Powdery mildew is a common problem in herb gardens, according to the Horticultural Development Company. This fungal disease, which looks like a fine, white fuzzy coating on the plants, is especially prevalent on sage, parsley and mint. The mildew may consist of a number of different types of fungi, which vary depending on the particular herb. Although the fungi might be different, the conditions that cause powdery mildew in an herb garden are the same.
Powdery mildew appears as spots of white, powdery or fuzzy material. The color might be creamy white or it might be gray. The "fuzzy" easily wipes off when rubbed. It is most commonly seen on the tops of the leaves of the plant, according to Colorado State University. In time, the spots will merge, and the entire leaf, as well as other parts of the plant, may be covered with the fungus.
There are many fungi that affect herbs in this manner. Mints and sage are usually infected by the fungus Erysiphe biocellata. Parsley is the favorite host for Erysiphe heraclei. Oregano is especially susceptible, as it can be infected by any of the Erysiphe species of fungi and also Neoerysiphe galeopsidis. Rosemary is also affected by Neoerysiphe galeopsidis. For that reason, it is not recommended to plant rosemary near oregano.
The fungi that cause powdery mildew thrive in warm, dry (not rainy) areas. Unlike many fungi that develop when water sits on the leaves of plants (usually causing leaf spot), these species do not travel on water. What they do need, however, is high humidity, according to Colorado State University. As humidity increases, so does the likelihood of powdery mildew developing.
Plants that are crowded together do not have good air circulation. This can increase the immediate humidity in the area, leading to the development and spread of the fungal spores that cause powdery mildew. Herbs should be spaced so that there is plenty of room for air to circulate around the plant. Do not use nitrogen-based fertilizers late in the summer, as this will promote new growth, which is especially susceptible to these fungi.
Removal of infected parts of the plant can limit the spread of the fungus. Do not compost the leaves, however, as this does not usually kill the spores, which can then spread to other plants when the compost is used. Prune overcrowded plants back so that the air can circulate. Fungicides that contain neem oil, sulfur, potassium bicarbonate or triforine can be effectively used, according to Colorado State University.