Mycoherbicides

Interest in research and development of indigenous fungal pathogens as mycoherbicides is increasing at an accelerating pace for several reasons. Increasing costs of chemical herbicide research and development, persistent lack of confidence in synthetic chemicals by a discerning society and presence of successful commercial examples have been major motivators. Four mycoherbicides are available for commercial use and five are in advanced stages of development. Only naturally occurring strains of fungi have been tested for mycoherbicide potential and only a meager portion of those found have been deemed promising enough to seek regulatory approval. Ideal weed targets are species derived from transient weed communities in temperate regions that have passed through one or more genetic bottlenecks while becoming weedy. Ideal fungal pathogens are those whose poor capacity to disseminate is their single most important epidemiological constraint. Important barriers to commercialization include absence of spore production and formulation technology, loss of virulence in culture, innate low virulence, fastidious environmental requirements for spore germination, host penetration or disease development and excessively narrow host-range with consequent low market potential or excessively broad host-range with non-target hosts of economic or ecological importance. Several advances have been made in the technology of solid state spore production and formulation of dry and wet spore preparations. These include spore production on mycelium in trays or gel encapsulated mycelium, and formulation as dry mixes with inert carriers. Wet preparations containing antibiotics or fungistatic compounds may be possible. Efforts to overcome innate biological deficiencies of weed pathogens with genetic modification have been modest and DNA transformations have yet to yield improved strains. In most cases, lack of fundamental knowledge of disease at the molecular level limits application of recombinant DNA techniques for enhancement of mycoherbicide effectiveness.