Abstract
Previous sporulation studies with Colletotrichum truncatum NRRL 13737, a fungal pathogen of the noxious weed Sesbania exaltata, showed that the carbon-to-nitrogen (CN) ratio of the conidiation medium influenced spore yield, morphology, and efficacy in inciting disease in S. exaltata. Spores produced in a medium with a CN ratio of 10:1 were more effective than were spores produced in a 30:1 or 80:1 ratio in causing disease in S. exaltata. With a basal salts medium supplemented with glucose and Casamino Acids, substrate utilization, spore production, biomass accumulation, and biomass and spore composition were compared in submerged cultures of C. truncatum grown in media with CN ratios of 80:1, 30:1, and 10:1. All cultures were sporulating by day 2, and spore concentrations in 5-day-old cultures were significantly different: 30:1 > 10:1 > 80:1. Amino acid and glucose utilization was balanced in cultures grown in media with a CN ratio of 10:1, whereas cultures grown in media with a CN ratio of 30:1 or 80:1 depleted amino acids prior to glucose. Conidia produced in media with a CN ratio of 10:1 contained significantly more protein (32% of dry weight) and less lipid (17% of dry weight) than conidia produced in media with a CN ratio of either 30:1 (15% protein, 33% lipid) or 80:1 (12% protein, 37% lipid). The higher lipid content of spores produced in media with a CN ratio of 30:1 or 80:1 was associated with the presence of increased numbers of lipid droplets. Optimization studies on conidia produced in media with CN ratios between 30:1 and 10:1 which compared yield, attributes, and efficacy in inciting disease in S. exaltata suggest that media with a CN ratio of 15:1 to 20:1 may be optimal for conidium production.