Efficacy of Phoma macrostoma, a bioherbicide for control of Dandelion (Taraxacum officinale) following simulated rainfall conditions

Original Research Paper: This study evaluated the efficacy of the bioherbicide P. macrostoma under simulated rainfall conditions to determine whether bioactivity was retained in the soil and whether bioherbicidal metabolites would be soluble in water possibly posing a risk to off-site hosts. Various application rates of the bioherbicide were leached with water equivalent to 25-250 mm of precipitation and tested for efficacy before and after leaching using a dandelion bioassay. The percolated water was collected in six fractions (F1-F6) and similarly tested for bioactivity using a dandelion bioassay. Relative concentrations of macrocidin A, the main bioherbicidal metabolite, were estimated by HPLC. DNA-specific primers were used to detect the presence of the fungus in soil. Of three soil types treated with the bioherbicide, all lost the ability to control dandelion after being leached. Clay, as compared to sandy loam and greenhouse soil mix, retained the most bioherbicidal activity. The bioherbicidal metabolite was soluble in water with up to 80% macrocidin A being released in 75 mm of water and bioactivity occurring in fractions F1-F3, with much less or no activity in fractions F4-F6. The impact of water on the distribution of the living component of the bioherbicide was not clearly determined. The study shows that water releases macrocidins from the bioherbicide allowing the compound to be taken up by the roots of the plant, subsequently resulting in plant death. When soils are at field capacity or drier, this amount of rainfall is of little concern as the bioactivity is localized. However under saturated soils, macrocidins may be released in the soil water and if not taken up by the plants, run-off water may pose a risk to off-site hosts. Factors that mitigate the risks associated with off-site movement are discussed.