Selection and Improvement of Bioherbicide Agents

On average, chemical companies screen more than 60,000 compounds before a new active ingredient of pesticide can be determined. The number required for screening biocontrol agents should be less due to a relatively smaller range of variations amongst naturally occurring fungal populations. However, if we are to identify "nature's best," a systematic approach is essential during the exploration and discovery phase to thoroughly evaluate the biodiversity. This diversity provides excellent opportunities for finding fungal strains with potential suitable traits for biocontrol (Avis et al. 2001; Weidemann and TeBeest 1990). Substantial variations may exist amongst different strains of a fungal species in terms of its virulence and responses to environmental variables (Sands et al. 1997; Tessmann et al. 2001). To be effective, critical traits for selection should be clearly identified and sensitive bioassays developed. Pathogen strains with high levels of virulence may exist in nature at low frequencies due to higher extinction rates (Yang and TeBeest 1992). Results by Yang and TeBeest (1992; 1993) indicated that pathogens showing high virulence along with important epidemiological traits such as rapid infection rates and dispersal are more likely to be candidates of a successful mycoherbicide agent. The success

Figure 1 Strategic framework for evaluation and development of mycoherbicides.

of C. gloeosporioides f. sp. aeschynomene for control of northern jointvetch was partially attributed to its ability to easily spread as an endemic pathogen. Yang and TeBeest (1995) further demonstrated a rapid rate of mortality of the weed as the number of pathogen lesions per plant increased from a single lesion. Therefore, more aggressive and virulent isolates of a pathogen with high infection efficiency, shorter latent periods, and better sporulation from diseased tissues should be selected from amongst the pathogen population. Chemical and physical methods have been used to create fungal mutants with acquired new traits such as elevated antibiotic production (Graeme-Cook and Faull 1991) or increased biocontrol efficacy (Palani and Lalithakumari 1999). Stability or low reversion frequency was observed with some mutants but, in general, stability can be a concern with chemical and physical mutagenesis (Wibowo et al. 1999). Ziogas et al. (1995) reported UV-induced mutants of Nectria haematococca with variable tolerance to fungicides that showed the same level of fitness as wild types as expressed by the rate of growth and virulence on squash seedlings. Mutagenesis is apparently a quick way of creating new fungal strains with variable traits. However, efficient bioassay systems based on the understanding of critical constraints are needed for an effective selection strategy. It is not uncommon for induced mutants to have lower competitiveness than the wild type due to reduced infectivity or reproductivity (Yang and TeBeest 1995), but judicious use of this technique may help develop new mycoherbicide strains that overcome critical hurdles such as those demonstrated with the plurivorous pathogen Sclerotinia sclerotiorum (Miller et al. 1989).

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