Azaleas are often attacked by mildew, which makes the leaves look like they've been dusted with white powder. Infected plants will probably drop leaves and some of next year's flower buds. Powdery mildew occurs most often during the fall, especially if weather conditions are particularly humid following extended periods of summer root dryness. While the plant is not likely to die from powdery mildew, the disease is unsightly and is known to cause damage to some of the more susceptible cultivars, such as Hinodegiri. However, mildew on azaleas is controllable with the use of some readily available products and appropriate cultivation practices.
Treat affected specimens with an azalea-safe fungicide such as Ortho RosePride Funginex at the first signs of mildew on the leaves. The earlier you initiate aggressive action, the easier it will be to effectively bring the disease under control. Dissolve 1 tbsp. of the product in a gallon of water and spray the plants as per packaging instructions. Multiple applications will be necessary.
Water deeply throughout the growing season so that the top 8 to 12 inches of soil is irrigated, but never soggy or wet. The medium should be evenly moist and the plant's roots must not dry out. Azaleas should never be allowed to wilt because prolonged root dryness is the major cause of mildew infection. Water early in the morning so that your azaleas will benefit from the moisture before it begins to evaporate during the midday hours.
Mulch azaleas 2 to 4 inches deep and as far around each plant as the branches reach with organic vegetative material in the spring. Don't use animal manure, which isn't appropriate for plants that love acidic soils. Mulching will help to retain moisture, discourage weeds and keep the soil warmer in the winter and cooler in the summer.
Use an appropriate slow-release fertilizer such as Miracle-Gro Shake N Feed Azalea Food throughout the season to keep your plants healthy. Miracid is also a good product for plants that require acidic soils. Fertilizing will help to increase their resistance to disease.
Test your soil for acid levels on a monthly basis. Azaleas grow and perform best when provided an acidic medium. The ideal pH for these plants is 4.5 to 6.0. Although your grandma used to just dump used coffee grounds or pour pickle juice around her azaleas, today most people apply sulfur to their plants to raise pH levels.
Locate new azalea specimens in landscape areas that are not heavily shaded. Space each plant well apart from any others and from structures to allow for adequate air circulation. Dwarf and evergreen varieties need 2 to 3 feet of space, deciduous azaleas should be 3 to 7 feet apart, and larger specimens will need to be at least 7 to 14 feet apart.