Cylindrotheca closterium Extract Protects Crops from Botrytis cinerea via Modulation of Leaf Surface Hydrophobicity

• Cylindrotheca closterium green extract protects crops from Botrytis cinerea infection. • The extract increases leaf surface hydrophilicity, reducing fungal spore adhesion. • Protection lasts up to one week without affecting plant growth or photosynthesis. • Diatom green extracts can contribute to sustainable crop protection. The overuse of chemical pesticides has accelerated the spread of resistant pathogen strains and raised major human and environmental health concerns, urging the search for sustainable alternatives to control crop diseases. Here, we show that foliar treatments with an aqueous extract of the mass cultivated microalga Cylindrotheca closterium (AEC), obtained without the use of chemical solvents, protect plants against the fungal pathogen Botrytis cinerea , causal agent of grey mould. When sprayed on leaves, AEC suppressed disease symptoms in the model plant Arabidopsis thaliana , as well as in crops like tomato, pepper, and eggplant. This effect persisted for at least one week after treatment, providing long-lasting protection without impairing plant growth and fitness. Notably, AEC appears to exert its protective action by increasing leaf surface hydrophilicity, causing a significant and dose-dependent reduction of water contact angle that correlates to impairment of early stages of B. cinerea infection. Spectroscopic and microscopic analyses indicated that AEC contains significant amounts of Si, as well as polysaccharides, proteins, and other amphiphilic molecules potentially able to modify the physicochemical properties of the leaf surface. Extract fractionation showed that this effect likely arises from the combined action of multiple constituents. By acting directly on the physical properties of the leaf surface, AEC is a promising candidate for novel, sustainable products for crop protection.