Significance of sulfur compounds in the protection of plants against pests and diseases

The effectiveness of targeted nutrition in the protection of plants against pests and diseases is called nutrient-induced resistance. Though the significance of individual nutrients for maintaining or promoting plant health prompted some interest in the 1960s and 1970s, research in the field of nutrient-induced resistance mechanisms has been scarce because of its complexity and limited practical significance due to the availability of effective pesticides, Interactions between mineral elements and plant diseases are well known for essential macro and micro plant nutrients, as well as aluminum and silicone. In the case of sulfur (S), the fungicidal effect of foliar-applied S has been exploited since the end of the 19th century, while the significance of soil-applied S for disease resistance became evident only a century later. Sulfur was neglected for a long time as an important plant nutrient because it was the main constituent of the toxic acid rain. Only during the 1990s, when S deficiency developed into a widespread nutrient disorder after clean air legislation came into force, was S also investigated with respect to nutritional aspects and plant health. Understanding the mechanisms of nutrient-induced resistance will contribute substantially to maintaining plant health in organic farming and minimize the input of pesticides in conventional farming systems. Sulfur-containing metabolites that are supposed to be involved in pathogen resistance are glutathione, glucosinolates, the gaseous release of volatile S, phytoalexins, S-rich proteins, and the formation of elemental S. Current knowledge about the influence of S nutritional status on these metabolites and their relationship to pathogenesis is summarized and discussed in the present article.