Potassium phosphite effectively controls rubber tree anthracnose by inhibiting melanin biosynthesis of Colletotrichum...

Anthracnose disease, caused by Colletotrichum siamense, is a major disease affecting rubber trees (Hevea brasiliensis), leading to significant yield losses in tropical regions. Current management strategies primarily rely on a limited selection of fungicides, which are costly due to the persistently low price of natural rubber. This study shows that Potassium phosphite (Phi) markedly suppresses melanin biosynthesis and aborts the formation of melanized appressoria and their primary hyphae in C. siamense. Notably, at the low concentration of 50 mg/L, Phi outperformed tricyclazole in both protective and curative applications for controlling anthracnose on rubber tree leaves. Transcriptomic analysis revealed that Phi and tricyclazole target distinct biological processes, with Phi specifically inhibiting melanin-related pathways and tricyclazole affecting the energy metabolism of C. siamense. Especially, Phi interfered with tyrosine metabolism and catalytic enzyme activities essential for melanin biosynthesis by downregulating several key genes involved in the DHN- and DOPA-melanin pathways. Functional validation confirmed that overexpression of CsSCD partially restored melanin synthesis and appressorial melanization under Phi treatment, suggesting that it may be a molecular target of Phi inhibition. These findings elucidate the molecular mechanisms by which Phi suppresses fungal melanin biosynthesis, offering a cost-effective alternative for managing anthracnose disease in rubber trees.