Insecticidal and antifungal activities of Thymus vulgaris and Cymbopogon citratus essential oils against Tuta absoluta and Geotrichum candidumon tomato production in Foumbot-Cameroon,

Abstract

Tomato (Solanum lycopersicum L.) yield has recently dropped in Cameroon, failing in supplying the national and neighbouring countries demands. Severe bug infestations, such as Tuta absoluta, has favoured tomato rot caused by Geotrichum candidum, which is one of the main reasons of decrease productivity. The most common method against damage management is the application of synthetic chemical pesticides, which are well-known for their harm to humans and the environment. Therefore, alternative strategies based on the use of natural products to increase production yield are required. Essential oils, as one of the most reliable biological pesticide sources, have shown promise in terms of efficiency, sustainability, and compatibility with other control measures in the context of Integrated Management Programs. The purpose of this study was to survey on Tuta absoluta in Foumbot, the main Cameroonian tomato production basins, and to determine the insecticidal and antifungal activities of Thymus vulgaris and Cymbopogon citratus essential oils. Data were collected through an interview and field sampling. Interview was performed on the basis of an open and closed response questionnaire with 56 tomato producers from five villages. The quantification of damages was carried out in 19 commercial tomato fields spread in the Foumbot subdivision. The incidence and severity of Tuta absoluta infestation were assessed using quadrats drawn in a W pattern and a 1 to 5 rating scale respectively. Furthermore, some factors related to the damage indexes were assessed. Hydro-distillation was used to extract essential oils from Thymus vulgaris and Cymbopogon citratus, and the oils chemical composition was evaluated using Gas Chromatography coupled with Mass Spectrometry. The insecticidal potential of essential oils was determined by testing their contact and fumigant toxicity against T. absoluta larvae. Geotrichum candidum was isolated from rotten-infected tomatoes fruits perforated by Tuta absoluta. Morphological and molecular traits were used to identify it. Essential oils were tested for antifungal efficacy on pathogens mycelia growth and conidia germination through dilution in solid medium and microdilution in liquid medium, respectively. As results, Tuta absoluta occurred in all the surveyed farms in different villages with mean incidence and severity of 93.20% and 4.40 on 1 to 5 rating scale, respectively. The epidemic damages were closely related to the variety and growth stage. The highest values been recorded with hybrid variety (incidence: 88.26%, severity: 4.46) and at the mature fruiting stage (incidence: 97.61%, severity: 4.90). The chemical composition analysis revealed that Thymus vulgaris oil was predominently composed of Thymol (21.53%) and α-Pinene (Dextro) (17.43%) whereas, Neral (34.48%) and Geranial (34.37%) were prominent in Cymbopogon citratus oil. Statistical analysis indicated that the both oils exhibited similar knockdown and insecticidal efficiencies through direct contact and fumigation routes. The resulting biological parameters for Cymbopogon citratus and Thymus vulgaris oils were KD50 values of 0.19 μL/mL and 0.59 μL/mL and LD50 values of 0.33 μL/mL and 0.61 μL/mL for contact toxicity and KD50 of 1.29 μL/mL and 2.57 μL/mL and LD50 of 1.48 μL/mL and 3.05 μL/mL for Cymbopogon citratus and Thymus vulgaris oils respectively for fumigant toxicity. Essential oils exerted larvicidal activity through their anti-acetylcholinesterase effect with similar IC50s value of 2.57 μL/mL and 3.06 μL/mL for Cymbopogon citratus and Cymbopogon citratus oils respectively. The sequencing of the ITS1-5.8S rDNA-ITS4 gene region and phylogenetic analysis confirmed the identity of the fungi pathogens to be the isolates of Geotrichum candidum (Geotrichum candidum G1, Geotrichum candidum G2 and Geotrichum candidum G3). The antifungal assay has shown that Minimal Inhibitory Concentration values of Cymbopogon citratus and Thymus vulgaris oils were 0.71 μL/mL and 5.67 μL/mL respectively on mycelia growth of all pathogens. The inhibition of conidia germination revealed that Minimal Inhibitory Concentration was undetermined with Thymus vulgaris oil and were 0.71 μL/mL, 1.42 μL/mL and 5.67 μL/mL for Cymbopogon citratus oil against Geotrichum candidum G1, Geotrichum candidum G2 and Geotrichum candidum G3 respectively. The results obtained demonstrate that Cymbopogon citratus oil could be used in the control of Tuta absoluta and Geotrichum candidum.