Rosemary plants are versatile shrubs that may be used as either decorative landscape plants or in an herb garden. Rosemary is an evergreen plant that develops blue flowers in the spring and summer to accompany its needle-like leaves. Caring for rosemary shrubs is usually easy, but there are several diseases that may infect these plants. Several of these diseases may actually be fatal for rosemary plants, and are often very difficult to get rid of once they infect the shrub.
Botrytis blight is a fungal infection that begins by rotting older leaves near the centre of the rosemary plant. It thrives in high humidity, cloudy weather and poorly circulated air, and can quickly spread over the plant. As the fungus progresses, it may appear as yellowish brown leaf spots or water spots on the stems of the plant. When humidity is particularly high, botrytis blight creates brown or grey fuzz that spreads over the decaying parts of the rosemary plant. If disturbed, the fungus gives off spores that help spread the disease. When left untreated, botrytis blight can overtake an entire rosemary plant and kill it. If you observe signs of the disease, remove any infected plants or debris, so the fungal infection does not spread. To prevent botrytis blight, place plant containers further apart for better air circulation. In addition, use crushed stone, gravel or rocks for mulching instead of organic material, which may contain fungal spores that allow the disease to enter your rosemary plants.
Root rot is another disease that can kill your rosemary plant. There are typically two main causes: over-watering or a fungal infection. Over-watering your rosemary plant robs the roots of oxygen and may cause some to die. These roots begin to decay and spread the rot to healthy roots, possibly killing the entire plant. Fungal cases of root rot are caused by dormant fungus in the soil that takes hold when excess water is added to the plant. You may suspect root rot if your rosemary plant begins to wilt or turn yellow for no obvious reason. Check the roots and base of the plant immediately. Infected roots appear black and are soft to the touch. In some cases, the affected roots may break free from the plant when touched. Root rot is typically incurable, so it is best to take preventive measures. Water your rosemary plant only when the top soil is dry, and make sure it is in a container that drains well so excess water does not build up.
Rosemary plants may also die as a result of leaf spot. Leaf spot is a fungal disease that attacks the leaves of the plant, turning them yellow or brown. They may also become covered with small black fruiting bodies that contain fungal spores that spread the disease. In most cases, leaf spot attacks leaves that are already weakened. When left untreated, the fungus may spread throughout your rosemary plant and kill it. To prevent leaf spot, be sure to fertilise your plant so that the soil is enriched with minerals. Using a copper fungicide before the plant begins growing in the spring may also help fight off leaf spot.