Like animals and humans, plants possess a kind of immune system. It can e.g. recognize pathogenic fungi by the chitin in their cell walls, triggering disease resistance. Some fungi hide from the immune system by modifying some of the chitin building blocks, converting chitin into chitosan. Researchers of the University of Münster now found that plants can react to a certain pattern in this chitosan, stimulating their immune system. They are already developing a chitosan-based plant immune-stimulant in order to reduce the use of chemical pesticides in agriculture.