Control by bacteria: strategy for managing nematodes in tomato plants

The tomato plant (Solanum lycopersicum L.) has great economic importance, being one of the most cultivated and consumed due to numerous factors such as versatility in terms of use, nutritional and commercial value (Fernandes et al, 2007). Soil pathogens are of great importance to this crop due to the damage caused and the difficulties in controlling them. Plant parasitic nematodes stand out, which in many cases make production and cultivation in infested areas unfeasible.

Tomato plants, when severely attacked by root-knot nematodes, Meloidogyne spp., have a completely disorganized root system and few functional roots. In high infestations at the beginning of the crop, seedlings tend to die in the field, and in surviving plants, production is strongly affected in quantity and quality (Charchar & Aragão, 2005).

To manage these plant parasites, chemical control is often used, which is increasingly limited in use. In this sense, biological control emerges as a viable alternative for the management of phytonematodes. Actinobacteria constitute an important group of bacteria, belonging to the class Actinobacteria, strict aerobics, commonly found in soil. The gender Streptomyces is the most studied among actinobacteria, being known worldwide for its capacity to produce antibiotics (Padilha, 1998), lytic enzymes and for the decomposition of organic matter, (Getha et al.

Studies have demonstrated the potential of actinobacteria as biological control agents for nematodes M. javanica, M. incognita, Rotylenchulus reniformis e Pratylenchus penetrans (Jonathan et al, 2000; Coimbra et al, 2004; Sousa et al, 2006). There is a great diversity of secondary metabolites produced by actinobacteria, which can have a nematicidal effect (Pollak & Berger, 1996; Blackburn et al, 1996; Garabedian & Van Gundy, 1983).

With the aim of evaluating the control of Meloidogyne javanica in tomato culture with actinobacteria, experiments were carried out vitro e in vivo at the Center for Agricultural, Environmental and Biological Sciences, Postgraduate Program in Agricultural Sciences, at the Federal University of Recôncavo da Bahia, in the municipality of Cruz das Almas.

    For the experiment vitro 17 isolates of actinobacteria and two controls (distilled water and culture medium) were evaluated by adding 50µl of a suspension containing 25 juveniles of M. javanica together with 500µl of liquid culture medium containing the metabolites produced by the actinobacteria isolates, this assay being incubated for 24 hours, 48 ​​hours and 72 hours, to evaluate the percentage of immobile and dead nematodes.

To evaluate the effect of actinobacteria on the control of M. javanica in tomato seedlings grown in commercial substrate, an experiment was carried out in a greenhouse with the same actinobacteria isolates mentioned above and a control, in tomato seedlings inoculated with M. javanica and without inoculation with M. javanica. The substrate was enriched with actinobacteria and incubated in polyethylene bags for 40 days at room temperature and with humidity maintained close to field capacity. After this incubation period, the tomato plant was sown and 15 days after the germination of the tomato seeds, the seedlings were inoculated with three thousand eggs and/or juveniles per plant. M. javanica. When collecting the plants, the height, stem diameter, dry mass of the aerial part and roots and the level of damage (number of galls and egg mass per plant and per gram of roots) caused by the nematode were evaluated.

There was a nematicidal effect provided by the metabolites produced by all actinobacteria isolates, with a nematode mortality rate above 50%. The actinobacteria isolates that caused the highest percentages of nematode mortality M. javanica were: BFT 4, BFT 11, BFT 41, PD3, BFT 58, BFT 25, which presented 88,85%, 87,27%, 86,85%, 86,12%, 85,75% and 85,57% , respectively (Table 1).

The effect of secondary metabolites on nematode motility and mortality varies depending on the actinobacteria isolate. Furthermore, intrinsic differences between actinobacteria species, as well as other characteristics of the growth medium (pH, temperature and nutrient availability), can interfere in both the quantity and composition of metabolites produced (Moura et al, 1998). These characteristics may have influenced, in part, the different degree of mortality provided by the actinobacteria isolates tested.

There was a significant difference in the number of galls and egg mass on the roots of tomato seedlings grown in the substrate, when compared to the roots of seedlings grown in a substrate without actinobacteria (Table 2). There was a reduction of up to 42,7% (BFT 104) in the number of galls/plant in relation to the control inoculated with nematodes. The actinobacteria isolates BFT 7, BFT 26, BFT 41, BFT 58, BFT 66, BFT 71, BFT 88, BFT 104, BFT 106 and PD3 promoted a significant reduction in the number of galls.

Isolates PD3, BFT 26 and BFT 104 reduced egg mass per plant in tomato roots by 61,9%, 62,3% and 68,4%, respectively. In relation to egg mass per gram of root, all isolates showed a nematicidal effect, with a reduction of up to 76,4% (Table 2). Isolate BFT 104 stood out among the others, with the highest percentages of reduction in the number of galls/plants, galls/g root, egg mass/plant and egg mass/g root. The reduction in egg mass has a direct effect on the nematode cycle and the reduction of its population in the soil, being a control strategy for this pathogen.

The substrate used was infested and incubated for 40 days before planting with actinobacteria. During incubation, the production of extracellular enzymes and secondary metabolites may have occurred in the substrate, which caused a reduction in the infectivity of nematodes in tomato roots.

There was no significant interaction between the inoculation of the substrate with actinobacteria isolates and the inoculation or not with nematodes for the parameters plant height, stem diameter and dry mass of the shoot. However, an interaction was obtained for root dry mass, with greater production of root dry mass in tomato plants. inoculated with nematodes and grown on substrate treated with actinobacteria (Table 3).

The increase in the development of the root system of tomato plants, in the presence of nematodes, can be explained by the greater supply of nutrients in the substrate to be absorbed by the plant roots, favoring the formation of secondary roots, which contributed to better development of the tomato plant. root system and increased infection and gall formation. The plant becomes more tolerant and undergoes metabolic changes that can favor root growth.

The use of biological agents constitutes a strategy with positive results against soil pathogens for which control measures are restricted. Thus, the results of these works demonstrated that actinobacteria have great potential for the biocontrol of M. javanica in tomato cultivation.

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