Ultrastructural Alterations in Fusarium sambucinum and Heterobasidion annosum Treated with Aluminum Chloride and Sodium Metabisulfite

Aluminum chloride (AlCl3) and sodium metabisulfite (Na2S2O5) have received increasing attention as antifungal agents for the control of plant diseases. In an effort to understand their toxic action on fungi, ultrastructural changes and membrane damage in Fusarium sambucinum (Ascomycota) and Heterobasidion annosum (Basidiomycota) in response to salt exposure was investigated using transmission electron microscopy. Conidial membrane damage was quantified using SYTOX Green stain, which only enters altered membranes. The results showed that mortality of the conidia was generally closely associated with SYTOX stain absorption in F. sambucinum treated with Na2S2O5 and in H. annosum treated with AlCl3 or Na2S2O5, suggesting that these salts cause membrane alterations. For both fungi, ultrastructural alterations in conidia treated with AlCl3 and Na2S2O5 included membrane retraction, undulation, and invagination. At higher concentrations or exposure periods to the salts, loss of membrane integrity, cytoplasmic leakage, and cell rupture were observed. Ultrastructural alterations and increased SYTOX stain absorption in salt-treated conidia appear consistent with a mode of action where AlCl3 and Na2S2O5 alter membrane integrity and permeability.