The antifungal activity of Ocimum leaves, extracts, essential oils and their components is frequently studied, mostly in warm countries where the need for protection of plants and stored crops against fungi is of great importance. Also the effect of Ocimum oils against a number of dermatophytes has been studied.
Protection of plants and stored crops
An ethanolic extract of Ocimum sanctum was used to treat healthy ripe tomato fruits prior to and after inoculation with Aspergillus niger in the presence of Drosophila busckii. The treatment kept the fruits free from rotting for 5 to 7 days. The essential oil of Ocimum canum was effective against damping-off disease causing fungi, Pythium aphanidermatum, P.debaryanum and Rhizoctonia solani. Ocimum canum could control damping-off disease of tomato up to 50% in soil infected with P.aphanidermatum and up to 43% in soil infected with P.debaryanum. The essential oil was not phytotoxic and showed superiority over commonly used synthetic fungicides such as Agrosan G.N. and Captan.
Pandey and Dubey determined the fungitoxic spectrum of Ocimum canum oil (500 μl/1) and found 100% inhibition of the growth of the following fungi: Fusarium oxysporum f. sp. ciceri, F.sesami, F.semitectum, Alternaria brassicae, A.solani, A.tenuissima, Cladosporium cladosporioides, Helminthosporium oryzae, Penicllium citrinum, Colktotrichum sp. and Drechslera auntii.
Ocimum sanctum leaf extract inhibited the radial growth of the rice pathogens Pyricularia oryzae, Cochliobolus miyabeanus (Helminthosporium oryzae) and Rhizoctonia solani (). Asthana et al. () found that the essential oil of Ocimum adscendens was fungicidal and inhibited the growth of Helminthosporium oryzae at a concentration of 200 μl/1. The aqueous extract of Ocimum basilicum leaves inhibited the growth of Tricboconiellapadwickii in paddy (Oryza sativa) seeds.
A leaf extract of Ocimum canum inhibited the germination of Cercospora moricola, a leaf spot causing pathogen on mulberry.
A crude steam distillate of Ocimum gratissimum sprayed onto infection courts on detached cocoa pods moments after inoculation with Phytophtora palmivora completely inhibited the pathogen and blackpod lesion development on 75% of the infection courts. In the field the extract also suppressed lesion development, although to a significantly lower extent and the fungitoxicity was lost within 3 h of application.
The essential oil of Ocimum adscendens protected stored chilli (Capsicum annuum) seeds completely from fungal development and gave better control than the synthetic fungicides Bavistin, Blitox-50 and Dithane M-45 for 12 months.
Exudates of Ocimum basilicum decreased the population of various fungi, including Aspergillus spp. and Fusarium spp. in the phyllosphere of beans. The essential oil of Ocimum basilicum exhibited fungitoxic properties against aflatoxin-producing strains of Aspergillus flavus and A.parasiticus. The oil was fungistatic at a dose of 1.5 ml/1 and fungicidal at 6.0 ml/1. These doses are much lower than those of commercial fungicides and fumigants, and the effect remained unaffected by temperature, storage, and increased inoculum.
Ocimum sanctum leaf extracts inhibited germination of Pestalotia psidii spores in vitro and on guava fruits dipped in the extract. The extract does not affect the fruit flavour. An aqueous leaf extract of Ocimum sanctum gave good control of the disease development (Botryodiplodia tbeobromae, Fusarium oxysporum, Helminthosporium spiciferum [Cochliobolus spicifer], Curvularia lunata [Cochliobolus lunatus], Aspergillus flavus and Trichothecium roseum) in banana (Singh et al., 1993). Sweet basil oil was highly effective in checking the growth of Pénicillium italicum (blue mold) on tangerine (Citrus reticulata). The appearance and flavour of the fruits were unchanged.
Protection against fungi causing diseases in humans
The growth of the common fungus Candida albicans was inhibited by the essential oil of Ocimum gratissimum showing an inhibition zone > 30 mm. The effect of Ocimum sanctum on C.albicans was none according to Grover and Rao and according to Prasad et al. () it inhibited the growth for 7 days. The essential oil of Ocimum basilicum showed some inhibitory effect against C.albicans (). However, the minimum inhibitory concentration was 1250 μg/ml compared to 312.5 μg/ml for Ocimum gratissimum ().
A group of fungi often used to study the antifungal effects of different oils is the dermatophytes. Singh et al. () report that the oil of Ocimum gratissimum was active at 1000 ppm against Epidermophyton floccosum, Microsporum canis and Trichophyton mentagrophytes. Clinical isolates of Trichophyton rubrum, T.mentagrophytes, M.canis and E.floccosum were used to test the antifungal activity of the essential oil of Ocimum gratissimum. It inhibited over 80% of the studied strains and produced inhibition zones > 10 mm in diameter. A later report confirmed the effect of Ocimum gratissimum against T.rubrum and T.mentagrophytes (). Ndounga and Ouamba report a very good effect of Ocimum gratissimum against T.mentagrophytes. The oil caused an inhibition zone of more than 30 mm. The minimum inhibitory concentration was 625 μg/ml. The oil of Ocimum gratissimum was active against T.mentagrophytes var. interdigitale at a dilution of 1:6400.
Essential oils of Ocimum basilicum (French, Indian and Niazbo) were effective against Trichophyton mentagrophytes, T.rubrum and T.verrucosum, except for the Indian oil where some growth of T.verrucosum was detected within seven days. Janssen et al. (1988) determined maximum inhibitory dilutions for Ocimum basilicum oil against Epidermophyton floccosum, T.mentagrophytes var. interdigitale and T.rubrum and report the values 1:3200–1:1600. On the other hand, Ndounga and Ouamba report the minimum inhibitory concentration of Ocimum basilicum oil against T.mentogrophytes to be 5000 μg/ml.
The activity of other Ocimum species against T.mentogrophytes var. interdigitale is characterized by the maximum inhibitory dilution values as follows: Ocimum canum 1:3200, Ocimum trichodon 1:3200 and Ocimum urticifolium 1:3200. One strain of Ocimum urticijblium was active at the dilution 1:6400. The essential oils of Ocimum sanctum and Ocimum kilimandscharicum were effective against T.mentagrophytes, T.rubrum and T.verrucosum without exceptions.
Individual oil components with antifungal properties are:
1. Eugenol, active against Absidia glauca, Aspergillus nidulans, A.niger, Colletotrichum capsici, Fusarium moniliforme, Pestalotiapsidii and Rhizopus nodosus as a pure isolate and at dilutions 1:100 and 1:200. At 0.1% concentration eugenol oil (Ocimum gratissimum) inhibited the growth of Sclerotium rolfsii, Rhizoctonia spp. and Alternaria alternata (). Ocimum suave oil (main component eugenol) inhibited the growth of Saccharomyces cerevisae with a minimum inhibitory concentration of 500 μg/ml.
2. Caryophyllene (pure isolate, 1:100 and 1:200) exhibited very strong antifungal activity against Absidia glauca ().
3. 1,8-Cineole showed very good activity against Alternaria alternata and Fusarium moniliforme ().
4. Thymol (Ocimum viride) checked the growth of Rhizoctonia solani and Sclerotium rolfsii ().
5. linalool (Ocimum canum) checked the growth of Rhizoctonia solani ().