Mildew is a fungal disease, which affects roses. The first signs of powdery mildew appear on young leaves, which hold their color but begin to crinkle. Then small patches of mold appear that develop into spore-bearing fungal filaments on foliage, stems and all other parts of the rose, even the buds (looks like a thin, white powdery substance sitting on growth, which steadily becomes deformed with the spread of the disease). It spreads in white strands, which anchor themselves to the foliage. From there the fungus will draw on the moisture and nutrients within the leaves. As soon as you see the crinkling of young rose leaves, be on the watch; the sooner mildew is arrested the better. Mildew can spread throughout the garden rapidly.
The disease is worst during hot, dry weather with cool, moist nights. Spores are dormant until they get the moisture required to germinate. Once the leaf surface is moist and remains moist (from overhead sprinkling or condensation) for about 3 hours, the mildew spores will begin to germinate. This can occur in damp, shady gardens where air circulation is poor, also where plants are stressed due to overcrowding or dehydration (plants insufficiently watered at the root level are often attacked by mildew). Plants grown in areas with not enough sunlight will produce thinner leaves making them more prone to infection. Also plants deficient in nutrients (especially calcium) have weaker leaf tissue and that makes them more vulnerable to disease.
It is important to keep roses well watered at the root level during hot days and prevent water from getting onto the leaves, especially before nightfall. Providing good air circulation between roses is vital. This helps dry up the roses more quickly. Good pruning methods and generous spacing between rose bushes when planting (generally 3 feet between hybrid teas and 4 feet between larger rose bushes is recommended). Treat dormant plants with lime sulphur in spring and spray with fungicidal soap.
Once mildew is noticed (keep an eye out for it in the early stages), spray the rose bushes with fungicidal soap or wettable sulphur (both products are readily available at the local nursery). Try to remove any diseased parts and bag and seal, or burn them. Do not compost any diseased parts as the spores will return to haunt your garden when recycled. There are some effective home remedies you can try to help fight mildew and stop it from spreading. The following recipe is also good for fighting blackspot and rust:
Baking Soda Spray:
Some folks have had certain success with stinging nettle spray. It is meant to help plants resist mildew and other diseases such as blackspot and rust. To make: gather 1 pound of stinging nettle plants (use long rubber gloves and wear protective clothing to prevent from being stung!); crush stinging nettle leaves and put into an old burlap sack or pillow case. Submerge the bag in a one-gallon bucket containing unchlorinated water, cover and let sit in a warm place for a week. Strain mixture through cheesecloth or mesh. Dilute liquid stinging nettle concentrate with 5 parts water to 1 part concentrate. Spray this over rose bushes every two weeks.