Submitted to: Canadian Journal of Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 11, 1996
Publication Date: N/A
Interpretive Summary: Interest in using plant pathogenic fungi to control weedy plants continues to increase in response to public preference for weed control strategies with reduced environmental risk. COLTRU, pathogenic fungus that kills the troublesome weed hemp sesbania, has potential for commercial development. We have previously demonstrated that we can induce this fungus to produce a life stage called "microsclerotia" in the laboratory. Microsclerotia are more durable than spores, and therefore, could be developed as a commercial product. In this study, we found that microsclerotia act as "spore production factories" when incorporated into soil, and that spores then germinate to infect seedlings of the target weed. We also determined that weed infection takes place very near the soil line when microsclerotia are added to weed infested soil in the laboratory. These discoveries will help us to make microsclerotia even more effective in killing weeds since we now know to search for formulations that increase the production of spores from microsclerotia and that microsclerotia do not need to be deeply incorporated in soil to be effective. The development and use of a biological weed killing product containing this fungus would contribute to a reduction in pesticide residues in the environment and on harvested agricultural crops.
Technical Abstract: Microsclerotial (MS) propagules of the bioherbicidal fungus Colletotrichum truncatum were produced in submerged culture. Seedlings of the weed Sesbania exaltata became infected when seeds were germinated in air-steam pasteurized (60 deg C, 30 min) field soil infested with 165 MS/ml. Infection was first noted three days after planting seeds, when the pathogen was recovered from 5% of plant segments taken from within 0.5 cm of the soil surface. By day 7, C. truncatum was recovered from 38% of stem and root segments within 0.5 cm of the soil surface, and from 60% of similar segments by day 8. Of all pathogen recovery, 66% came from segments within 0.5 cm of the soil surface and 92% of recoveries came from within 1.0 cm of the soil surface. All freshly produced MS particles on Nobel water agar had germinated after 24 h and conidial production from germinated MS was detected. Conidiation peaked in vitro after two days with approximately 3500 conidia being produced per MS. In situ, light microscopy showed ~40% of MS produced setal hypha after one day in pasteurized potting mix. This level was virtually unchanged after 2, 3, and 4 days of incubation though all MS remained viable. Scanning electron microscopy determined that, in situ, MS germinated sporogenically to produce conidia and setal hypha. Newly produced conidia germinated and formed appressoria on S. exaltata roots after two days, when root radicles were less than one day old.