Garden Time: Common Herb Diseases

2015 5-26 Oregano (1)By Jill Henderson – Show Me Oz

Spring is prime time for buying, starting, propagating and transplanting herbs into the garden. However, should you find that one of your brand new store-bought herbs (or one you’ve just started or have been growing indoors over the winter) isn’t looking so hot,  take a moment to thoroughly inspect it for pests and diseases before introducing it to the garden.

Although diseases of herbs are uncommon, they do occur. In this article, I will talk about the most common herb pests and diseases to help you identify the culprit behind lagging, fading or flat-out dying herbs.  Let me begin by pointing out that the number and complexity of diseases that might affect garden herbs are numerous, though most are uncommon.

Of course, it is impossible to discuss all of the possible diseases that can affect herbs in a single article. If you suspect that your herb has a disease and you cannot identify it, take a sample of the diseased portion to your local cooperative extension office or call the nearest chapter of master gardeners and ask them to have a look at it for you.

With that in mind, all of the following diseases are fungal in nature, which gives us a clear insight into their prevention. Maintain excellent drainage, avoid overwatering and overhead watering when possible, and use mulch to prevent fungal spores from spreading to the leaves. Additionally, strong, healthy plants rarely succumb to these types of diseases, which means we should direct our attention to improving soil health, garden sanitation, plant rotation, and variety selection whenever possible.


Rust is a fungal disease that can lay dormant in the soil for a very long time. It emerges only when the conditions for its growth are favorable. Rust spores are carried by wind and water and are often introduced into the garden via raw or partially-composted manure.

By Downtowngal - Own work, Public Domain,

This disfiguring disease is characterized by rust-colored pustules, which first appear on the undersides of the lowest leaves on a plant. As rust progresses, the symptoms move towards the top of the plant.  As this happens, the pustules often appear on the surface of the leaves as well as the undersides.  Eventually, the leaves will turn yellow and fall off.

In order to halt transmission of the disease, remove all affected leaves and remove all debris, including mulch, from around the plant. Allow the soil to dry and place 4 in. (10 cm) of fresh straw mulch around the plant. Spray the leaves with an organic antifungal product. If the problem persists, you may have to destroy all of the infected plants and replant new ones elsewhere.

Mint rust (Puccinia menthae) is a type of rust that specifically affects members of the mint family. The symptoms of mint rust are very similar to other rusts in that yellowish to rust-colored pustules appear on leaves and stems.

This disease often strikes when roots have become over-dense and tangled. To avoid inviting mint rust, divide mints every several years to allow for proper drainage and air circulation and maintain good garden sanitation, disposing of dead plant material at the end of each growing season. If this fungal disease should strike, try dusting the plants with sulfur as directed.

Powdery Mildew

If your herbs develop white, circular, powdery spots on their leaves and stems, they are likely infected with another fungal disease known as powdery mildew.

CC BY 3.0 us,

Although mildew sounds like a disease that thrives in moist conditions, this particular disease actually strikes most often during periods of dry weather punctuated by evening dew or fog.

Powdery mildew spreads through wind, water, and infected tools and hands. To help prevent powdery mildew, avoid overhead watering and do not water at night or late in the day. To help prevent mildew from starting in the first place, try spraying plants regularly with a dilute combination of liquid kelp and milk mixed with water. These types of sprays are believed to change the pH of the leaf surface to an unacceptable level for the growth of mildew.

Downy Mildew

Another mildew to watch for is downy mildew. The small angular yellowish spots on the upper leaf surfaces and the “downy” white spores underneath characterize this fungal disease. Older leaves are usually the first to show the signs of wilting and yellowing.

By The original uploader was Agne27 at English Wikipedia(Original text: David B. Langston, University of Georgia, United States) - USDA ARS, Public Domain,

If allowed to continue its course, powdery mildew could prove to be fatal to the host plant. This disease, unlike its powdery counterpart, occurs most often during cool, damp weather. Treat it as you would powdery mildew. Space plants according to recommendations, increase air circulation by removing nearby plants, avoid overhead watering, and use a light mulch such as hay or straw.

Root Rot

Root rot occurs periodically in herbs of Mediterranean origin, such as rosemary, oregano, and marjoram. This particular fungal disease is most prevalent during periods of hot, humid weather and can persist in the soil for a very long time.

By Bjornwireen - Own work, CC BY-SA 3.0,

Plants affected by rot generally show symptoms of yellowing, wilting leaves starting at the top of the plant and moving downward as the disease progresses. The ultimate symptom, if left untreated, is death. And because root rot works so slowly, symptoms are not often evident until the condition has become critical.

This drastic disease calls for drastic measures. If the situation involves poor drainage (and it most often does) dig up the plant, trim off one-third of the leaves, wash all soil from the roots, and replant the herb in a new location with excellent drainage or in a pot with an equal mix of potting soil and coarse sand. Spray leaves with an organic foliar fertilizer such as fish emulsion.

Understand that if the plant is at the point of collapse, saving it is almost impossible. Avoid this disease altogether by providing drought-tolerant herbs with excellent drainage, especially during cold, wet winter and spring months. To improve drainage, amend the soil with equal amounts of compost, chopped leaves, and coarse sand or pea gravel. Consider planting these herbs in raised beds or plant them on raised mounds and mulch with 2 in. (5 cm) of pea gravel. Compost may discourage root rot.


Verticillium and fusarium wilts are both fungal diseases of great concern to gardeners everywhere. These dreaded diseases are notoriously difficult to control once in motion and generally lead to the death of plants infected with them.


The soil borne fungi directly affect the roots of plants, causing systemic failure of the vascular system. Infections cause stunting, wilting, yellowing leaves, and ultimately plant collapse.

Prevalent in warm, dry weather, wilt spreads through infected seeds, insects, tools, shoes, and hands. Wilt is highly contagious: Immediately remove and destroy all infected plants to avoid spreading the disease further. Avoid this disease by increasing soil drainage, rotating crops, and enacting sanitary garden practices. Basil and peppers are particularly vulnerable to fusarium wilt.

Many fungal diseases affecting herbs tend to appear later in the growing season, but those grown in a greenhouse or in warm, humid indoor environments are also susceptible.  So, before you pop that new, or newly sprouted herb, into the garden, take a few moments to give it a thorough checkup for any obvious signs of pests or diseases to help prevent spreading them throughout your garden.

Happy gardening!

© 2016 Jill Henderson  Feel free to share with a link back to the original article.

THPOKH-214x321_thumb7The Healing Power of Kitchen Herbs

Learn how to grow and use the world’s oldest, safest, and most medicinal herbs with this easy step-by-step guide!  From starting seeds to preparing home remedies, The Healing Power of Kitchen Herbs is a treasured resource that you will turn to time and time again.

Available in the Show Me Oz Bookstore.
Look inside!

Jill Henderson is an artist, author, and the editor of Show Me Oz . Her books, The Healing Power of Kitchen Herbs, The Garden Seed Saving Guide and A Journey of Seasons can be found in the Show Me Oz Bookstore.  Jill is a contributing author for Acres USA and Llewellyn’s Herbal Almanac and her work has appeared in The Permaculture Activist and The Essential Herbal.

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