Efficacy of new fungicide molecules against Colletotrichum siamense causing arecanut leaf spot

Introduction

Arecanut (Areca catechu L.) is a commercially important plantation crop of the family Arecaceae, widely grown in the tropical regions of South and Southeast Asia. In India, it plays a crucial role in supporting small and marginal farmers while contributing significantly to domestic trade and export earnings. The crop also holds social, cultural, and religious importance and has traditionally been used for oral hygiene and digestive purposes, with recent studies reporting antimicrobial activity against oral pathogens. India leads global arecanut production, with cultivation exceeding 1.01 million hectares and a production of about 1.44 million tonnes during 2024-25, of which Karnataka contributes the major share (ANON., 2025) [1] . However, productivity is severely affected by several diseases caused by fungal and bacterial pathogens, among which leaf spot disease has recently re-emerged as a major constraint in Karnataka. The disease causes necrotic lesions, premature leaf drying, and significant yield loss, and is now predominantly associated with Colletotrichum siamense. Since cultural practices alone are insufficient under favourable conditions, fungicidal management remains essential, with fungicides such as Captan, Chlorothalonil, Carbendazim, Propiconazole, Tebuconazole, and Tricyclazole + Hexaconazole showing effective inhibition of the pathogen.

Material and Methods

A total of 20 fungicides, comprising six contact, eight systemic and six combi products, were assessed for their efficacy against the leaf spot pathogen by following the poisoned food technique. The isolate Colletotrichum siamense (CsHn2) was utilized for in vitro studies based on its sporulation and virulence. The PDA medium was prepared and amended with the appropriate quantity of fungicides as per the concentration. Twenty ml of media amended with fungicides was added to sterile Petri plates and was inoculated with 9 mm mycelial disc of the pathogen. The control plates were maintained without fungicides. The inoculated plates were maintained at 27 °C in a BOD incubator. The growth of the pathogen in fungicide amended plates were measured when the pathogen growth in control plates attained full growth. The radial growth and percent inhibition in each concentration was calculated as per Vincent (1947). Inhibition percent = [C-T/C] X100, Where, I = Percent inhibition, C = Growth of test fungus in control plate (mm) and T = Growth of the test fungus in treatment plate (mm).

Results and Discussion

The screening of the fungicides revealed that all the test fungicides significantly inhibited the radial mycelial growth of Colletotrichum siamense at various tested concentrations. Among the fungicides evaluated, carbendazim 50% WP, propiconazole 25% EC and tebuconazole 25% EC consistently recorded the highest mycelial growth inhibition across all concentrations (100-200 ppm), with inhibition exceeding 90 percent. Difenoconazole 25% EC and hexaconazole 75% WG showed moderate efficacy, while tricyclazole 75% WP and thiophanate methyl 70% WP were comparatively less effective. Azoxystrobin 23% SC exhibited the lowest inhibition at all concentrations, even at 200 pp. The superior performance of triazole fungicides is attributed to their inhibition of sterol biosynthesis, whereas carbendazim acts by disrupting mitosis through inhibition of β-tubulin assembly. These results are in agreement with Burgute and Magar (2019), who reported high mycelial growth inhibition of Colletotrichum fungi by carbendazim, hexaconazole and difenoconazole under in vitro conditions. Among the contact fungicides, Captan 50% WP and chlorothalonil 75% WP consistently recorded the highest mycelial growth inhibition across concentrations (500-1000 ppm), with inhibition exceeding 93 percent. Propineb 70% WP and mancozeb 75% WP showed moderate efficacy, whereas zineb 75% WP was comparatively less effective.

Copper oxychloride 50% WP recorded the lowest inhibition at all concentrations. The high efficacy of chlorothalonil is attributed to its multisite contact action, while captan disrupts thiol-containing enzymes, impairing fungal metabolism. These findings are in agreement with Anwar et al. (2017) [4] .

Among the contact fungicides, Captan 50% WP and chlorothalonil 75% WP consistently recorded the highest mycelial growth inhibition across concentrations (500-1000 ppm), with inhibition exceeding 93 percent. Propineb 70% WP and mancozeb 75% WP showed moderate efficacy, whereas zineb 75% WP was comparatively less effective.

Copper oxychloride 50% WP recorded the lowest inhibition at all concentrations. The high efficacy of chlorothalonil is attributed to its multisite contact action, while captan disrupts thiol-containing enzymes, impairing fungal metabolism. These findings are in agreement with Anwar et al. (2017) [4] . Among the combi-fungicides evaluated, tricyclazole 45% + hexaconazole 10% WG consistently recorded the highest mycelial growth inhibition (90.74-92.96%) across all concentrations. This was followed by azoxystrobin 11% + tebuconazole 18.3%, which showed high efficacy, particularly at higher concentrations. Tebuconazole 50% + trifloxystrobin 25% WG and azoxystrobin 18.2% + difenoconazole 11.4% SC exhibited moderate inhibition, whereas carbendazim 12% + mancozeb 63% WP and hexaconazole 4% + zineb 68% WP were comparatively less effective.The superior efficacy of azoxystrobin-based combinations is attributed to their QoI-mediated inhibition of fungal respiration at Complex III, while triazole-based combinations act as demethylation inhibitors. These results are in agreement with the findings of Mahesh et al. (2020) [5] . In vitro evaluation of fungicides provides effective and preliminary information about fungicide efficacy against pathogens in a short period, and thus serves as a model for field testing. In the present investigation, the poison food technique was used for testing fungicide efficacy. Eight systemic, six contacts and six combi fungicides were screened for their efficacy against Colletotrichum siamense. in vitro evaluation of fungicides provides effective and preliminary information about fungicide efficacy against pathogens in a short period, and thus serves as a model for field testing. In the present investigation, the poison food technique was used for testing fungicide efficacy. eight systemic, six contacts and six combi fungicides were screened for their efficacy against Colletotrichum siamense.

Conclusion

All the tested fungicides significantly inhibited the mycelial growth of Colletotrichum siamense under in vitro conditions. The in vitro evaluation revealed that triazole fungicides such as tebuconazole and propiconazole were highly effective against Colletotrichum siamense due to their inhibition of sterol biosynthesis, while carbendazim effectively suppressed mycelial growth by disrupting mitosis through inhibition of β-tubulin assembly. Contact fungicides like chlorothalonil and captan also showed high efficacy owing to their multisite action and interference with thiol-containing enzymes, respectively. Combi-fungicides, particularly tricyclazole + hexaconazole and azoxystrobin + tebuconazole, exhibited superior inhibition due to the combined action of demethylation inhibition and QoImediated disruption of fungal respiration. The effectiveness of these fungicides and validating in vitro screening is a reliable preliminary tool for selecting fungicides for field evaluation in leaf spot management.

Acknowledgement

AcknowledgmentThe authors are highly grateful to the Department of Plant Pathology, College of Agriculture, Keladi Shivappa Nayaka University, of Agricultural and Horticultural Sciences, Shivamogga. We would also like to give special thanks to major advisor Dr. B. Gangadhara Naik for his valuable guidance and support throughout the experiment.

References

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