Herbs are generally healthy plants, and a herb that is given the sun, soil and water it wants will rarely be troubled by disease. When disease does strike, it is usually because the conditions were wrong: soil that stays wet, air that cannot move, plants packed too tightly, or leaves that are wet overnight. Prevention, not cure, is the whole story with herb diseases, because once a disease has taken hold there is often little to do but remove the affected growth and improve the conditions.
This guide covers the diseases that matter for herbs — powdery mildew, root rot, damping off, leaf spot and rust — and the cultural practices that prevent them. Use our plant disease checker if you are unsure what you are looking at, and check your local extension service for a diagnosis if a problem does not match anything here — they can confirm from a sample or a photo, and they know which diseases are common in your area this season.
Powdery mildew on herbs
Powdery mildew is the most common herb disease, and it is easy to recognise: a white, powdery coating on the leaves, usually starting on the older leaves and spreading. Mint is the classic victim, and sage, oregano, lemon balm and chives are all susceptible, especially in late summer when the days are warm and the nights are cool and damp. Mildew does not usually kill a herb, but it weakens the plant, spoils the leaves, and makes them unusable for cooking.
Prevention is about airflow and dry leaves. Give herbs space — do not pack mint and sage into a dense clump where air cannot move — and water at the base in the morning, never over the leaves in the evening. Cut back mint hard when mildew appears, and it will usually regrow with clean leaves. Remove badly affected leaves and dispose of them in household waste, not compost. Our guide to powdery mildew treatment covers the options, and our plant spacing calculator can help you work out spacing that gives herbs room to breathe.
Root rot and wet soil
Root rot is the disease that kills rosemary, lavender and thyme more often than any other problem, and it is caused by wet soil, not by a pest. The signs are yellowing leaves, wilting even though the soil is wet, a soft, dark stem at the base, and roots that are brown and slimy instead of white and firm. Mediterranean herbs are the most vulnerable, because they evolved in dry, stony soil and their roots cannot cope with standing water. Once the roots have rotted, the plant rarely recovers.
Prevention is drainage, drainage, drainage. Grow Mediterranean herbs in raised beds or pots with gritty compost, never in heavy, waterlogged soil. Make sure pots have drainage holes and are raised off the ground so water can escape. Do not mulch the crowns of rosemary and lavender with moisture-holding materials — a gravel mulch is better, because it keeps the crown dry. Water deeply but infrequently, and let the soil dry out between waterings. Our guide to root rot treatment explains what to do if you catch it early, and our guide to clay soil improvement covers how to improve a heavy bed before you plant herbs in it.
Damping off and seedling diseases
Damping off is the disease that kills herb seedlings in cold, damp compost: the stem goes thin and dark at soil level, and the seedling collapses and dies, often overnight. Basil and parsley seedlings are particularly vulnerable, because they are sown indoors in spring when the compost is cool and damp. The cause is fungal organisms in the compost, and they thrive when the compost is wet, cold, and airless, and when seedlings are sown too thickly.
Prevention starts with clean materials: use fresh seed compost, clean pots and trays, and do not reuse old compost for seedlings. Sow thinly, so air can move between the seedlings, and give them light and ventilation as soon as they appear — a windowsill with the window open on mild days is better than a sealed propagator. Water from below, or with a fine rose, and let the surface of the compost dry a little between waterings. Our guide to damping-off seedlings covers the full prevention steps, and our seed starting calculator can help you time your sowings so seedlings are not sitting in cold compost for weeks.
Leaf spot, rust and other leaf diseases
Leaf spots are brown or black spots on the leaves, sometimes with a yellow halo, and they affect basil, parsley, mint and sage. They are usually fungal, they spread in wet weather, and they are rarely fatal, but they spoil the leaves and weaken the plant. Remove affected leaves promptly, do not wet the leaves when you water, and improve airflow by thinning the plant. Do not compost diseased leaves — put them in household waste. Downy mildew is a related problem on basil: yellow patches on the upper leaves with a grey, fuzzy growth underneath, worst in warm, humid weather. It spreads fast, so remove affected plants early and do not grow basil in the same spot the following year.
Rust appears as orange or brown pustules on the undersides of leaves, and it affects mint and chives in damp summers. Remove affected leaves, improve airflow, and cut mint back hard — it will regrow with clean leaves. For all leaf diseases, the pattern is the same: dry leaves, moving air, space between plants, and prompt removal of affected growth. Use our plant disease checker to compare symptoms, and check your local extension service if a leaf problem does not match the common diseases.
Good habits that prevent most herb diseases
Most herb diseases can be prevented by a handful of habits. Buy healthy plants from a reputable source, and quarantine new plants for a week before you put them with your other herbs. Rotate annual herbs — do not grow basil in the same pot or bed year after year, because soil-borne diseases build up. Clean your pots and tools between uses, and do not reuse compost from a diseased plant. Water at the base in the morning, give plants space, and mulch with compost or gravel rather than with materials that hold moisture against the stems.
Keep the herb garden tidy: remove dead leaves, spent annuals and fallen debris, because they shelter slugs and harbour disease over winter. Do not over-feed, because soft, sappy growth is more susceptible to disease than sturdy growth. And choose resistant varieties where they exist — some basil varieties are bred for resistance to downy mildew, and they are worth seeking out if mildew has been a problem in your garden. A clean, airy, sunny herb garden with healthy plants is the best disease prevention there is, and it costs nothing but a little attention each week.
When to remove a plant, and what to do next
Sometimes the right answer is to remove the plant. A rosemary with root rot, a basil plant covered in downy mildew, or a mint clump that is badly rusted will not recover, and leaving it in place lets the disease spread to its neighbours. Remove the whole plant, dispose of it in household waste — never in the compost heap, because home compost rarely gets hot enough to kill disease organisms — and do not plant the same herb in the same spot for a season or two. Clean the pot or the bed, refresh the compost, and start again with a healthy plant.
Before you replant, ask what went wrong. Was the soil too wet? Were the plants too close together? Were the leaves wet every evening? Fix the cause, not just the symptom, or the next plant will suffer the same fate. Take cuttings from any healthy growth before you remove a diseased plant, so you have a replacement ready — but only from growth that is clearly healthy, because a cutting taken from a diseased plant carries the disease with it. If you are unsure whether a plant can be saved, a photo sent to your local extension service will usually get you a clear answer.
Common mistakes
The most common mistake is watering over the leaves in the evening, which is an open invitation to mildew and leaf spot. The second is packing herbs too tightly, so air cannot move and the leaves stay damp. The third is growing Mediterranean herbs in heavy, wet soil, where root rot is only a matter of time. The fourth is composting diseased material, which spreads the disease to next year's beds. And the fifth is ignoring the first spots — a few mildewed leaves removed promptly is a small job, while a plant left until it is covered in mildew is a lost plant. Check your herbs weekly, remove the first signs of trouble, and keep the leaves dry and the air moving.
| Disease | Herbs most affected | Signs | Prevention |
|---|---|---|---|
| Powdery mildew | Mint, sage, oregano, lemon balm | White powdery coating on leaves | Airflow, space, dry leaves, morning watering |
| Root rot | Rosemary, lavender, thyme | Wilting in wet soil; yellow leaves; brown roots | Sharp drainage; gritty soil; do not overwater |
| Damping off | Basil, parsley seedlings | Seedlings collapse at soil level | Fresh compost; sow thinly; light and ventilation |
| Downy mildew | Basil | Yellow patches above, grey fuzz below | Remove affected plants; rotate; resistant varieties |
| Leaf spot | Basil, parsley, mint, sage | Brown or black spots with yellow halo | Dry leaves; remove affected leaves; airflow |
| Rust | Mint, chives | Orange-brown pustules under leaves | Airflow; cut mint back hard; remove leaves |
Frequently asked questions
What is the white powder on my mint leaves?
That is powdery mildew, a fungal disease that thrives in warm days, cool damp nights and still air. Remove affected leaves, improve airflow by thinning the plant, water at the base in the morning, and cut mint back hard — it usually regrows with clean leaves.
Why is my rosemary wilting even though the soil is wet?
Wilting in wet soil usually means root rot — the roots have rotted in waterlogged soil and can no longer take up water. Improve drainage, stop watering until the soil dries, and repot into grittier compost if the plant is still alive. Prevention is sharp drainage from the start.
What is damping off in herb seedlings?
Damping off is a fungal disease of cold, damp compost that makes seedlings collapse at soil level, often overnight. Use fresh compost and clean trays, sow thinly, give seedlings light and ventilation, and let the compost surface dry a little between waterings.
Can I compost diseased herb leaves?
No — put diseased leaves and plants in household waste. Home compost rarely gets hot enough to kill disease organisms, so composting diseased material spreads the disease to next year's beds.
How do I prevent diseases on herbs?
Give herbs sun, space and airflow, water at the base in the morning, keep leaves dry overnight, grow Mediterranean herbs in free-draining soil, rotate annual herbs, and remove the first affected leaves promptly. Healthy herbs in the right conditions rarely get sick.