How can chitosan protect plants from disease?

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It is long known that chitosan treatments can render crop plants resistant to diseases, but the effect used to be unreliable unless excessive amounts of chitosan (ca. 40 kg/ha) were used. However, scientific studies of the last decade have revealed detailed structure/function relationships of chitosans in plant protection, identifying which chitosans are best suited to protect plants from disease, and such well-defined 'second generation' chitosans (of which less than 200 g/ha may suffice) with reliable functionalities are now becoming available on an industrial scale. Importantly, chitosan hydrochloride has been declared a 'basic substance' by the European Commission in 2014 for the use in plant strengthening products. Chitosans may protect plants from disease in at least two different ways. On the one hand, the anti-microbial activities of chitosans can directly impede the growth of pathogenic micro-organisms. On the other hand, chitosans can trigger induced disease resistance reactions in plants, strengthening their own 'immune system'. It is not yet fully understood how plants perceive chitosan, but the chitin-receptor which allows plants to recognize chitin-containing fungi and insects seems to be involved. Generally, chitosans with high degree of polymerization and intermediate degree of acetylation are best to induce plant disease resistance.