Powdery mildew

Powdery mildew is a fungal disease that affects a wide range of plants. Powdery mildew diseases are caused by many different species of fungi in the order Erysiphales.

Identifying Features
It is one of the easier diseases to spot, as its symptoms are quite distinctive. Infected plants display white powdery spots on the leaves and stems. The lower leaves are the most affected, but the mildew can appear on any above-ground part of the plant. As the disease progresses, the spots get larger and denser as large numbers of asexual spores are formed, and the mildew may spread up and down the length of the plant.

Treatment
As powdery mildew does not like wet conditions, wetting the leaves reduces the severity of attack. A "wetter", e.g. detergent, has to be added (just a squirt); plain water will just run off the leaves. Weeds can over-winter the fungus, so good garden hygiene is essential. Remove and bin or burn infected leaves, do not compost them.

Prevention
Ensure plants have adequate water, especially those in containers, and apply mulches over moist soil if grown in open soil. Avoid overcrowding plants to give good air circulation and minimise humidity. Don't feed plants too much nitrogen-rich fertiliser because this encourages lush sappy growth which is more susceptible to infection.

Erysiphe species

 * Erysiphe spp.
 * Oregano
 * Erysiphe artemisiae
 * Tarragon
 * E. biocellata
 * Bergamot, mint and mint species, sage
 * E. heraclei
 * Dill, parsley

Neoerysiphe species

 * Neoerysiphe galeopsidis
 * Bergamot, oregano, Rosemary, sage