Evaluation of Pathogens as Potential Biocontrol Agents of Hydrilla

During a 1-year survey for micro-organisms associated with hydrilla, approximately 2200 microbes were recovered from hydrilla, surrounding water and sediment collected from 10 man-made ponds and three natural lakes in Florida, USA. A representative selection of fungi (651), bacteria (330) and actinomycetes (38) from this collection was screened against hydrilla in bioassays. Severity of damage (DS) was determined on a scale of 0 to 4, where 0 ¼ healthy, 1 ¼ 1–25% damage, 2 ¼ 26–50% damage, 3 ¼ 51–75% damage, and 4 ¼ 76–100% damage (100% ¼ complete kill). Of the 1019 isolates that were examined in this manner, none of the actinomycetes were pathogenic on hydrilla, however two strains of bacteria (0.6% of the bacteria tested), and 42 isolates of fungi (6.5% of the fungi tested) were effective at DS level 4 (DS4) on hydrilla 3 weeks postinoculation. Fungi and bacteria capable of killing hydrilla are considered to have potential as biocontrol agents or as sources of novel herbicidal metabolites. These included Acremonium sp., Botrytis sp., Curvularia sp., Cylindrocarpon sp., Fusarium culmorum and F. moniliforme. The biocontrol potential of F. culmorum has been established in several tests. For maximum bioherbicidal activity of this fungus against hydrilla, its inoculum should be applied at a rate of 5 · 105 conidia/ml at 20–25 C water temperature