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Purine nucleotides are indispensable compounds for many organisms and participate in basic vital activities such as heredity, development, and growth. Blocking of purine nucleotide biosynthesis may inhibit proliferation and development and is commonly used in cancer therapy. However, the function of the purine nucleotide biosynthesis pathway in the pathogenic fungus <i>Magnaporthe oryzae</i> is not clear. In this study, we focused on the <i>de novo</i> purine biosynthesis (DNPB) pathway and characterized MoAde8, a phosphoribosylglycinamide formyltransferase, catalyzing the third step of the DNPB pathway in <i>M. oryzae</i>. MoAde8 was knocked out, and the mutant (∆<i>Moade8</i>) exhibited purine auxotroph, defects in aerial hyphal growth, conidiation, and pathogenicity, and was more sensitive to hyperosmotic stress and oxidative stress. Moreover, ∆<i>Moade8</i> caused decreased activity of MoTor kinase due to blocked purine nucleotide synthesis. The autophagy level was also impaired in ∆<i>Moade8</i>. Additionally, MoAde5, 7, 6, and 12, which are involved in <i>de novo</i> purine nucleotide biosynthesis, were also analyzed, and the mutants showed defects similar to the defects of ∆<i>Moade8</i>. In summary, <i>de novo</i> purine nucleotide biosynthesis is essential for conidiation, development, and pathogenicity in <i>M. oryzae</i>.