Ferric Chloride Controls Citrus Anthracnose by Inducing the Autophagy Activity of Colletotrichum gloeosporioides

Colletotrichum gloeosporioides causes citrus anthracnose, which seriously endangers the pre-harvest production and post-harvest storage of citrus due to its devastating effects on fruit quality, shelf life, and profits. However, although some chemical agents have been proven to effectively control this plant disease, little to no efforts have been made to identify effective and safe anti-anthracnose alternatives. Therefore, this study assessed and verified the inhibitory effect of ferric chloride (FeCl3) against C. gloeosporioides. Our findings demonstrated that FeCl3 could effectively inhibit C. gloeosporioides spore germination. After FeCl3 treatment, the germination rate of the spores in the minimum inhibitory concentration (MIC) and minimum fungicidal concentration (MFC) groups decreased by 84.04% and 89.0%, respectively. Additionally, FeCl3 could effectively inhibit the pathogenicity of C. gloeosporioides in vivo. Optical microscopy (OM) and scanning electron microscopy (SEM) analyses demonstrated the occurrence of wrinkled and atrophic mycelia. Moreover, FeCl3 induced autophagosome formation in the test pathogen, as confirmed by transmission electron microscopy (TEM) and monodansylcadaverine (MDC) staining. Additionally, a positive correlation was identified between the FeCl3 concentration and the damage rate of the fungal sporophyte cell membrane, as the staining rates of the control (untreated), 1/2 MIC, and MIC FeCl3 treatment groups were 1.87%, 6.52%, and 18.15%, respectively. Furthermore, the ROS content in sporophyte cells increased by 3.6%, 29.27%, and 52.33% in the control, 1/2 MIC, and MIC FeCl3 groups, respectively. Therefore, FeCl3 could reduce the virulence and pathogenicity of C. gloeosporioides. Finally, FeCl3-handled citrus fruit exhibited similar physiological qualities to water-handled fruit. The results show that FeCl3 may prove to be a good substitute for the treatment of citrus anthracnose in the future.