First report of Colletotrichum gloeosporioides causing leaf spot in Bougainvillea (Bougainvillea spectabilis Willd.) in India

Abstract

Bougainvillea (Bougainvillea spectabilis Willd.), a widely cultivated ornamental plant valued for its vibrant bracts and resilience, is increasingly threatened by foliar diseases that compromise its aesthetic and physiological functions. This study reports the incidence of Colletotrichum gloeosporioides associated with leaf spot disease on Bougainvillea in Karnataka, India. In April 2024, symptomatic leaves manifested as water-soaked lesions progressing into necrotic spots with chlorotic halos, primarily affecting older foliage, were collected from ornamental gardens. The symptomatic leaf disease index ranged from approximately 20.50 to 41.00 at the University of Agricultural Sciences, GKVK, Bangalore, and other surveyed locations in Karnataka. Pathogen isolation on PDA revealed colonies with characteristic cottony to fluffy, whitish mycelial growth. The conidia were cylindrical, hyaline, and aseptate, with an average conidial size of 15.29 × 4.27 μm. Cultural and morphological characterization revealed that the association with Colletotrichum spp. is the cause of the disease. The targeted multilocus sequencing of the internal transcribed spacer (ITS) region, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and chitin synthase 1 (CHS1) gene confirmed the identity of the pathogen as C. gloeosporioides. Phylogenetic analysis using Maximum Likelihood method further supported this identification. Pathogenicity assays fulfilled Koch's postulates, with inoculated plants exhibiting the same natural symptoms as those observed in the field conditions. The confirmed presence of C. gloeosporioides highlights its emerging role as a foliar pathogen in Bougainvillea under subtropical conditions in India.