FACTORS INFLUENCING THE GERMINATION OF PATHOGENIC AND WEAKLY PATHOGENIC ISOLATES OF COLLETOTRICHUM-GLOEOSPORIOIDES ON LEAF SURFACES OF STYLOSANTHES-GUIANESIS

Isolates of Colletotrichum gloeosporioides which showed considerable variation in pathogenicity were obtained from the same anthracnose lesions on leaves of Stylosanthes guianensis accessions in Pucallpa, Peru. Conidia of weakly pathogenic isolates germinated more rapidly and to a significantly higher percentage than pathogenic isolates on detached leaves of S. guianensis CIAT 136 and 1283. Germination of conidia of pathogenic isolates was generally lower on leaves of the more resistant CIAT 136 than on the more susceptible CIAT 1283. Leachates from leaves of CIAT 136 suppressed germination of all four test isolates of C. gloeosporioides. Bacteria isolated from the phylloplane of S. guianensis in Pucallpa, Peru, particularly Pseudomonas spp. and Bacillus spp., inhibited both mycelial growth on agar and germination of conidia of pathogenic and weakly pathogenic isolates of C. gloeosporioides on leaves of S. guianensis. Germination in the pathogenic isolates was inhibited to a greater extent than in the weakly pathogenic isolates, especially on CIAT 136. Antagonistic activity of these bacteria appeared to be based in antibiotic production to which the weakly pathogenic isolates were less sensitive. The weakly pathogenic C. gloeosporioides isolates appear to have evolved as highly competitive saprophytes competing with pathogenic isolates. The possibility that weakly pathogenic isolates induce a resistance mechanism in the host is discussed.