Conidium germination and appressorium penetration of Colletotrichum gloeosporioides on Stylosanthes guianensis

Pre-penetration and early penetration phases of two pathogenic strains of Colletotrichum gloeosporioides (HM 502, HM 514)from Zaire were investigated quantitatively on four Stylosanthes guianensis genotypes (cv. Schofield, cv. Graham, CIAT 184 and CIAT 10136) of various levels of susceptibility. The results demonstrated control of the rates and type of conidial germination by the fungus, influenced to a lesser extent by the host genotype. Cell reactions were visible as papillae and cytoplasmic aggregations closely associated with the penetration peg. No fungal growth was observed when deposits of papillae occurred under appressoria-forming infection pegs, whereas the effect of the cytoplasmic aggregations varied according to the host genotype. In the interactions with strain HM 502, the highest rates of plant reaction were recorded on the resistant CIAT 184 genotype, while the highest level of penetration without reaction was observed on the most susceptible cv. Schofield. Cytoplasmic aggregations were also observed under a few ungerminated appressoria on this genotype. Differences were thus detected in the rates and types of conidial germination, in the rates and types of cell reactions under appressoria and in the appressorial penetration rates without cell reactions. However, these differences did not completely explain the range of macroscopic symptoms observed on the various genotypes.