Development ofColletotrichum gloeosporioidesf. sp.ClidemiaeandSeptoria passifloraeinto Two Mycoherbicides with Extended Viability

Abstract

Norman, D. J., and Trujillo, E. E. 1995. Development of Colletotrichum gloeosporioides f. sp. clidemiae and Septoria passiflorae into two mycoherbicides with extended viability. Plant Dis. 79:1029-1032. Two potential mycoherbicides were formulated for extended viability: one, containing Colletotrichum gloeosporioides f. sp. clidemiae as the active ingredient, was effective against Koster's curse (Clidemia hirta); and the other, containing Septoria passiflorae, was effective against banana poka (Passiflora tripartita var. tripartita). Microcycle conidiation of both fungi occurred on the surface of solid media inoculated with spore suspensions ≥1 x 10 6 conidia per ml, ca. 1.67 x 10 4 conidia per cm 2 . In 4 days, C. g. f. sp. clidemiae produced 5 x 10 6 conidia per cm 2 after incubation at 25°C under continuous illumination on the surface of potato-dextrose agar adjusted to 3% agar. In 3 weeks, S. passiflorae produced 8.6 x 10 7 conidia per cm 2 after incubation on 10% Gerber Mixed Cereal for Baby agar, while on agitated potato-dextrose broth the production was 3.9 x 10 7 conidia per ml at 21°C under continuous illumination after 4 days incubation. Spores of C. g. f. sp. clidemiae and S. passiflorae, harvested by scraping the surfaces of solid cultures, were mixed in kaolin, dried, and stored at -18 and 1°C. They maintained greater than 84% viability for over 4 months and greater than 95% viability for 6 months, respectively. Spores of S. passiflorae harvested from liquid culture and stored at ≤1°C, mixed in kaolin and/or by lyophilization, maintained 97% viability for <1 year. C. g. f. sp. clidemiae spores produced in liquid culture had low viability and were killed by lyophilization. Viability was optimally maintained in both fungi when they were stored at -18°C. Viability of spores of both fungi stored as a kaolin formulation at 22°C was short-lived. No significant differences in pathogenicity were found in spores as a kaolin formulation after 4 months of storage. The shelf life of stored C. g. f. sp. clidemiae-kaolin was not affected by rehydrating in a 30% sucrose solution; whereas significant loss of viability occurred when the spore-kaolin mixtures were rehydrated in sterile distilled water (SDW). Rehydration of the mycoherbicide containing S. passiflorae in SDW did not decrease its activity. Both fungi produced a significantly higher number of lesions when applied to host plants suspended in 2% sucrose-0.5% gelatin solution than in SDW. Also, the number of lesions produced increased linearly with increases in inoculum. There was no significant difference in pathogenicity between freshly harvested spores and kaolin-spore mixtures stored for 4 months.