The rhizosphere bacterium ZKB-2 showed strong nematostatic activity against Meloidogyne incognita. Our study aimed to identify the nematostatic metabolites and evaluate the biocontrol efficiency in pot experiments. As the bacterial culture filtrate showed 100% nematostatic activity against M. incognita juveniles in 12 hr, we isolated and identified six compounds following activity guiding. 3-methoxycyclobutane-1, 2-dione showed 58.9% and 72.2% nematostatic activities against juveniles of M. incognita in 12 and 48 hr, with strong LC₅₀ value at 447 μg mL⁻¹. In pot experiments, treatments with the bacterial culture filtrate of strain ZKB-2 showed significant efficacy, especially at doses of 150 mL/pot, which were close to that of avermectin (positive control) at 0.01 g kg⁻¹ soil. The most effective treatment inhibited 85.1% population of juveniles of M. incognita in the roots and 76.9% in the rhizosphere soil after 30 days. Furthermore, the promoting tomato growth also significantly increased in a dose-dependent manner. Our results revealed the potential of strain ZKB-2 to act as a biocontrol agent in the integrated management of root-knot nematodes on tomatoes.

Pochonia chlamydosporia is widely applied in many countries as a biocontrol fungus against parasitic nematodes in plants. In a field experiment, the combined use of Bacillus nematocida B16 increased the biocontrol efficiency of P. chlamydosporia ZK7 against Meloidogyne incognita. Further study indicated that the colonization of P. chlamydosporia ZK7 in the rhizosphere soil and the roots of tomatoes was significantly higher in the combined use group than in the control group. Gas chromatography was conducted to determine the effects of signaling substances. Five compounds, hexanal, (E)-2-hexenal, furfural, benzaldehyde, and 2-nonanone, were found to be highly altered in the volatile compounds produced in the soil under the combined application. The changes in benzaldehyde and 2-nonanone were the main factors that resulted in an increase in the colonization of fungi P. chlamydosporia ZK7 in the tomato roots. Furfural was the main volatile substance that affected the colonization of fungi P. chlamydosporia ZK7 in the soil. The combined use of B. nematocida B16 and P. chlamydosporia ZK7 altered the volatile ranges and resulted in increased colonization of biocontrol fungi and improved biocontrol efficiency against nematodes. This combined model could be used to promote the ability of biocontrol fungi to control root-knot nematodes.

The AWA neurons of Caenorhabditis elegans mainly perceive volatile attractive odors, while the ASH neurons perceive pH, penetration, nociception, odor tropism, etc. The perceptual neurons of Meloidogyne incognita have been little studied. The number of infestations around and within tomato roots was significantly reduced after RNA interference for high-homology genes in AWA and ASH neurons compared between M. incognita and C. elegans. Through in situ hybridization, we further determined the expression and localization of the homologous genes Mi-odr-10 and Mi-gpa-6 in M. incognita. In this study, we found that M. incognita has neuronal sensing pathways similar to AWA and ASH perception of C. elegans for sensing chemical signals from tomato roots. Silencing the homologous genes in these pathways could affect the nematode perception and infestation of tomato root systems. The results contribute to elucidating the process of the plant host perception of M. incognita.

Rhizosphere microorganisms and the volatile organic compounds (VOCs) produced by them take part in the regulation of the chemotaxis of nematodes. A total of 150 strains of rhizosphere bacteria were screened via a chemotaxis experiment with Meloidogyne incognita. Some isolates affected the behavior of the nematodes, including attraction, randomness, and repulsion. Volatile metabolites produced via the selected bacteria were associated with the chemotaxis of nematodes. M. incognita was highly attracted to decanal. In addition, dimethyl disulfide, 2,5-dimethylpyrazine, pentadecanoic acid, and palmitic acid were found to attract weakly M. incognita. Furthermore, the chemotaxis of M. incognita was tested in a pot experiment. The bacteria Bacillus sp. 1-50, Brevibacillus brevis 2-35, B. cereus 5-14, Chryseobacterium indologens 6-4, and VOC decanal could regulate the movement of M. incognita in the pot with or without plants. The results provide insights into rhizosphere microorganisms and their VOCs and how they regulate the chemotaxis of the nematodes.