The fungi that cause mildew usually appear when there is too much humidity, in those places where sunlight does not reach or when the plants are very close to each other. These microorganisms also affect other plants, since the disease known as mildew is caused by a large number of fungal species, both fungi themselves and the so-called fungoidal protists (molds that resemble true fungi, but in reality do not are related to them).
These are fairly specialized pathogens, with several host-specific species. This means that the type of fungus that infects one type of plant may not be the same species of fungus that infects another plant, even though the symptoms may appear to be the same. Downy mildew spores are easily spread through wind drift and air movement, although they can be carried by insect pests moving from plant to plant. As the spores settle on a host plant, they can germinate and infect in as little as eight to 12 hours if the plant or its leaves are wet.