Unexpected Side Effects of Herbicides: Modulation of Plant-Pathogen Interactions

Herbicides are widely used as an important alternative to prevent excessive growth of weeds in agricultural crop land, particularly where conservation tillage is adopted. Weeds reduce crop yield and quality, interfere with cultivation and harvest operations and seem to be the most economically important of all pests with respect to sales of pesticides worldwide. As an interesting side effect, the biological activity of herbicides extends beyond their effect on target organisms and herbicides may influence plant-pathogen interactions through their effect on the pathogen, the plant, or on the surrounding soil organisms including symbiotic interactions. This phenomenon was first observed in the early 1940s by Smith et al. (1946) and described in more detail since 1960. Several studies examining the direct effects of various herbicides on plant pathogens and disease development have been published. The objective of this chapter is to summarise publications in which herbicide applications have resulted in a direct effect on fungal plant pathogens in vitro or in the field or on disease development by influencing the metabolism of the plant. The question that arises is if the direct or indirect effect of herbicides on microorganisms is a general feature of these agrochemicals, also provoking some kind of stress that leads to a reprogramming of the plant’s physiology, or if the observed effect relates to a distinct mode of action within a given plant-pathogen relationship. In some cases, herbicide-resistant plants were used to specify the effect on pathogens and plants when herbicides are applied at field rates. Even though the application of field doses may represent the true situation in the field where herbicide-resistant crops are planted, the effect of sublethal doses on non-transformed plants and pathogens remains more or less obscure. Thus, some hormetic effects, although defined as side effects of putatively toxic compounds but playing an important role regarding plant health, plant growth, or even harvest, can not be explained. Some, if not many herbicides seem to provoke hormetic effects (Duke et al., 2006). Hormesis refers to stimulatory effects caused by toxic compounds. Paracelsus, an ancient leader in toxicology, declared that all things are poison and are not poison. Only the dose matters and it is only the dose that makes a thing not to be a poison. He considered that substances, although toxic at higher concentrations or doses, can be stimulatory or even beneficial when used at low doses. Even though this phenomenon was recognised a long time ago, hormesis was mainly discussed in the biomedical literature, especially in toxicology and radiation biology. Sublethal doses of