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Recognizing Diseases Common to Herbs

Although less susceptible to them, herbs are not completely immune to plant diseases. Perhaps it is due to their undomesticated nature or their high concentrations of aromatic oils, but young or weak plants can still become a target. It is important to protect the seedlings of your herbs as well as the more mature plants from rots, mildews, viruses, fungi, and wilts. Good gardening practices, including prompt clean up and plant rotation, will take care of many problems. This list will show the most common diseases, what they look like, what plants they attack, and how you can control them.

Anthracnose – Small, water-soaked spots on the aboveground parts. Turns a light color and may drop out. Forms elongated tan cankers. It is a fungus.

  • Herbs Affected – Foxglove, mint, and violets.
  • To Control – Rotate your plants every two years. Do not cultivate when plants are wet. Remove diseased plants right away.

Bacterial Soft Rot – Soft decay of the fleshy tissues, causing them to become slimy or watery. Shoots will wilt and blacken at the base.

  • Herb Affected – Orris.
  • To Control – Remove diseased plants very quickly. Avoid injury to plants. Keep the soil as acidic as the plants can tolerate.

Bacterial Wilt – No yellowing occurs, but plants will wilt and die. Stem sap produces strings while the vascular system turns brown.

  • Herbs Affected – Coriander (cilantro), foxglove, nasturtium, sage, and scented geranium.
  • To Control – Remove diseased plants very quickly. Avoid injury to plants. Keep the soil as acidic as the plants will tolerate.

Black Spot – Black circles form on both sides of the leaves, up to ½ inch in diameter, with indistinct, fuzzy edges. The tissue around the spots turns yellow and leaves drop. It is a fungus.

  • Herbs Affected – Black hellebore.
  • To Control – Space plants to allow for good ventilation. Do not cultivate when plants are wet. Mulch heavily. Removed diseased leaves promptly. Water plants in the morning.

Botrytis Blight – Small yellow, orange, or brown splotches on the leaves, flowers, roots, or bulbs. Once it spreads, the plants become coated with fuzzy, gray mold and hard, black blisters on the plants stems. It is a fungus.

  • Herbs Affected – Rosemary and scented geranium.
  • To Control – Remove all yellowing foliage and dead flowers right away.

Crown Rot – Fibrous fans of white fungus near the base of the plants. Also appears as a red or light brown crust on the soil’s surface around the plants. Plants turn yellow, wilt, and die.

  • Herbs Affected – Aconite, angelica, black hellebore, parsley and violets.
  • To Control – Rotate your plants every three years. Dig up and burn the diseased plants.

Curly Top – The leaves pucker and curl down, cupping or looks like little balls. The young plants are stunted and die. It is a virus.

  • Herbs Affected – Foxglove.
  • To Control – Protect the plants from beet leafhoppers, which carry the disease, by closely spacing and covering the plants with gauze or a Reemay garden blanket.

Damping Off – This disease kills the seedling’s roots. The affected plants are water-soaked and look shriveled.

  • Herbs Affected – Sweet marjoram.
  • To Control – Pasteurize the soil. Provide warm, well-drained seedbeds.

Downy Mildew – Yellow spots appear on the upper surface of the leaves. Downy or violet-gray mold appears on the undersides. Leaves wither and die.

  • Herbs Affected – Calendula, coriander (cilantro), germander, queen of the meadow, tarragon, and violets.
  • To Control – Do not crown the plants. Cultivate only when plants are dry. Remove any diseased plants quickly. Rotate plants every three years.

Leaf Smut – Black, sooty spots or stripes on the leaves. Leaves will twist and curl. Plants eventually die. It is a fungus.

  • Herbs Affected – Autumn crocus.
  • To Control – Removed diseased plants quickly. Do not plant in infected soil.

Leaf Spot – Tiny, greenish yellow spots on the upper side of the leaves, which forms a white spot surrounded by a red band. This may drop out to give the appearance of a shot-hole. It is a fungus.

  • Herbs Affected – Echinacea, foxglove, germander, lavender, nasturtium, sunflowers, and violets.
  • To Control – Remove diseased plants quickly. Disinfect garden tools. Do not cultivate when plants are wet. Allow wide spacing for ventilation. Water the soil, not the foliage.

Mosaic Virus – Yellow and green blotched, curled leaves and stunted plants. Leaves grow upright in rosettes with misshaped flowers.

  • Herbs Affected – Aconite and foxglove.
  • To Control – Control aphids and keep your garden and surrounding areas free of weeds. Remove diseased plants quickly.

Powdery Mildew – White, powdery mold on upper surfaces of leaves and petioles. The foliage will brown and wilt, then drop. It is a fungus.

  • Herbs Affected – Aconite, agrimony, bee balm, calendula, germander, lemon balm, queen of the meadow, sunflowers, tarragon, and yarrow.
  • To Control – Remove diseased plants quickly. Thoroughly clean up your garden in the fall.

Root Rot – Rotted, yellowish brown to black roots and underground stems. Outside layers of root slough off, leaving a central core. It is a fungus.

  • Herbs Affected – Clary, fenugreek, lavender, mullein, myrtle, oregano, rosemary, sage, salad burnet, tarragon, thyme, and winter savory.
  • To Control – Rotate plants every three years. Provide good drainage to the beds. Remove all diseased plants quickly. Thoroughly clean up your garden in the fall.

Rusts – Reddish, orange, or black waxy pustules on the stems and foliage. Leaves may drop prematurely. The plants look unsightly. It is a fungus.

  • Herbs Affected – Beebalm, germander, mints, sunflowers, and yarrow.
  • To Control – Remove diseased plants quickly. Dust plants with sulfur. Thoroughly clean up your garden in the fall.

Stem Rot – Dead areas will develop at the bases. Plants will wilt, fall, and die. It is a fungus.

  • Herbs Affected – Sunflowers and yarrow.
  • To Control – Remove diseased plants quickly. Rotate plants every three years.

Verticillium Wilt – Yellowed blotches on the leaves, gradually turning to brown. The entire plant is affected at once, causing midday wilting. Leaves drop, beginning at the bottom of the plant. Also causes chlorosis – or greensickness – and stunting. It is a fungus.

  • Herbs Affected – Aconite, coriander (cilantro), foxglove, mints, nasturtiums, and sage.
  • To Control – Rotate plants every three years. Avoid fertilizers high in nitrogen. Remove diseased plants quickly.

By knowing more about the things that attack your plants, you will have better gardens year by year. Happy gardening!

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