The Dominance of Chitosan Hydrochloride over Modern Natural Agents or Basic Substances in Efficacy against Phytophthora infestans, and Its Safety for the Non-Target Model Species Eisenia fetida

Growing pressure to reduce the environmental pesticide burden has the greatest impact on agriculture and crop protection. There is an enormous increase in the demand for research on new, effective, naturally based agents that do not pose an environmental risk. Phytophthora infestans is one of the most destructive phytopathogens, especially in cases where synthetic fungicides are not allowed. This paper describes the high efficacy and safety of the natural polymer chitosan under in vitro and in vivo conditions and its dominance over other natural agents or products. Chitosan demonstrated the highest efficacy against P. infestans. A concentration of 0.2–0.4% was highly effective. The protective effect of chitosan was 99.3% in natural conditions. Direct activity, equivalent to synthetic fungicides (MIC50 0.293 mg/mL), was confirmed. Chitosan was rated non-toxic to useful non-target species. We promote further chitosan expansion within legislation and implementation of chitosan as a safe substance that could reduce the pesticide burden, particularly in eco-friendly plant protection and production of non-harmful foods.