Powdery mildew is an easy disease to diagnose because of the white powdery growth occurring on leaf surfaces [1]. It can occur on many different types of plants including deciduous trees, shrubs, and perennials. The most commonly affected plants are rose, alpine currant, oak, phlox, and lilac. This diseases is caused by different species of host specific fungi. In other words, the fungus causing powdery mildew on oak trees cannot infect roses.

Powdery mildew can occur throughout the growing season, but is most severe in the spring and fall during cool, humid weather. It does not usually damage plants, but repeated infestations can decrease plant vigor.

To manage powdery mildew increase airflow and light penetration by pruning or thinning infected plants [2]. Adequately space new plantings. Water, mulch, and fertilize plants to maintain their vigor. And clean up fallen leaves and plant debris in late fall to reduce the number of spring infections.

Fungicides are available for severely stressed plants, however they are only preventative and not curative, so treatment needs to be started as soon as the disease appears. Remember, always read the label before purchasing, opening, and using fungicides.