Green Patch: 5 Common Herb Diseases

| February/March 2011

  • "It is possible for herb gardeners in the Eastern United States to develop a successful garden spot for sage."

1. Powdery mildew (Erysiphe cichoracearum). This fungus forms white, powdery mold on the upper surfaces of leaves and petioles; foliage then wilts and browns. Promptly remove the diseased plants and follow through with a fall cleanup. May affect bee balm, lemon balm and yarrow.

2. Downy mildew (Phytophthora spp.). Leaves will wither and die after this fungus takes hold. It forms yellow spots on the upper surface of leaves and violet-gray mold on the undersides. To defend, don’t crown plants; cultivate only when plants are dry; and rotate every three years. May affect calendula, coriander, tarragon and basil. See Page 64.

3. Damping-off (Rhizoctonia solani and Pythium debaryanum). This disease kills seedling roots, leaving them water-soaked and looking shriveled. Provide warm, well-drained seed beds to manage. May affect sweet marjoram.

4. Root rot (Rhizoctonia solani). This fungus causes rotted, yellowish brown to black roots and underground stems. Outer layers of the root will slough off, leaving a central core. To control, rotate plants every three years and provide with good drainage. Then, promptly remove the diseased plants and follow through with a fall cleanup. May affect lavender, oregano, rosemary and sage.

5. Anthracnose (Colletotrichum spp.). Watch out for small, water-soaked spots on aboveground parts with this fungus. Elongated tan cankers may also form on stems. Rotate plants every two years afterward and don’t cultivate when wet. Promptly remove the diseased plants. May affect foxglove, mints and violets.

According to Rodale’s Illustrated Encyclopedia of Herbs (Rodale Press, 1998), edited by Claire Kowalchik and William H. Hylton.

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