In vitro antifungal activity of the tea tree (Melaleuca alternifolia) essential oil and its major components against plant pathogens

AIMS: The aim of this study was to examine the effect of Melaleuca alternifolia essential oil (TTO) and its principal components on four cereal-pathogenic fungi. METHODS AND RESULTS: The antimycotic properties of TTO and of terpinen-4-ol, gamma-terpinen and 1,8-cineole (eucalyptol) were evaluated in vitro on Fusarium graminearum, Fusarium culmorum and Pyrenophora graminea. Moreover, barley leaves infected with Blumeria graminis were treated with whole TTO. All the tested fungi were susceptible to TTO and its components. CONCLUSIONS: TTO exerted a wide spectrum of antimycotic activity. Single TTO purified components were more active than the whole oil in reducing in vitro growth of fungal mycelium and, among the tested compounds, terpinen-4-ol was the most effective. SIGNIFICANCE AND IMPACT OF THE STUDY: TTO and its components can be considered potential alternative natural fungicides.     

Efficacy of plant essential oils on post-harvest control of rot caused by Botrytis cinerea on kiwi fruits

Antifungal activity of the essential oils of Carum carvi and Pimpinella anisum against Botrytis cinerea fruit rot of key kiwi fruit was studied. In vitro experiments, antifungal activities of essential oils were tested on potato dextrose agar media. Results of an in vitro experiment showed that these essential oils, at all applied concentrations, inhibited grey mould growth. Black caraway essential oil at concentrations of 600 and 800?μL?L^?1 inhibited germination spores of grey mould. Then, the fruits were artificially inoculated with a suspension at 1?×?10⁵?conidia/ml and then treated with different concentrations of these essential oils. The results of in vivo conditions showed that black caraway and anise essential oils applied at all concentrations were increasing the shelf life and inhibited the grey mould growth on kiwi fruits completely in comparison to control. The result showed that black caraway and anise oils at a concentration of 800?μL?L^?1 had higher total soluble solids, ascorbic acid, titrable acidity and antioxidant content compared to untreated fruits.     

In vitro antifungal activity and mode of action of selected polyphenolic antioxidants on Botrytis cinerea

Six selected antioxidants (catechin, quercetin-3-galactoside, cyanidin-3-glucoside, pelargonidin-3-glucoside, ellagic and gallic acids) were evaluated in vitro for their antifungal activities and mode of action on Botrytis cinerea Pers., one of the most important pathogens of strawberries. Inhibitory effects were found for all the tested antioxidants, but varied at different fungal developmental stages. Catechin and quercetin-3-galactoside showed linear inhibitory effects on germ tube elongation, with the highest suppression ratios of 54.8% and 58.8% respectively. No significant effect was found on spore germination between treatments and control. Gallic acid showed very strong and linear inhibition on spore germination (r = ?0.95), but the effect diminished after spore germination. Cyanidin-3-glucoside and pelargonidin-3-glucoside provided effective control on the fungi as concentrations increased. The arresting effect of ellagic acid on development of B. cinerea was quadratic. Ellagic acid inhibited germ tube elongation and mycelial growth at its highest and lowest concentrations, while no effects were observed at its medium concentration used in this study.     

The antimicrobial peptaibol trichokonin IV promotes plant growth and induces systemic resistance against Botrytis cinerea infection in moth orchid

Trichokonins (TKs) are antimicrobial peptaibols that are extracted from Trichoderma pseudokoningii strain SMF2. We discovered that TK VI, the primary active constituent of TKs, not only promotes growth, but also induces systemic resistance against grey mould caused by Botrytis cinerea in moth orchid (Phalaenopsis). Firstly, following treatment with TK VI, the growth of several varieties of Phalaenopsis increased relative to the control treatment. Both the aboveground and belowground biomass increased, particularly the length, superficial area, volume and root branching. Secondly, treatment with TK VI by either root or foliar application controlled grey mould on the moth orchids. Following irrigation with TK VI, the activities of defence‐related enzymes, including peroxidase, polyphenol oxidase and phenylalanine ammonia‐lyase, and the resistance‐related enzymes, including superoxide dismutase and catalase, all increased, while those of harmful substance malondialdehyde decreased. These findings indicated that induced systemic resistance was the primary mechanism of control.     

Antimycotic activity of Melaleuca alternifolia essential oil and its major components

AIMS: The aim of this study was to analyse the antimycotic properties of Melaleuca alternifolia essential oil (tea tree oil, TTO) and its principal components and to compare them with the activity of 5-fluorocytosine and amphotericin B. METHODS AND RESULTS: The screening for the antimycotic activity was performed by serial twofold dilutions in Roswell Park Memorial Institute medium with the inclusion of Tween-80 (0.5%). TTO and terpinen-4-olo were the most active compounds. CONCLUSIONS: The majority of the organisms were sensitive to the essential oil, with TTO and terpinen-4-olo being the most active oils showing antifungal activity at minimum inhibitory concentration values lower than other drugs. SIGNIFICANCE AND IMPACT OF THE STUDY: This study provides a sample large enough to determine the antifungal properties of TTO and terpinen-4-olo and suggests further studies for a possible therapeutic use.     

Influence of essential oil of Hyssopus officinalis on the chemical composition of the walls of Aspergillus fumigatus (Fresenius)

The cell walls of the growing hyphae of Aspergillus fumigatus (Fresenius) cultured in the presence or absence of the essential oil of Hyssopus officinalis were isolated and their chemical composition analysed. The presence of the essential oil led to a reduction in levels of neutral sugars, uronic acid and proteins, whereas amino sugars, lipids and phosphorus levels were increased. HPLC analysis of the neutral sugars showed that they consisted mainly of glucose, mannose and galactose, while the amino sugars consisted of glucosamine and galactosamine. The presence of the essential oil in the culture medium induced marked changes in the content of galactose and galactosamine. Cell walls were fractionated by treatment with alkali and acid. The essential oil induced similar alterations in the various fractions with a more marked effect on the major constituents. The alterations were related to changes in the structure of the cells. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effects of nanostructured essential oils against subterranean termites (Coptotermes gestroi)

Currently, the control of termites is mainly carried out with the use of chemical pesticides, which although effective, pose risks to the environment and to human beings. An alternative to using such chemical pesticides is natural products such as essential oils in insecticidal action. Despite the fact of being effective, essential oils are unstable, poorly soluble in water, and degrade from the action of light, oxygen even at moderate temperatures. Thus, the nanostructuring of essential oils could circumvent such problems and ensure its effectiveness. The following study aimed to assess for the first time the effects of nanostructured systems of essential oils of Cymbopogon flexuosus, Eucalyptus globulus and Melaleuca alternifolia in subterranean termites (Coptotermes gestroi). The results showed that the solid lipid nanoparticles of M. alternifolia have both repellent and insecticide action. The following study showed that nanostructuring of essential oils with insecticidal action is a promising tool in the fight against termites.     

Functional analysis of endo‐1,4‐β‐glucanases in response to Botrytis cinerea and Pseudomonas syringae reveals their involvement in plant–pathogen interactions

Plant cell wall modification is a critical component in stress responses. Endo‐1,4‐β‐glucanases (EGs) take part in cell wall editing processes, e.g. elongation, ripening and abscission. Here we studied the infection response of Solanum lycopersicum and Arabidopsis thaliana with impaired EGs. Transgenic TomCel1 and TomCel2 tomato antisense plants challenged with Pseudomonas syringae showed higher susceptibility, callose priming and increased jasmonic acid pathway marker gene expression. These two EGs could be resistance factors and may act as negative regulators of callose deposition, probably by interfering with the defence‐signalling network. A study of a set of Arabidopsis EG T‐DNA insertion mutants challenged with P. syringae and Botrytis cinerea revealed that the lack of other EGs interferes with infection phenotype, callose deposition, expression of signalling pathway marker genes and hormonal balance. We conclude that a lack of EGs could alter plant response to pathogens by modifying the properties of the cell wall and/or interfering with signalling pathways, contributing to generate the appropriate signalling outcomes. Analysis of microarray data demonstrates that EGs are differentially expressed upon many different plant–pathogen challenges, hormone treatments and many abiotic stresses. We found some Arabidopsis EG mutants with increased tolerance to osmotic and salt stress. Our results show that impairing EGs can alter plant–pathogen interactions and may contribute to appropriate signalling outcomes in many different biotic and abiotic plant stress responses.     

Baseline sensitivity of Botrytis cinerea to fluazinam and cross‐resistance

Grey mould, caused by the fungus Botrytis cinerea Pers ex Fr., is a very destructive and important disease worldwide. Fluazinam is a phenylpyridinamine fungicide with broad‐spectrum activities. The baseline sensitivity of B. cinerea to fluazinam is yet to be established in Henan Province, China. In this study, a total of 117 field isolates of B. cinerea were collected from 49 commercial greenhouses in different locations of Henan Province, in 2016, and the sensitivities of these isolates to fluazinam were determined based on mycelial growth. The effective concentration for 50% (EC₅₀) values ranged from 0.0038 to 0.0441 μg/ml, and the mean EC₅₀ value was 0.0201 ± 0.0081 μg/ml for mycelial growth. The frequency distribution range presented a unimodal curve. To define the cross‐resistance relationships, the linear correlation coefficients of the EC₅₀ values between fluazinam and carbendazim, procymidone, pyrimethanil or boscalid were analysed. The results showed that no correlation was observed between fluazinam and the other tested fungicides. These results provide important information to growers for the prevention and control of grey mould.