Microcomputer Measurements of Pathogen Injury to Weeds

The impact of chemical or biological herbicides in weed control may or may not be absolute. The traditional goal in chemical herbicide evaluation has been the identification of materials and application rates resulting in absolute (100%) kill of susceptible weeds. However, many insects or plant pathogens affecting weeds do not kill their weed hosts but decrease their growth and or reproduction in the field. Obligate plant pathogens dependent upon their weed hosts for their own survival derive no evolutionary advantage from killing their weed hosts and seldom do so under natural conditions. Under field conditions where weeds are subjected to stresses imposed by competition with crop and/or other wild plants, these biological agents can decrease the impact of a weed on crop or wildland productivity to tolerable economic levels. The recognition that the impact of chemical and/or biological herbicides on weeds need not be absolute to be effective has been accompanied by a search for an accurate, efficient, and economical method to measure quantitative changes in weed growth or health. While much of this report will concentrate on measuring impact of plant pathogens on weeds or other plants, all comments and techniques will apply also to other stresses such as herbicide injury.

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