Rosemary Disease Control – How To Treat Sick Rosemary Plants

Rosemary Disease Control – How To Treat Sick Rosemary Plants

By: Bonnie L. Grant, Certified Urban Agriculturist
Image by Michael Deemer

Mediterranean plants like rosemary impart herbal elegance to the landscape and aromatic flavor to cuisine. Rosemary is a relatively stoic plant with few pest or disease issues but occasionally they do have some problems. Sick rosemary plants need an accurate diagnostic prior to treatment for adequate control. Learn about the most common rosemary diseases and how you can combat any problems.

Is My Rosemary Sick?

Rosemary disease control is almost unnecessary since they are naturally resistant to almost all common plant plagues. However, fungal diseases of rosemary do occur as well as a couple of bacterial infections. The best defense is good cultural care and proper siting.

Questions regarding whether your rosemary is sick or not can be answered by first giving a thorough inspection of the plant. If plant stems, leaves or tissues are discolored, it could be from the feeding activities of certain pests. Check carefully for tiny invaders.

If you see no insects, a closer look is required

to decide which common rosemary diseases might be infecting the plant. To prevent disease, make sure your plants have plenty of circulation and are planted in a well-draining area. If overly wet soil occurs frequently, consider moving the plants to containers or raised beds.

Fungal Diseases of Rosemary

The most common fungal diseases are root rot and powdery mildew. The latter occurs in warm, wet periods and is characterized by a dusting of whitish, fine spores on all parts of the plant. It is most prevalent when the plant is in semi-shade and temperatures are 60 to 80 degrees Fahrenheit (16-27 C.). An organic fungicide spray or a DIY mixture of baking soda and water can help combat the fungus.

Root rot will almost always kill the plant. The rosemary will become limp and terminal leaves and stems die off. This is because the roots are no longer able to uptake and move nutrients and water to the plant. Dig up the plant and prune out any infected roots and dust with fungicide powder. If the entire root system is black and mushy, discard the plant.

Sick Rosemary Plants with Bacterial Disease

Bacterial diseases are less common but may arise in favorable conditions and in contaminated soils.

Blight infections are both fungal and bacterial, and result in patchy leaf growth and yellowish spots. High humidity, too little sun and lack of circulation are promoting factors. Prune to increase circulation and ensure the plant is in a sunny location.

Leaf spot is another disease that may stem from fungal or bacterial pathogens. Brownish black spots appear and the stems will wilt. Avoid watering plants overhead.

In most cases, rosemary disease control is a simple matter of correctly siting the plant, good care and common sense. These are hardy perennials and rarely have any issues.

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