Herb Plant Diseases

According to Rodale’s Illustrated Encyclopedia of Herbs (Rodale Press, 1988) there are several diseases you need to worry about with your herbs.  I tell you this now, because if you begin shopping in the nursery soon, you want to check plants for signs of these issues to avoid buying diseased plants.

1. Damping-off (Rhlzoctonia solani and Pythium debaryanum). This disease kills seedling roots, leaving them water-soaked and looking shriveled. Provide warm, well drained seed beds to manage this issue.  May affect sweet marjoram and sweet basil, especially the hybrids like purple and lemon basil.
   
   
   
2. 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. Check new nursery plants by popping them put of the pot before purchase.  If you happen to have this in the garden, then control it by rotating plants every three years and provide with good drainage. Promptly remove the diseased plants. May affect lavender, oregano, rosemary and sage.
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4. Powdery mildew (Erysiphe cichoracearom). This fungus forms white, powdery mold on the upper surfaces of leaves and petioles; foliage then wilts and browns. Promptly remove the diseased plant. May affect bee balm, lemon balm and yarrow.
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6. 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 against it, don't crown plants; cultivate only when plants are dry; and rotate your plants every three years. May affect calendula, coriander. tarragon and basil.  
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8. Anthracnose (Leaf spot) (Colletotrichum spp.). You can see this fungus on mints and scented geraniums.  It looks like a water-soaked spot on leaves and stems of the plant.  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.
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