Agro Carregal Pesquisa e Protecção de Plantas, municipality of Rio Verde, GO, during the 2018/19 harvest. The reaction of 17 weed species to M. unexplained, as described in table 1. The experimental design used was a completely randomized design (DIC) with 17 treatments, four replications and two pots per plot.

The weed seeds came from the herbology sector of Agro Carregal Pesquisa e Protecção de Plantas and were sown in commercial Bioplant substrate contained in Styrofoam trays. Sowing was started with the seeds of weeds with a slower germination and development speed and ended with species with rapid germination. Thus, homogenization of development was maintained at the time of inoculation. The seedlings were transplanted 12 days after emergence into 300 cm³ pots containing only the sterile commercial substrate Bioplant, maintaining five plants per pot. Inoculation was carried out on the fifth day after transplantation.

The inoculum used in the test came from previously infested Santa Clara tomato seedlings and used as a substrate to maintain nematode breeding in a greenhouse. All pots containing invasive seedlings received 2 mL of inoculum containing 5.000 specimens (eggs + second stage juveniles (J2)). To this end, three holes approximately 2 cm deep were made, opened around the plants. ‘Santa Clara’ tomato plants were used as witnesses for the viability of the inoculum used.

The population of M. unexplained was analyzed 30 days after inoculation (DAI). The number of eggs and J2 (second stage juvenile) of M. unexplained After extraction, it was analyzed and quantified using a Peters chamber under a microscope. 

Reproduction factors were estimated through the ratio between the final and initial population (Fr = final population / initial population). The 30-day population was considered as the final population and the inoculated population (5.000 eggs + juveniles) as the initial population. Those that presented Fr or = 1 were considered susceptible and Fr = 1, immune (Table 0).

According to the analysis of the data obtained in the present study, it was found that more than 85% of the weed species tested were susceptible to the nematode M. incognita, presenting a reproduction factor greater than one unit (table 1). The viability of the inoculum can be proven by the average values ​​of the reproduction factor found in Santa Clara tomato plants, which was around 21,40, indicating intense multiplication of the nematode.

Most gall-causing nematode species can be easily identified in the field in relation to the genus, even by people who are experts in the subject, since the galls (cell hypertrophy and hyperplasia) are visible to the naked eye. Identification at species level must be carried out in a nematology laboratory by qualified people. However, in some cases, galls are not induced on the plant, even if it is host and multiplies the nematode, which can make visual identification of the genus difficult.

Among the species studied here, Digitaria insularis, Eleusine indica, Commelina benghalensis, Nicandra physaloides, Conyza bonariensis, Ipomoea grandifolia, Cenchrus echinatus, Bidens pilosa, Amaranthus viridis, Digitaria horizontalis, Alternanthera tenella, Euphorbia heterophylla e Portulaca oleracea behaved as susceptible to root-knot nematode. Only the species Spermacoce latifolia, Chloris polydactyla e Crotalaria ochroleuca did not multiply the nematode M. unexplained (table 1).

 Some weed species reported here as resistant, immune or susceptible may differ from other studies due to intraspecific variability of weeds or even physiological variation (race 1, 2, 3 and 4) of weed populations. M. unexplained.

Knowledge of the nematode reproduction factor in weeds is extremely important, as it represents the ability of this nematode to multiply in the soil, even during the off-season. This implies the need to adopt practices to control weeds that are susceptible to the nematode in areas where it is present. It is worth mentioning that in areas infested with nematodes, the damage caused by weeds tends to be greater, as in addition to multiplying the nematode in the soil, it also competes with the crop for abiotic factors such as water, light and nutrients. Another fact that should be mentioned is that some species are resistant to certain herbicides, such as Horseweed (conyza bonariensis), can be an aggravating factor when it comes to nematode management, as these species will remain in the soil, maintaining or multiplying the parasite.

Management strategies are specific to each species of nematode and, therefore, an agronomist should always be consulted.

*By Celso Mattes de Oliveira, from Agro Carregal Research and Plant Protection, and Luiz Henrique Carregal