Fungus on a Rosemary Bush

Powdery Mildew

• As with most plants, rosemary is subject to its own particular brand of powdery mildew. This fungus exists in many different forms that attack specific plants, such as rosemary. Rosemary is most vulnerable to this fungus when it is moved. Outdoors, it is much less vulnerable. Bringing plants inside, however, will make the rosemary susceptible to attacks by the fungus, which looks like a dry, powdery substance dusted over the leaves.

Rhizoctonia

• Rhizoctonia is a fungus that affects rosemary in different ways, depending on where the fungus attacks the plant. It attacks some rosemary in seed form, preventing germination. In cases where the plant is more developed, it attacks the plant itself and causes web blight. This makes the plant matter turn soft and limp, with a web-like fungus growing on it. Don't water plants excessively, and provide space between each plant to prevent rhizoctonia outbreaks.

Phytophora Diseases

• Phytophora is a category of fungus that attacks rosemary in many different forms. It attacks the entire plant and, if left unchecked, will stunt and eventually kill the plant. The primary sign of phytophora is cankers on the plant. Usually, these appear near the base of the plant where it meets the ground. Good soil drainage is important to prevent outbreaks of phytophora from attacking your rosemary.

Downy Mildew

• Downy mildew is a fungus that mirrors the look of powdery mildew, with some key differences. The downy fungus is rougher and more fibrous than the powdery variety. Yellow spots on a leaf are another sign of downy mildew. Water your plants early in the day so the sunlight and wind dry them before cooler night temperatures set in. Cull plants that are very sick with downy mildew from the garden to reduce transmission.