There are many of types of mildew, with one or more, occurring on almost every known flowering plant there is, including roses.
Mildew is a fungal disease.
This particular type appears as small patches of chalky coloured mould on the upper and lower surfaces of the foliage, and on the stems and any buds that have formed.
The mould develops into spore-bearing fungal filaments which adhere to the stems and foliage.
From there, the fungus will draw on the moisture and nutrients within the leaves and stems.
Spores are dormant until they get the moisture required to germinate.
The disease can spread quite quickly throughout the garden particularly if the weather is hot, dry with a light breeze during the day, and cool and moist overnight.
Condensation, overhead watering, or rain, wets the leaves and creates the ideal conditions for the windborne spores to stick to the plant and germinate.
The fungus can be encouraged in a number of other ways, for example:
Damp shady gardens, plants grown in areas with too little sunlight will produce thinner leaves making them more prone to infection.
Where plants are stressed due to overcrowding and air circulation is poor.
Where plants have been insufficiently watered at root level and have become dehydrated.
Plants deficient in nutrients, in particular calcium, have weaker leaf tissue making them more vulnerable to disease.
Infections cause distortion, chlorosis, and browning of the foliage.
It generally affects lush new plant growth as opposed to mature growth, and young leaves may hold their color, but begin to crinkle when squeezed.
Keep roses well watered at root level during hot days and prevent water from getting onto the leaves, especially before nightfall.
Providing good air circulation between plants, this helps dry up the roses more quickly.
Good pruning methods and generous spacing between bushes will help.
In spring spray plants with fungicidal soap.