The volatile oil from Eucalyptus camaldulensis (syn. E. rostrata) has been the subject of several studies where the target organisms were dermatophytic fungi. Singh et al. () tested the oil against four human pathogens, Trichophyton mentagrophytes, Epidermophyton floccosum, Microsporum cants and M. gypseum, as well as two storage fungi, Aspergillus nidulans and A. terreus. At concentrations of 10,000 ppm (1 per cent) the oil showed fungicidal activity towards all the test organisms. In a second study (), a combination of oils from E. camaldulensis and Juniperus communis was found to be more effective than either single oil against Epidermophyton floccosum, M. gypseum and Paecilomyces variotii. The minimum inhibitory concentration (MIC) and time taken to inhibit mycelial growth were less with the mixture than with the individual oils, suggesting that there were synergistic interactions between the components present in the two oils.
In a wide-ranging study Pattnaik et al. () tested ten essential oils, one of them from Eucalyptus citriodora, against twelve test fungi (mostly human pathogens, with a few plant pathogens): Alternaria citrii, Aspergillus fumigatus, A. oryzae, Candida albicans, Cryptococcus neoformans, Fusarium oxysporum, E solani, Helminthosporium compactum, Macrophomina phaseolina, Sclerotium rolfsii, Sporothrix schenckii and Trichophyton mentagrophytes. The eucalyptus oil was effective against all the fungi except M. phaseolina. Seven of the oils (citronella, lemongrass, patchouli, palmerosa, geranium, orange and aegle) were effective against all twelve fungi, with lemongrass performing the best.
E. pellita oil was found to be active against the human dermatophytes M. gypseum and T. mentagrophytes, and exhibited greater inhibition than the pine oil from Pinus caribaea ().
Organisms involved in food spoilage
The range of fungi that have been screened using eucalyptus oils is very extensive. It has been shown that inhibition of members of the genera F’enicillium (such as the mycotoxigenic P. citrinum) and Aspergillus (including aflatoxin-producing A. flavus) is readily achievable. Food spoilage fungi are also inhibited, making eucalyptus a potentially useful oil in the preservation of foodstuffs.
Eucalyptus globulus has been tested for its inhibitory effect on the growth of a large number of mould species frequently involved in food spoilage. Benjilali et al. () tested it using a micro-atmosphere method, along with five other oils, against thirteen Penicillium spp., six Aspergillus spp. and sixteen other species. However, while thyme oil was consistently the best performer, the eucalyptus oil was the least effective. Illustrative results for eucalyptus, rosemary and thyme oils, including compositional data for the oils, are shown in Table Antifungal activity of eucalyptus, rosemary and thyme oils, with composition in terms of selected constituents. Similar results were obtained using the same oils but an alternative method of testing: Eucalyptus globulus oil was moderately effective against Byssochlamys nivea, Geotrichum candidum, Paecilomyces variotii, Penicillium purpurogenum and Stachybotrys sp., all spoilage organisms, but was the least effective of the oils overall ().
The mycotoxigenic Aspergillus flavus, responsible for the production of aflatoxins in groundnuts and other crops, has received much attention from researchers. Several groups have included eucalyptus oil in screening volatile oils for possible use as antifungal agents although the results have not been particularly encouraging. Montes-Belmont and Carvajal () investigated the inhibitory effects of eleven volatile oils, including Eucalyptus globulus, against A. flavus growing on maize kernels. The antifungal effect of the eucalyptus oil was described as scanty and no further evaluations of it were carried out. Ansari and Shrivastava () examined the inhibitory power of eucalyptus oil at three concentrations: the lower two concentrations caused A. flavus growth and toxin production to be inhibited while the highest concentration resulted in complete inhibition of growth. After twelve days incubation, however, toxin production was greater than in the control. It was speculated that the conidia and mycelium were initially under stress, showing poor growth and low toxin production, but that this was followed by high toxin production in the late phase of incubation. Masood and Ranjan () have reported on the ineffectiveness of fungi-toxicant after advanced incubation which stimulated synthesis of aflatoxin, and the correlation between stress and aflatoxin biosynthesis clearly requires further investigation.
Table Antifungal activitya of eucalyptus, rosemary and thyme oils, with composition in terms of selected constituents ()
|Aspergillus flavus||-||+||+ + + + +|
|A. fumigatus||-||+ +||+ + +|
|A. niger||+||+||+ + + + +|
|A. repens||+ +||+ + +||+ + + + +|
|Gliocladium roseum||+||+||+ + + + +|
|Mucor hiemalis||+ +||+ +||+ + +|
|M. racemosm||+||+||+ + + + +|
|Venicillium clavigerum||+ +||+ +||+ + + + +|
|P. notatum||+||-||+ + + + +|
|P. purpurogenum||+ +||+||+ + + + +|
|Stachybotrys sp.||+ +||+ + +||+ + + + +|
a Measured by volume of oil required to give complete inhibition: >100µl (—), 100µl ( + ), 50µl (++), 20µl (+ + + ), 10µl (+ + + +), 5 µl (+ + + + + ).
b Relative abundance, %.
In tests against the spoilage organisms Aspergillus niger, Penicillium italicum and Zygorrhyncus sp., Eucalyptus globulus oil was found to cause reversible inhibition of spore germination (). At the lowest concentration (0.01 per cent), however, the oil actually caused stimulation of germination in A. niger conidia and there was evidence to suggest that the fungus responded positively to the stress caused by low concentrations of oil on the mycelium. Both of the other oils tested, origanum and mugwort, were more effective at inhibiting spore germination than the eucalyptus oil and this seems to be in keeping with the observations made by others on the effectiveness of different types of oil, namely, phenolic oils > ketone-rich oils > cineole-rich oils.
A. niger and Zygorrhyncus sp., as well as the yeasts Candida albicans and Saccharomyces cerevisiae, and some bacteria, were included by Hajji et al. () in a more comprehensive screening process involving twenty-one different Eucalyptus oils:
(syn. E. salicifolia)
|E. diversicolor||E. occidentalis|
|E. eximia||E. oleosa|
|E. astringens||Eucalyptus globulus subsp. maidenii||E. paniculata|
|E. calophylla||E. gomphocephala||E. piperita|
|E. camaldulensis||E. longifolia||E. sideroxylon|
|Eucalyptus citriodora||E. macarthurii||E. viminalis|
|E. cladocalyx||E. macrorhyncha|
|E. dealbata||E. melliodora|
E. cladocalyx oil was the most effective of the oils tested although its yield from the leaves was relatively poor (0.4 per cent) and not indicative of one that could be produced commercially. Eucalyptus citriodora oil was obtained in the highest yield (2.5 per cent) and was moderately effective.
Eucalyptus citriodora oil has been tested, along with palmarosa (Cymbopogon martinii), against a variety of organisms, including human and plant pathogens as well as food spoilage organisms: Alternaria alternata, three Aspergillus spp. (A. flavus, A. fumigatus and A. niger), Cladosporium cladosporioides, Curvularia lunata and two Fusarium spp. (F. oxysporum and F. solani) (). Both oils contain geraniol and citronellol. Palmarosa was more effective than the eucalyptus oil at controlling fungal growth, although there was some variability towards different organisms. Both oils caused complete inhibition at 0.01 per cent concentration. In another study involving Eucalyptus citriodora oil and Cymbopogon martinii (var. motia), Baruah et al. () investigated their antifungal activity towards Fusarium moniliforme, a post-harvest pathogen of cereal crops. Mentha piperita and Cinnamomum tamala oils were also tested. Using a disc assay and measuring zones of inhibition on the surface of agar plates, all four oils exhibited activity, with Cymbopogon martinii the most effective and Eucalyptus citriodora the next most effective.
It would be of great benefit to be able to employ eucalyptus oil as a natural fungicide, one which was biodegradable and able to control some of the important plant pathogens. The potential use of eucalyptus oils in agriculture has been investigated by Singh and Dwivedi () in attempts to control Sclerotium rolfsii, the causative organism of foot-rot of barley. Four concentrations, 1000–4000 ppm, were used in the poison food technique, where a number of inhibition parameters were used as a measure of efficacy, including diameter of the colony on agar media, dry weight of hyphal mat and reduction in the viability of sclerotia. Of five different oils tested, Eucalyptus globulus and Ocimum americanum (syn. O. canum) were the most effective, with MICs of <4000 ppm. However, further studies showed neem oil (from Azadirachta indica) to be more effective against S. rolfsii than both Eucalyptus globulus and 0. americanum oils (). Nevertheless, Eucalyptus globulus oil showed considerable activity towards ten soil fungi, including the mycotoxigenic Penicillium citrinum; it was most active against Trichoderma viride ().
The leaf oil from E. camaldulensis was part of a selection of plant oils assayed for antimycotic activity against the soil-borne fungi Eusarium moniliforme, Phytophthora capsici, Rhizoctonia solani and Sclerotinia sclerotiorum (). The eucalyptus oil, however, was found to have only slight inhibitory effects in the initial bioassay and was not studied further. Oils from Origanum minitiflorum, Thymbra spicata and Satureja thymbra were the most active and fungitoxicity was shown to be due to the presence of carvacrol and/or thymol. Salvia fruticosa, which contained twice as much 1,8-cineole as E. camaldulensis, had a similar, weakly active profile to the eucalyptus oil.
Eucalyptus oil, along with turpentine and clove oil, completely checked rotting when guava fruits (Psidium guajava) inoculated with Pestalotiopsis versicolor and Rhizoctonia solani were dipped in them. However, they had adverse effects on the keeping quality of the treated fruits, in contrast to mustard oil which checked the rotting and preserved keeping quality ().
The widespread planting of the so-called ‘Eucalyptus hybrid’ in India (essentially E. tereticornis), and its leaf oil, have been referred to elsewhere (). The oil contains variable amounts of pinenes, 1,8-cineole and p-cymene. In their work to produce true hybrids with superior growth and wood and oil quality characteristics, Chaudhari and Suri () reported the oil properties of F1 hybrids of reciprocal crosses E. tereticornis x E. camaldulensis and E. camaldulensis x E. tereticornis, together with those of the parent species. The data include results from screening the oils for activity against six fungal species: Aspergillus niger, Candida albicans, Epidermophyton rubrum, Malbranchea pulchella, Microsporum gypseum and Penicillium notatum. The four oils were active against all six fungi at a dilution of 1: 500 and the findings were deemed to bode well for the formulation of control measures against the organisms.