| Biological control using phytopathogenic fungi as mycoherbicides is a promising weed management strategy, but ecological complexity may limit effective application of the technology. Our model organisms include: Centaurea stoebe (an invasive weed), the fungi Sclerotinia sclerotiorum (potential mycoherbicide), the antagonist fungi Trichoderma harzianum (ThzID1), and Aphelenchoides, a fungivorous nematode. In this study, the efficacy of a formulation of S. sclerotiorum as a biological control agent against C. stoebe was evaluated. The greatest disease severity and plant biomass reduction was found when adding 5 and 10 formulated pellets containing mycelia of S. sclerotiorum. An addition of sucrose kept the formulated pellets of S. sclerotiorum viable and pathogenic after a year of storage at 4 °C. In another experiment, we examined the trophic interaction between ThzID1 and Aphelenchoides on the efficacy of S. sclerotiorum at killing C. stoebe. The ability of S. sclerotiorum to kill C. stoebe was not affected by either ThzID1 or nematodes alone. However, the addition of Aphelenchoides and ThzID1 together reduced the efficacy of S. sclerotiorum. We tested colonization of sclerotia of S. sclerotiorum by Aphelenchoides. Nematodes were inoculated into 5-day-old fungal cultures inoculated with sclerotia, mycelium, or alginate pellets of S. sclerotiorum, or ThzID1. The highest numbers of ematodes were found on S. sclerotiorum and the lowest on ThzID1. After six weeks, 300--600 nematodes were observed inside of sclerotia. In soil, sclerotia were also colonized by Aphelenchoides , and apothecial production was inhibited when nematodes were present. In another study, sixteen nematode taxa were found in soils infested with C. stoebe. The omnivorous/predatory nematodes were the most dominant, and plant feeders were less abundant. The most prevalent fungal feeders were Aphelenchus, and Aphelenchoides. Trichoderma spp populations varied from 9.8 x 101 to 3.6 x 102 CFUs/g of soil. Our understanding is that the synergistic relationship between Aphelenchoides sp and T. harzianum reported here may affect the potential efficacy of plant pathogens (e.g., S. sclerotiorum ) in the biological control of invasive knapweed. |
