Identifying Diseases that Harm Rosemary Plants

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What You'll Need
Garden pruners

Rosemary is a hardy perennial, but it can suffer with a variety of diseases from time to time. It's one of the most popular culinary herbs found in home gardens. It also has medicinal value. The leaves and flowers are used to produce a tonic which relaxes muscular tensions, stimulates the liver and gall bladder, and improves digestion. Rosemary is an easy herb to grow. Basic conditions for successful growth include well drained soils, moderate moisture, adequate sunlight and good air circulation. You can propagate it from seeds, cuttings or bare roots. The herb will do well whether planted in the garden or in containers. Keep an eye out for these plant diseases.

TIP: Our expert gardening advisor, Susan Patterson adds, "Keeping rosemary healthy will help ward off disease."

Powdery Mildew

This is a common disease, especially when rosemary is grown indoors. It is a white, powdery fungal infection that develops when air circulation is poor, encouraging high humidity. Shady conditions are also conducive to powdery mildew. This will not kill your rosemary but it weakens the plant. Control it by keeping the humidity at low levels. Allow the soil to dry out between watering sessions. Expose your herbs to plenty of sunlight. If necessary, create a breeze around the herbs using a fan. Dust the herbs with sulfur to bring mildew under control.

Root Rot

Soil with poor drainage can lead to root rot. High levels of soil moisture encourage fungal spores which attack the plant roots. Roots turn black and eventually rot while stems are discolored. The leaves wilt and die. Do not over-water your rosemary. Using minimal water will help prevent excessive moisture. Also avoid heavy application of fertilizer, as this encourages fungal spores in the soil. Remove all affected herbs, including the surrounding soil, and discard.

Blight Infection

This is both a fungal and bacterial infection which kills the plant tissues. Blight is encouraged where there is high humidity, bad air circulation and cloudy weather. Yellowish-brown spots and patches will appear on leaves and stems. These spread rapidly, resulting in wilting of foliage. Blight also results in water-soaked spots forming on the herb stems. Prune the herbs to prevent them from getting too bushy. When they overgrow, air circulation is restricted, which encourages humidity. You can also increase spacing between the containers to improve air circulation. Remove and discard all infected plants to contain spread of the disease.

TIP: Susan suggests, "Prune rosemary in the winter with clean and sharp garden pruners."

Leaf Spot

Leaf spot is caused by bacteria as well as fungi. Affected leaves are usually yellowish-brown in color. Brown-black spots or patches appear on the leaves. As the infection increases, small clusters of back fruiting bodies develop on the leaves. Wet conditions encourage the spread of leaf spot. Do not apply water to the foliage but restrict watering to soil. Apply a fungicide to affected leaves to help control the infection. Shake the plants so that dead and weak leaves drop off. Remove all dead leaves from the plant base and destroy them to contain the infection.

TIP: Susan recommends, "Repot rosemary at least once a year."