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Current study aims to explore natural anti-phytopathogenic compounds for plant disease management. Syzygium cumini essential oil and its isolated compounds were screened for their antifungal assay against two phytopathogenic fungi, Rhizoctonia solani AG 4HG-III and Choanephora cucurbitarum . The chemical composition of oil the isolated oil was identified by gas chromatography and gas chromatography/mass spectrometry. Compounds such as 7-hydroxycalamenene, 7-acetoxycalamenene, 1-epi-cubenol, and α-terpineol showed an inhibition, ca. 95%, 80%, 76%, and 82%, respectively, against R . solani whereas 95%, 70%, 92%, and 100% against C . cucurbitarum in contact phase. In addition, high ocimene rich column fractions showed 70% and 87% inhibition against R . solani and C . cucurbitarum , respectively, in volatile phase. The findings of the present study suggest that the isolated 7-hydroxycalamenene, 7-acetoxycalamenene, 1-epi-cubenol, α-terpineol, and ( Z )-β- & ( E )-β-ocimene have a potential to be used as antifungal agents for the effective control of fungal diseases in medicinal plants. We report 7-acetoxycalamenene as novel compound from natural source for first time.

Essential oil extracted from aerial parts of two Ocimum species (O. canum and O. kilimandscharicum) was investigated using gas chromatography and gas chromatography/mass spectrometry techniques on DB-5 (5% diphenyl-95% dimethylpolysiloxane) and β-cyclodextrin (6-tertiarybutyldimethylsiliyl-2,3-diethyl-β-cyclodextrin) capillary columns. Essential oil extracted from O. canum contained camphor, limonene, camphene and myrtenol as most abundant constituents among monoterpenoids, whereas β-selinene, α-selinene, maaliol and β-caryophyllene were identified under sesquiterpenoids class. On contrary, two folds higher camphor was recorded in O. kilimandscharicum. Maaliol (6.4%) was characterized in O. canum for the first time with the help of extensive 1D and 2D-NMR experiments. Therefore, maaliol can be considered as a marker constituent to differentiate both the camphor rich oils. Further, when the oils were subjected to chiral analysis on β-cyclodextrin column; a high enantiomeric excess for (1R)-(+)-camphor was recorded. This revealed a close relationship between these two species in terms of biosynthesis of single enantiomer. Apart from analysis, oils were assessed for antifungal activity against Rhizoctonia solani and Choanephora cucurbitarum. O. kilimandscharicum exhibited a complete inhibition against R. solani and C. cucurbitarum while O. canum was only effective against R. solani.

Chiral compounds have shown antimicrobial activity, enantioselectively. Leaf oil composition of Cinnamomum camphora was studied using GC-FID, SPME-GC-FID, enantio-GC and GC/MS techniques. Camphor exists in two chiral forms and contributes major percentage to C. camphora essential oil (68.4–81.2%). The results showed (1R)-(+)-camphor as major enantiomer (ee > 99%). The biological potential of essential oil, (1R)-(+)- and (1S)-(−)-camphor was investigated against Choanephora cucurbitarum, a Withania wet rot pathogen. The activity was evaluated against fungal growth and morphological changes in the hyphae of C. cucurbitarum, after 18, 21 and 24 h of treatments. A significant growth inhibition in C. cucurbitarum was observed against (1R)-(+)-camphor. Further, in comparison to (1R)-(+)-camphor, C. camphora oil revealed better activity. Fungal growth inhibition by (1R)-(+)-camphor and plant essential oil was due to cytoplasm coagulation and hyphal lysis of C. cucurbitarum. Moreover, C. camphora essential oil could be used in the preparation of strong fungistatic agent against C. cucurbitarum infection.

In vitro and in vivo experiments were carried out to determine the mechanisms involved in suppression of Choanephora cucurbitarum the causal pathogen of wet rot disease of okra by using the tea produced from agro-waste such as rice straw (RST) and empty fruit bunch (EFB) of oil palm composts. Non-sterilized, filter-sterilized and heat-sterilized compost teas were tested on the growth of C. cucurbitarum. The highest percentage of inhibition in conidial germination was obtained by both the non-sterilized RST and EFB compost tea with values of 84% and 79%, respectively. The inhibitory efficacy was reduced significantly ( P ⩽ 0.05) when the teas were subjected to millipore membrane filters or heat-sterilization. The mycelial growth of C. cucurbitarum was reduced by 100% in plates amended with both the non-sterilized compost tea. Light microscopy micrographs of mycelial samples of C. cucurbitarum exposed to non-sterilized RST and EFB compost tea showed alterations in mycelial morphology and lysis, resulting in inhibition in mycelial growth. Glass house trials indicated that induced host resistance was stimulated in okra plants treated with non-sterilized and filter-sterilized compost teas based on the detection of inducible resistance-related compounds. The mean values for inducible enzymes which have a role in disease resistance [peroxidase (PO), polyphenol oxidase (PPO) and phenylalanine ammonia lyase (PAL)] increased until day 8, which were significantly higher in okra plants pre-treated with non-sterilized tea in view of the high nutritive and microbiological properties. There was a further increase in inducible compounds when the pre-treated okra plants were challenged with C. cucurbitarum. Following the increase in total PO, PPO and PAL activities, development of Choanephora wet rot was delayed, confirming the possible involvement of induced resistance. However, resistance was not maintained since it decreased over time. This could be due to the highly stressed environment of the leaf surfaces which had a detrimental effect on the survival of the antagonists and subsequently affected the efficacy of compost tea. It was therefore suggested that plants should be sprayed at short intervals with the compost tea to sustain the suppressive effect.

The potential of water extracts produced from rice straw (RST) and empty fruit bunch of oil palm (EFB) composts fortified with Trichoderma harzianum for the control of Choanephora wet rot of okra caused by Choanephora cucurbitarum was studied under field conditions. Disease severity was lowest in plants treated with Trichoderma-fortified RST extracts (9.56%) with a disease index of 1, mancozeb (Dithane M-45® (2 g l−1 of water), Trichoderma-fortified EFB compost extracts, extracts of RST and EFB, and an aqueous suspension of T. harzianum recorded disease severity values of 10.25%, 19.38%, 37.56%, 53.71% and 56.36%, with a disease index of 1, 1, 2, 3 and 3, respectively. There was a reduction of 85.04% in Choanephora wet rot severity on okra treated with Trichoderma-fortified RST extracts during 12 weeks of assessment in the field, which was comparable to the conventional fungicide Dithane M-45®, suggesting that application of extracts produced from well-matured compost fortified with biocontrol agents could be an alternative control strategy.

Protoplasts of Choanephora cucurbitarum were obtained by treatment of germlings with commercial enzymes. Colonies originated from regenerated protoplasts showed the same mating type and colony type as their respective parent. Colonies from fused protoplasts of two (+) isolates were of the (+) type, while those from two (—) isolates were of the (—) type. Colonies derived from fused protoplasts of (+) and (—) isolates were self-fertile and produced azygospores and zygospores subsequently. This result supports the hypothesis that azygospores of C. cucurbitarum are single-suspensor zygospores. Mating type heterokaryons resulting from fused protoplasts of (+) and (—) isolates with different colony types segregated into component isolates indicating that karyogamy and meiosis did not occur.

Nine okra (Hibiscus esculentus Linn.) cultivars were evaluated in the field for 2 years for their susceptibility to premature fruit abortion induced by Choanephora cucurbitarum (Berk & Rav.) Thaxt. Results indicated differences in abortion varying from 24 to 73% between the cultivars tested.

Label from the radioactive chitin precursor, [ 3 H]- N -acetyl-glucosamine, accumulated at the hyphal apices and infection sites of Choanephora cucurbitarum and Phascolomyces artictulosus , a susceptible and resistant host, respectively, to a mycoparasite, Pipocephalis virginiana . The degree of incorporation varied as a function of host-parasite compatibility and duration of the incubation period. Autoradiographs of Phascolomyces articulosus prepared 18, 24 and 36 h after inoculation revealed silver grains over the infection zone. Labelling was extensive at 24 and 36 h. This pattern of label accumulation coincides with formation of a papilla (18 to 20 h) and early development of a well differentiated sheath (20 to 22 h) previously reported. In contrast, negligible levels of label accumulated at infection sites 18 h after inoculation in susceptible host, C. cucurbitarum . Although incorporation increased significantly by 24 h, levels of grain density comparable to those observed in the resistant host, Phascolomyces articulosus , were not apparent until 36 h following inoculation, coinciding with time when sheath appears in C. cucurbitarum . Significance of chitin synthesis stimulation in infection zone as a response to parasitism is discussed.

The haustorium of a mycoparasite, Piptocephalis virginiana, was found enclosed in an electron-dense sheath 30 h after inoculation into its host Choanephora cucurbitarum. The sheath was not present around the young haustorium 18 h after inoculation, and its progressive development is described. High resolution autoradiography showed that [3H]N-acetyl-glucosamine was incorporated preferentially into the host cell wall and in the sheath zone. Cell fractionation of label-fed hyphae showed that 83% of the label was in the cell wall and specifically in the chitin portion of the cell wall. Polyoxin D, a specific inhibitor of chitin synthetase, suppressed the label incorporation in the cell wall and sheath zone and resulted in a decrease of electron density. These results suggested that the sheath zone around the mature haustorium contained host cell wall material, i.e. chitin. Significance of cell wall material deposition in the sheath zone of the haustorium is discussed in the light of host resistance.

Eight systemic fungicides were tested for in vitro activity against Choanephora cucurbitarum causing blight of winged bean flowers. Triadimenol, vinclozolin and bitertanol were most effective in inhibiting spore germination and mycelial growth. Monthly applications of these fungicides in a field trial reduced blight.

Blight of winged bean flowers in Sri Lanka was caused by Choanephora cucurbitarum. Flowers were susceptible to infection at all stages of development, and wounding substantially increased infection, especially of young flower buds.

Evidence obtained in this study implicates chitosan in the cell wall and γ-linolenic acid in the cellular lipids of the mucoraceous fungi as determinants of host specificity of the mycoparasite, Piptocephalis virginiana . The cell wall of the susceptible host, young (1-day-old) culture of Choanephora cucurbitarum , is composed of a single layer of randomly arranged microfibrils. Mechanical penetration of this host by the parasite resulted in a compatible interaction. Resistant hosts, Phascolomyces arliculosis and 7-day-old Choanephora cucurbitarum , invariably possessed 2-layered cell walls with characteristic macromolecular architecture; an outer layer with random microfibril orientation and an inner layer with parallel microfibril arrangement. The penetration of these resistant hosts was through enzymatic dissolution of the outer layer. The failure of the parasite to establish nutritional contact with the resistant host was either due to the papilla formation by the resistant host in response to invasion by the parasite or due to the development of an extensive sheath around the invading parasite, thereby preventing its direct contact with host protoplast. The parasite did not attempt penetration of the non-host species, Linderina permispora and Chaetomium cochlioides . A hypothesis concerning the induced development of papilla is presented and the function of the latter in resistance to a mycoparasite is discussed.