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Phytotoxicity of Discula destructiva Culture Filtrates to Cornus spp. and the Relationship to Disease Symptomology

Discula destructiva culture filtrates and partially purified culture filtrates (PPCF) inhibited radish (Raphanus sativus) and dogwood (Cornus) species in a seedling root bioassay. Noninoculated potato-dextrose broth (PDB) extracted and separated in a similar manner also inhibited seedling growth. Reverse phase high performance liquid chromatography separated this inhibitory activity into two fractions, with one associated with the inhibitory action observed with PDB controls. The active fraction without interference with PDB, determined by bioassays, was extracted from cultures grown on Murishige-Skoog (MS) medium, which had no inhibitory activity associated with noninoculated controls. The active fraction was tested in a leaf overlay technique using 10 Cornus spp. All dogwood species were sensitive to the fraction and exhibited necrotic lesions bounded by a red margin, typical of dogwood anthracnose. The active fraction was translocated in Cornus alba to the leaf margin. C. canadensis showed minimal primary lesion formation but developed leaf curling and necrosis on leaf margins of newly emerging leaves, indicating apical translocation of the fraction from the application site. Comparison of three D. destructiva (Type 1) isolates and a Discula sp. (Type 2) isolate for production of the active fraction showed that the Type 2 isolate did not produce detectable amounts of the active component.