Conclusion

The identification of efficacious biological control agents is only the beginning in the development of mycoherbicide products. Continuing strain selection is essential to ensure that "nature's best" is employed. Likely there will always be limitations associated with naturally occurring organisms, therefore, enabling technologies such as fermentation, formulation, and application technology will be instrumental in determining whether a highly efficacious agent can be developed into an economically feasible mycoherbicide product. The ultimate goal is to incorporate mycoherbicides into agricultural production systems. More efforts should be directed into combining biocontrol agents with other weed control options including chemical herbicides, cultural practices, and use of the multiple pathogens to enhance the effectiveness and flexibility of integrated weed management systems and to reduce the chemical load on crops and in the environment (Boyetchko et al. 2002; Rosskopf et al. 1999). The challenge is to critically evaluate the merits of individual mycoherbicide candidates and to make realistic decisions whether they have all the essential characteristics required for successful application in agroecosystems. Based on discussions presented in this review, the following research priorities are suggested for development of mycoherbicide candidates:

(a) Assessment of natural strain variation and biodiversity within the pathogen population based on critical epidemiological characteristics, including environmental adaptation, virulence, dispersal, and infection efficiency.

(b) Evaluation of key areas for efficient scale-up mass-production based on fundamental elements relating to nutritional and physical requirements of specific fungal agents that facilitate selection of economical fermentation ingredient substitutes, along with down-stream processing procedures that are compatible with production methods.

(c) Selection of appropriate formulation technologies (i.e., liquid or solid-matrices) based on the mode of attack and critical efficiencies of candidate agents. Over-simplification of formulation by using single ingredients will not likely address the complex challenges that fungal organisms will encounter in the environment, including moisture constraints, temperature extremes, and UV irradiation.

(d) Critical factors in application technology related to placement and penetration of the crop canopy to the target weed in order to maximize application efficiency. These factors include leaf-wetting properties and ability to penetrate physical barriers (e.g., waxy cuticles and leaf hairs), retention and dispersal on the leaf surface, optimum dose transfer in various liquid droplets or solid-based granules, and selection of application equipment such as nozzle types and angle position or soil-application placement (e.g., within furrow application, side-banding, etc.). Application parameters should also be evaluated jointly with formulation ingredients.

(e) Integration of mycoherbicide agents into crop production systems using several weed management tools (e.g., synergy with chemical herbicides, combinations with other biological control agents, or weed control options) to optimize weed control effectiveness. Judicious use of mycoherbicides as one of the components in an integrated weed management system will enhance their value and practicality for control of multispecies weed communities in agroecosystems.

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