First Report of <i>Colletotrichum siamense</i> Causing Anthracnose on Banana in Mauritius

Banana (Musa acuminata, C.) is one of the most important cultivated fruits in Mauritius, with a production of 11,246t in 2023 (Anonymous 2024). In August 2023, brown necrotic lesions, suggestive of anthracnose, were observed on leaves of Dwarf Cavendish banana variety at Rivière des Anguilles (RA) after a period of heavy rainfalls. The infection began at the leaf margin and spread across the leaf blade. Similar symptoms were observed in Dwarf Cavendish at Petite Cabane (PC) and in Mamoul at Saint Antoine (SA). Disease incidence was 100 %, with more than 50 % leaf infection per plant in both varieties. Black lesions were also observed at PC on green immature fruits, rendering them unmarketable (incidence <5%). Six symptomatic leaves (three from RA, two from PC and one from SA) were collected from each region, and pieces of infected tissue were surface sterilized with 0.15% NaOCl for 1 min. Tissues were rinsed thrice in sterile water and dried on sterile filter paper. They were then cultured at 25°C in the dark on potato dextrose agar with 0.0025% chloramphenicol and 0.003% Penicillin G Sodium. After seven days, gray-white colonies were observed, producing single-celled, aseptate, hyaline, cylindrical conidia with rounded ends measuring 12.7 to 16.9 µm x 3.6 to 5.7 µm (avg. 14.8 x 4.7 µm), typical of the genus Colletotrichum. Orange masses of conidia were visible after 14 days. One isolate was recovered from each leaf. The ApMat and GAPDH genes, resolutive for the Colletotrichum species (Huang et al. 2021; Liu et al. 2016), were amplified for the six isolates using primers AM-F/ -R (Huang et al. 2021) and GPD1/2 (Berbee et al. 1999), respectively, and sequenced (GenBank accessions ApMat: PV679901 to -906, GAPDH: PV699518 to -523). Two sequence-types (ST) were obtained for ApMat (one for the RA isolates and one for the PC/SA isolates) and three for GAPDH (one for each of the RA, PC, and SA isolates). The most closely-related sequences found with BLAST analyses matched C. siamense (Prihastuti et al. 2009), regardless of the gene/ST analyzed (99.6% to 100% identity). Phylogenetic analyses based on ApMat, GAPDH and concatenated datasets using the maximum-likelihood algorithm showed that the six isolates clustered within C. siamense, together with those reported by Huang (Huang et al. 2021), thus supporting their morphological identification. Asymptomatic banana plants (1-2 months old) of variety Dwarf Cavendish were used to test pathogenicity of three RA isolates. Four plants were inoculated per isolate. Two young leaves per plant were rubbed with an abrasive wool before applying a sterile cotton impregnated with a conidial suspension (106 spores/ml). Sterile water served as a negative control. Plants were incubated in a greenhouse at 25-30°C under a 13h/11h photoperiod under plastic bags to maintain a high humidity. Necrotic lesions appeared 20 days post-inoculation on at least one of the two treated leaves per plant. No symptom was observed in controls. The fungus was re-isolated from the infected leaves and identified as C. siamense by microscopic observation and sequencing, fulfilling Koch's postulates. To our knowledge, this is the first report of Colletotrichum siamense causing anthracnose on banana in Mauritius. The newly detected fungus affecting banana leaves and fruits is compounding the existing challenges faced by banana plants, posing a significant threat to local production. This underscores the need for an integrated management approach to effectively mitigate the disease’s impact.