Biological control of weeds has been studied for over 100 years. The approach taken has been to use fungal plant pathogens to control weeds by postemergence application of the plant pathogen to weed foliage. The most commonly studied fungi are Colletotrichum, Phytophthora, Sclerotinia, and Puccinia. For an in-depth discussion on the use of microbial pesticide control of weeds, see TeBeest (1996).
Although many of the organisms discussed have shown excellent efficacy in pure culture or microcosm studies, their adoption for wide use in agriculture is minor (except for Bt). This has been due to strict environmental regulations regarding their release and the technical problems associated with introducing and maintaining populations in the soil. Technical problems include: (1) identification of factors that affect survival rates, (2) determining which strains are best for each crop of interest, and (3) field conditions, methods of application, and implementation of management practices that enhance biocontrol.
Significant disease suppression and alteration of soil microbial populations can also be achieved by manipulating the physiochemical and microbiological environment through management practices such as soil amendments, crop rotations, tillage (as discussed earlier), natural or synthetic compounds (soil fumigation), soil solarization, or the use of genetically resistant varieties. Soils that are suppressive to pathogens typically have neutral to alkaline pH, and liming of acid disease-prone soils has been shown to reduce the severity of fungal pathogens.
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