Rosemary (Rosmarinus officinalis) is an aromatic herb native to the Mediterranean region, where it's widely used in Italian and Spanish cuisines. This member of the mint family is reliably hardy in U.S. Department of Agriculture plant hardiness zones 8 through 10, but is grown as an annual in cooler climates. Although rosemary is a sturdy and resilient plant, it can become vulnerable to various diseases when subject to certain conditions. But you can prevent most problems with proper care.
Rosemary thrives in well-drained sandy or rocky soil as long as it receives at least six hours of full sun each day. It requires little supplemental watering. In fact, overwatering rosemary, especially in containers, leads to "wet feet" and root rot. This disease is caused by one of several species of Phytophthora fungus (Phytophthora spp.), which spreads via spores that can survive on garden tools and planters. Because conventional fungicides are not approved for use on herbs, prevention is key. Otherwise, plan to remove and destroy infected rosemary plants and sterilize potentially infected gardening equipment.
Rosemary is susceptible to powdery mildew, a fungal disease characterized by a white, powdery coating on leaves, which can be partially rubbed off with the fingers. This disease can be prevented by providing full sun but limited water, and spacing rosemary plants 18 to 24-inches apart to encourage air circulation. If you're growing rosemary indoors or in a greenhouse, avoid getting leaves wet or watering with overhead irrigation systems. Infected plants should be removed. However, some gardeners report success with the application of a water and baking soda paste, which alters the pH of the leaf surfaces.
Also known as gray mold, botrytis (Botrytis cinerea) is a fungal disease that commonly affects many vegetables and herbs, especially those grown in humid climates or in a greenhouse. Rosemary is one of the herbs affected by botrytis, and is often made vulnerable by broken stems or aggressive cutting combined with excess moisture. Improving ventilation and avoidance of overhead watering will help to prevent this disease. In addition, don't take cuttings from lower woody branches but from upper stems instead.
Crown gall (Agrobacterium tumefaciens) causes "galls" -- deformed overgrowths -- to develop on the roots of woody plants such as rosemary. Loss of nutrients and stunted growth may eventually cause the plant to die. To prevent this disease, avoid overwatering and provide adequate drainage. Also, because the bacteria invades the plant through wounds, do not take cuttings from lower branches.