Guttation droplets of sugarcane red rot pathogen Colletotrichum falcatum: Formation, toxigenic properties, and composition

Production of guttation droplets in the cultures of Colletotrichum falcatum was often noticed. Hence, formation of guttation droplets, exudation of toxin in guttation, and guttation droplet chemical composition were assessed to find the ecological importance of guttation. Among the tested culture media, more guttation was noticed in oat meal agar medium and host extracts broth medium. Assessing the effect of five different carbon sources on guttation of C. falcatum showed that among solid media, sparse guttation droplets were noticed in glucose-, fructose-, and starch-amended media, whereas in broth cultures except fructose-amended media no other carbon sources induced guttation in mycelium. The guttation phenomenon was noticed only in sporulating mycelia, and it also followed a specific pattern in connection with sporulation. Bioassay of guttation exudates on sugarcane leaves produced pronounced necrotic lesions with large, yellow halo similar to positive control, thus confirming the presence of toxin. The guttation fluid initiated browning symptom on sugarcane leaves at a concentration of 1000 ppm and produced pronounced necrotic lesions of 3.7 cm at 10 000 ppm. The toxicity of crude fraction was thermostable up to 60 C. The gas chromatography-mass spectrometry (GC-MS) analysis of crude guttation droplets identified the presence of 24 compounds belonging to fatty acids, derivatives of fatty acids, flavonoids, flavonoid glycosides, sterols, quinone derivatives, amides, and glycol ethers, and among them fatty acids were dominant. The two compounds, viz., hexadecanoic acid methyl ester and 9-octadecenoic acid (Z) methyl ester, were abundant (18.14% and 17.98%, respectively) and identified as probable toxic compounds.