Morpho-molecular characterisation of Colletotrichum siamense causing leaf blight disease of Acorus calamus from West Bengal, India

The present study investigated the leaf blight disease of Sweet flag (Acorus calamus L.) during the cropping seasons of 2019–20 and 2020–21. Field observations showed that all five tested varieties (Nagireddigudem, Munipalli, Gaddipalli, Aihagaripalli and Symbolia) were susceptible, with maximum disease severity recorded between February and March. The highest disease severity was recorded on Nagireddigudem (37.87 %) in 2019–20 and Munipalli (39.54 %) in 2020–21. The pathogen associated with the disease was identified morphologically as a Colletotrichum species, producing white to grey cottony mycelium, hyaline cylindrical conidia, melanised septate setae and dark brown appressoria. Pathogenicity tests confirmed symptom reproduction within 10-12 days post-inoculation. Molecular characterisation through Internal Transcribed Spacer (ITS) sequencing and phylogenetic analysis further confirmed the pathogen as Colletotrichum siamense (GenBank Accession: MT672519). For the management of disease, five isolates of Trichoderma as a bioagent and five botanical extracts were tested against the pathogen, along with a check. In vitro evaluations demonstrated significant inhibition of the pathogen by all Trichoderma isolates, with T5 (TriK5) showing the highest suppression (53.5 %). Among botanical extracts, Withania somnifera exhibited the maximum inhibition (57.4 %), whereas the fungicides carbendazim and copper oxychloride provided the highest suppression at 100 % and 67.4 %, respectively. The study confirms C. siamense as the causal agent of leaf blight in A. calamus and identifies potential biocontrol, botanical and chemical options for disease management.