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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!

©2012 Off the Grid News

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5 comments

  1. Wow it seems someone like myself has no chance at really growing herbs and I don’t have a PHD in plants so what am I suppose to do? I know after reading the long list (and I am somewhat sure not complete list of all the dangers). I am guessing I will have to buy dried form of these herbs. But I need to know what ones and what and how they can be used for. Any suggestions. Many thanks for all you do.
    David L.

    • Actually herbs are pretty hardy. I have been growing my own for many years and use no pesticides. I would start with the basics, Parsley, Oregano, Sage, Chives. There are a lot of good books you can purchase off the internet for cheap or web sites. my favorite is herbcompanion.com.
      The uses for herbs are almost endless, from culinary, medicinally to cosmetically to cleaning house.

      Good luck 🙂

      • theresa wilkinson

        hi, i am growing cilantro and it is producing many leaves but they are getting little tiny holes in the leaves but i don’t see any bugs unless they are so small to see. what should i do? natural remedy please.

    • Dawn is correct; herbs are very hardy and pretty easy to grow. I don’t grow that many of the ones mentioned in the article, but I don’t recall every having any herb die of a disease. Some that are hardy in my zone aren’t reliably so, but that’s a different story. I have grown, curly & Italian parsely, rosemary–if not hardy in your area, grow in pots and overwinter inside, many kinds of sage, Oregano, which will take over so don’t be afraid to pull it out when it exceeds it are, sage, chives, taragon, many kinds of basil, many types of thyme, nasturtiums, coriander (dies down in the heat of summer as do some others, dill, chevril, salad burnett. Many of the plants mentioned that get root rot, what well drained soil & do really well in raised beds. Bee Balm will indeed get powdery mildew, but I grow as an ornamental and don’t treat for it. Be very careful with Mint, it is very invasive. It’s roots spread and are very difficult to get rid of. I don’t grow it, but did grow some that would grow in my pond; and it self-seeded outside the pond. Big mistake, don’t think I’ll ever get rid of it. Don’t be afraid to try them; you’ll do great. Just start small!

    • Never b discouraged. Its easier than it looks. If your black thumbed start with oregano;)

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