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The antifungal activity of chitosan against Fusarium spp. was investigated based on in vitro and in vivo assays, and its possible modes of action were also explored. Chitosan applied at 4.0 g/L of acetic acid‐distilled water solution significantly decreased the mycelial growth of Fusarium oxysporum, Fusarium sambucinum and Fusarium graminearum by 88.4%, 89.0% and 89.8%, respectively. Tuber treatment by chitosan (4.0 g/L) of acetic acid‐distilled water solution, prior to inoculation, reduced dry rot severity induced by F. oxysporum and F. sambucinum by 60.0% and 48.2%, respectively. When tested as plant treatment, potato plants inoculated with Fusarium species, exhibited 33.5%–45.3% less wilting severity as compared to the control. This abiotic treatment improved the phenolic compounds activities and defence‐related enzymes such as peroxidase and polyphenoloxidase in potato tubers inoculated with Fusarium spp. Results clearly demonstrated that chitosan could be explored as an alternative agent to chemical fungicides for the control of tuber dry rot and Fusarium wilt through induction of the plant defence system.

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Nine isolates of Aspergillus spp., isolated from soil and compost were tested in vitro and in vivo for their antifungal activity against Fusarium sambucinum and Phytophthora erythroseptica, the causal agents of the Fusarium dry rot and pink rot of potato tubers. Tested using the dual culture method, the pathogen growth of F. sambucinum and P. erythroseptica was inhibited by 27 to 68% and 16 to 25% by all Aspergillus species, respectively. The highest inhibitory activity against both pathogens was induced by the isolate CH12 of A. niger. A significant reduction of the mycelial growth of both pathogens tested using the inverse double culture method involves the presence of volatile antifungal metabolites. Their effectiveness was also evaluated as tuber treatment prior to inoculation with the pathogens. The highest effectiveness in reducing Fusarium dry rot severity was recorded on tubers treated with the isolate CH12 of A. niger. This study also revealed that the efficacy of Aspergillus spp. as biocontrol agents may be enhanced by varying the timing of their application. In fact, the lesion diameter of dry and pink rots was reduced by 54-70 and 52% with preventive application, respectively. However, this parameter decreased by 21-48 and 47% when the Aspergillus spp. were applied simultaneously with pathogens, respectively. Similarly, diseases' severity, estimated based on average penetration of F. sambucinum and P. erythroseptica, was reduced by 57-77 and 55% with preventive treatments and by 29-68 and 44% with simultaneous application, respectively. This study reveals that Aspergillus spp., isolated from compost and soil, exhibits an interesting antifungal activity toward F. sambucinum and P. erythroseptica and may represent a potential source of biopesticide. Testing of their culture filtrates, their organic extracts and their toxicity may give additional information on their safe use as biocontrol agents.