Interactions of chemical additives, pH and temperature on conidia germination and virulence of Colletotrichum truncatum, a bioherbicide of Sesbania exaltata

We studied several factors influencing the conidial germination and pathogenicity of fungus Colletotrichum truncatum, a potential bioherbicide for hemp sesbania. The conidia were germinated on 2% water agar over pH (3.5 to 9.0) and the optimal pH for the germination depended on the specific buffer used. Citrate and acetate buffers of low pH inhibited the germination. Germination decreased as buffer concentrations increased from 1 to 100 mM and as conidial density increased. Washing the conidia improved the germination. Temperatures of 15 and 20°C proved more favourable for germination and infection of hemp sesbania than 35 and 40°C. However, the emergence of germ tubes from each cell of two-celled conidia and growth and branching of germ tubes were greater at 35°C than at lower temperatures. Increased germination and branching from both conidial cells did not increase the infectivity. Germination was similar under light or dark conditions. Additions of 10 mM amino acids (alanine, glycine, phenylalanine, and tryptophan), 10 mM sugars (glucose, galactose, and xylose) and extracts of various plants (hemp sesbania and pigweed) stimulated the germination, but cystine drastically inhibited the germination. The alanine or xylose amino acids enhanced the infectivity, when applied with conidia to hemp sesbania seedling tissues.