Chitosan has been identified as an excellent rainfastness aid for a model agrochemical. We use fluorescent labelling and microscopy to show that chitosan deposits on Vicia faba leaves are able to resist wash off by rain, both in a lab-scale test and via simulated rain. Furthermore, via the same methods it has been demonstrated that the rainfastness of azoxystrobin, a common fungicide which is fluorescently active, can be improved by formulating with chitosan. These findings via fluorescent microscopy were in agreement with a quantitative LC-MS/MS method in which the exact quantity of active ingredient on a leaf surface was determined before and after a rain wash. The potential of chitosan as a rainfastness aid is further supported by positive comparison with a commercial tank-mix standard, with almost 100% retention of the model active ingredient achieved in both cases after 1 hour of washing with 10 mm h−1 intensity rain
