Introduction Plant pathogenic fungi are major pathogens in

Introduction Plant pathogenic fungi are major pathogens in agricultural diseases and the cause of large crop losses worldwide. There are many kinds of plant pathogenic fungi, with various modes of action. For example, Botrytis cinerea (B. cinerea) is the disease pathogen in the economically important gray α-Conotoxin PIA disease, which can make fruits and vegetables rot shortly after harvest (Li, Shao et al., 2017). Phytophthora capsici Leonian (P. capsici) is a soil borne destructive disease, which can infect roots, crowns, and even leaf surfaces rapidly at different developmental phases (Ozgonen & Erkilic, 2007; Wang, Sun, Zhang, Zhang, & Feng, 2016). It can seriously affect the yield and quality of various vegetable crops by causing Phytophthora blight. Fusarium solani (F. solani) and other Fusarium fungi are one of the main pathogens causing great economic losses in the world, and parasitize many kinds of plants (peanut, tomato, potato, etc.), causing plant root rot disease, Fusarium wilt and other diseases (Chohan & Perveen, 2015; Rojo, Reynoso, Sofia, & Torres, 2007). These kinds of fungi are highly infectious and cause great harm to crops (Hao et al., 2017; Zhang et al., 2013). At present, the use of chemical fungicides is the primary measure used to prevent and control plant fungi diseases (Nguyen et al., 2013; Wang et al., 2016). However, because heavy application of said fungicides has resulted in serious ecological, environmental and health problems (Yuan et al., 2014), and fungi can be easily induced to develop resistance to fungicides (Dias, 2012), chemical fungicide use is limited as a method of prevention and control for disease (Hsu, Wang, Sun, Hu, & Chen, 2017; Jiang et al., 2015; Koç, Arici, & İşlek, 2016; Shi & Sun, 2017). Therefore, it is urgent to research low-toxicity and green fungicides. Marine resources are rich in antibacterial active substances from which fungicide alternatives can be developed. Chitosan, which is extracted from shrimp and crab shells, is a marine-sourced antibacterial active substance. It is the only alkaline polysaccharide currently found in nature with good biocompatibility and biological explanation for antimicrobial activity. According to the reports, chitosan can not only regulate plant resistance, but is also active against bacteria, fungi, yeasts and viruses (Hu et al., 2016), which makes chitosan an excellent alternative to synthetic pesticides in reducing negative influences on environment (Muxika, Etxabide, Uranga, Guerrero, & De La Caba, 2017). Because of these desirable properties, chitosan has attracted much attention and has broad application prospects in the field of plant protection. However, its weak antifungal activity and water solubility restrict its application in agriculture. Thus far, more and more studies have shown that the antimicrobial activity of chitosan may be related to its amino protonated groups (NH3+): the interaction between these with the negatively charged groups of cell membranes could disrupt cell structure (Pavinatto, Caseli, & Oliveira, 2010) and lead to microbial death (Je & Kim, 2006; Kong, Chen, Xing, & Park, 2010; Zhang et al., 2017). Hence, in order to improve prospects for practical use, many papers have proposed protonated chitosan as an enhancement for chitosan’s antimicrobial ability and water solubility in the field. Protonation is achieved by turning the primary amino groups into positively charged groups (Chen et al., 2016; Wiarachai, Thongchul, Kiatkamjornwong, & Hoven, 2012) or directly grafting positively charged groups, such as quaternary ammonium (Li, Zhang, Tan, Gu, & Guo, 2017; Mohamed, Sabaa, El-Ghandour, Abdel-Aziz, & Abdel-Gawad, 2013), guanidine (Li, Gao et al., 2017; Sang, Tang, He, Hua, & Xu, 2015), or quaternary phosphonium (Nikitina et al., 2016; Vasu, Kiran, Suresh, & Devendranath, 2004) on the chitosan. However, the strong protonated groups lead to several unexpected problems: for example, they are cytotoxic to human cells (Chen et al., 2016).