This study evaluated the efficacy of the combined application of well-characterized chitosan polymer (degree of acetylation = 10%, degree of polymerization [DPn] = 90, and dispersity [Ð_DP] = 2.8) and oligomers (partially acetylated chitosan polymers and oligosaccharides [paCOS]) (DP = 2 to 17) on conidia germination and mycelial growth of Fusarium graminearum, the major causal agent of Fusarium head blight in wheat. The polymer alone showed a higher inhibitory effect than the paCOS mixture alone, with half-maximal inhibitory concentrations of less than 50 µg ml⁻¹ and more than 100 µg ml⁻¹, respectively. Using time-lapse microscopy, we also showed that paCOS did not affect conidia germination at 50 µg ml⁻¹, whereas chitosan polymer at the same concentration led to a delay in germination and in elongation of germ tubes. Scanning electron microscopy was used to observe the chitosan-induced changes in hyphal morphology. Surprisingly, the combination of chitosan polymer and paCOS led to strong synergistic effects in inhibiting conidia germination and fungal growth, as quantified by both the Abbot and Wadley equations. To our knowledge, this is the first report on a synergistic effect of a combination of chitosan polymers and oligomers, also highlighting for the first time the importance of Ð_DP when studying structure–function relationships of functional biopolymers such as chitosan. The consequences of this finding for the improvement of chitosan-based antimicrobial or plant protective products are discussed. Given the economic importance of F. graminearum, this study suggests that the combination of chitosan polymer and oligomers can be used to support an efficient, sustainable plant protection strategy. [ ] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY 4.0 International license .

Chitosan is a derivative of chitin that is one of the most abundant biopolymers in nature, found in crustacean shells as well as in fungi cell walls. Most of the commercially available chitosans are produced from the exoskeletons of crustaceans. The extraction process involves harsh chemicals, has limited potential due to the seasonal and limited supply and could cause allergic reactions. However, chitosan has been shown to alleviate the negative effect of environmental stressors in plants, but there is sparse evidence of how chitosan source affects this bioactivity. The aim of this study was to investigate the ability of chitosan from mushroom in comparison to crustacean chitosan in enhancing drought stress tolerance in tomato plants (cv. MicroTom). Chitosan treatment was applied through foliar application and plants were exposed to two 14-day drought stress periods at vegetative and fruit set growth stages. Phenotypic (e.g., fruit number and weight), physiological (RWC) and biochemical-stress-related markers (osmolytes, photosynthetic pigments and malondialdehyde) were analyzed at different time points during the crop growth cycle. Our hypothesis was that this drought stress model will negatively impact tomato plants while the foliar application of chitosan extracted from either crustacean or mushroom will alleviate this effect. Our findings indicate that drought stress markedly decreased the leaf relative water content (RWC) and chlorophyll content, increased lipid peroxidation, and significantly reduced the average fruit number. Chitosan application, regardless of the source, improved these parameters and enhanced plant tolerance to drought stress. It provides a comparative study of the biostimulant activity of chitosan from diverse sources and suggests that chitosan sourced from fungi could serve as a more sustainable and environmentally friendly alternative to the current chitosan from crustaceans.