Powdery mildew is probably the most common garden fungus around. It is not too terribly picky about where it spreads, it likes humid weather, thrives in the heat of the summer and is hard to control once it has started.
The trick is to prevent it from happening by proper plant selection, spacing, pruning and treatment before it spreads.
Powdery mildew is caused by several different fungi, I won’t bore you with their names because they all act the same way.
The spores of the fungus are carried by the wind and by splashing water.