POTENTIAL FOR THE BIOLOGICAL-CONTROL OF ST-JOHNSWORT (HYPERICUM-PERFORATUM) WITH AN ENDEMIC STRAIN OF COLLECTOTRICHUM-GLOEOSPORIOIDES

An endemic pathogen of St. John's-wort (Hypericum perforatum) identified as Colletotrichum gloeosporioides, was evaluated as a potential mycoherbicide. On St. John's-wort the pathogen caused typical sunken anthracnose lesions that girdled stems and killed plants when they occurred near the base. Variability in virulence among isolates was noted. The pathogen also occurred naturally on Hypericum canadense and was found to infect H. virginicum, and tomato fruit (Lycopersicon esculentum), but only when wound-inoculated. Infection failed to occur on H. calycinum and 21 other crop species. The optimum temperature for mycelial growth on agar was 24-degrees-C. On St. John's-wort, infection occurred after 25, 9, and 6 h at 16, 22, and 28-degrees-C, respectively. Disease severity increased with increasing wetness duration and was greatest at 28-degrees-C. Disease on plants of various ages inoculated with C. gloeosporioides and then maintained at various postinoculation temperatures increased with temperature and decreased with host age. Plants inoculated 5 and 10 weeks after transplanting were killed within 2 days when incubated at 21 and 28-degrees-C, respectively. Lesions on 15-week-old transplants incubated at 7-degrees-C remained small, but the disease significantly reduced the weight of green tissue of plants incubated at higher temperatures. In a field trial of transplanted St. John's-wort, mortality was 72.2% and 83.3% 10 weeks after spraying with suspensions of 2 x 10(6) and 8 x 10(6) spores / mL, respectively. Mortality in the control plots was 24.4% due to secondary spread of the disease. In a natural stand of St. John's-wort seedlings, 53.2% and 70.5% of plants were killed within 6 and 9 weeks after inoculation, respectively; all remaining plants were infected and failed to survive the winter.