Introduction
Oomycetes are among the most important and devastating pathogens of our crop plants. Around 35% of all plant diseases of global economic importance are caused by species belonging to this phylum. Oomycetes are characterised by short incubation times, spreading epidemically if not checked by appropriate plant protection measures. On a global scale, these diseases are treated using copper based formulations. Compared to organic compounds, copper based fungicides have the advantage of a low danger of resistance development even after prolonged large scale uses due to their multi site inhibitor action on diverse enzymatic reactions in the pathogens. However, the rather high application doses typically required for conventional copper fungicides may lead to unwanted build-up of copper residues in the soil. We therefore propose to develop copper containing chitosan nanoparticles in order to retain this important active ingredient which is even approved for organic farming. We will make use of the superior copper binding properties of chitosan, a renewable resource which for reasons of solubility and bio-availability will be used in the form of nanoparticles as an active carrier material for the copper ions. Chitosan itself possesses plant strengthening properties so that it can synergistically support the action of copper. Also, we are planning to increase the efficacy of the copper-chitosan nanoparticles by combining them with resistance inducing soil microorganisms. Such fungal or bacterial ‘bio-pesticides’ (bio-control agents, BCA) are often used by subsistance farmers in tropical and subtropical agriculture. However, these biological agents are difficult to combine with chemical means of plant protection, and not at all with the broad acting copper formulations. Copper-chitosan nanoparticles might represent a true breakthrough in this area, as BCA at least in part rely on chitin and chitosan degrading enzymes for their antagonistic action against soil borne pathogens, thus potentially being protected from the antimicrobial action of chitosan. In fact, the chitosan fragments generated in this process can in turn induce disease resistance in plants.