It is known that some plant essential oils have pesticide activities. Among the 29 oils evaluated in this study, 14 showed nematicidal activities of 8 to 100% at the concentration of 1,000 μg/ml, compared with a control of 0.01 g/ml Tween 80®. At a lower concentration of 500 μg/ml, only Dysphania ambrosioides oil caused >90% mortality of second‐stage juveniles (J2) of Meloidogyne incognita. The LC50 and LC95 values for D. ambrosioides oil were 307 μg/ml and 580 μg/ml, respectively. M. incognita eggs placed in D. ambrosioides oil solutions had a significant reduction in J2 hatching compared with controls. Therefore, the oil had a toxic effect on both eggs and J2 of M. incognita. This was in contrast to nematicides on the market that act efficiently only on J2. When J2 were placed in D. ambrosioides oil at its LC50 concentration and inoculated onto tomato plants, the reduction in numbers of galls and eggs was 99.5% and 100%, respectively. Dysphania ambrosioides oil applied to the potting substrate of plants at a concentration of 1,100 μg/ml significantly reduced the number of galls and eggs of M. incognita, whereas a concentration of 800 μg/ml only reduced the number of eggs compared with the controls (Tween 80® and water). The main components of the D. ambrosioides oil detected by gas chromatography–mass spectrometry were (Z)‐ascaridole (87.28%), E‐ascaridole (8.45%) and p‐cymene (3.35%), representing 99.08% of the total oil composition. Given its nematicidal activity, D. ambrosioides oil represents an exciting raw material in the search for new bioactive molecules for the pesticide industry.

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The search for natural nematicides that are biodegradable with little or no human toxicity has intensified in recent years. In this context, the use of essential oils has the potential to function as an alternative plant–parasitic nematode control strategy, and their characterization may identify new nematicidal molecules. In this study, the nematicidal activity of Cinnamomum zeylanicum essential oil, its most abundant biochemical component and its analogue were evaluated against Meloidogyne incognita. Mean LC50 and LC95 values for C. zeylanicum oil were 49 µg/ml and 131 µg/ml, respectively. When J2 placed in C. zeylanicum oil at its LC50 concentration were used to inoculate tomato plants, the reduction in numbers of galls and eggs versus samples inoculated using J2 and no C. zeylanicum oil was 98%. C. zeylanicum essential oil reduced levels of M. incognita J2 that hatched 38%–54%, while carbofuran (positive control) did not prevent hatching. C. zeylanicum oil applied at a concentration of 800 µg/ml to a potted substrate containing infested tomato plants significantly reduced numbers of M. incognita galls. The cinnamaldehyde molecule within C. zeylanicum oil had LC50 and LC95 values of 64 and 768 µg/ml, respectively, while LC50 and LC95 values for cinnamaldehyde oxime were 323 and 529 µg/ml, respectively. Both cinnamaldehyde and its oxime inhibited hatching in M. incognita J2. These findings indicate that C. zeylanicum essential oil, its major biochemical component, cinnamaldehyde and cinnamaldehyde oxime (a cinnamaldehyde analogue) can be used to reduce levels of M. incognita.

Summary Incorporation of castor bean cake into the soil results in the emission of four main compounds: skatole, γ-decalactone, 4-methylphenol and phenol. The toxic effects of these compounds on the life cycle of Meloidogyne incognita were studied here. Only phenol did not reduce hatching of M. incognita second-stage juveniles (J2) at any of the concentrations tested. The other compounds reduced J2 hatching by 42.1 to 59.3% at the highest concentrations. When J2 were immersed in Tween 80® solutions of each compound, at their respective LC50 values and inoculated into soil with tomato plants, reductions in the numbers of galls (48.5-69.3%) and eggs (42.8-62.5%) were observed in comparison with the control; this result was similar to that of carbofuran. The highest reductions in the numbers of galls (21.8-88.5%) and eggs (49.6-98.9%) occurred with all compounds when the highest concentration (500 mg l−1) was applied to the substrate at the moment of J2 inoculation. Volatile compounds of γ-decalactone and skatole failed to cause J2 immobility or mortality. However, phenol caused 95.8% and 4-methylphenol caused 100% mortality. With fumigation, the highest concentration (1000 mg (l substrate)−1) of any tested compound reduced gall numbers at a similar rate to the nematicide dazomet (98.7-100%) and reduced egg numbers (87.1-99.7%). The volatile compounds skatole, γ-decalactone, 4-methylphenol and phenol have a nematicidal effect on the life cycle of M. incognita and alter its pathogenic behaviour on the plant.