Wondering about that white film on the leaves of your plant?

As I have been visiting gardens over the past few weeks, I am seeing more and more examples of powdery mildew – a fungal disease that is a common problem in gardens, infecting a wide variety of plants.

There are many different species of powdery mildew and some plants are more susceptible to it than others.

The most commonly affected vegetables include anything in the squash family such as cucumber, melon and pumpkin as well as tomatoes, eggplants and peppers.

A few of the ornamental plants that are most susceptible are roses, peonies, summer phlox, bee balm, magnolia, ninebarks and lilacs.

Powdery mildew spores drift into your garden with the wind, or if you’ve had powdery mildew occur in the past, new outbreaks may also come from dormant spores in old vegetation or weeds nearby.

When the fungus begins to take over one of your plants, a layer of mildew made up of many spores, forms across the top of the leaves. These spores are then carried to other plants by the wind.

The mildew can slow down the growth of your plant and, if the infection is severe enough, it can reduce flower or fruit yield and quality. It will not kill your plant, but it makes it look unsightly.

We often associate mildew with moisture, but powdery mildew thrives in warm (15C to 27C), dry climates, though it does require high relative humidity (i.e., humidity around the plant) to spread.

So, after a long dry period or in areas with little air circulation, powdery mildew will be more of an issue. Young foliage is most susceptible to damage as leaves turn yellow and dry out.

The best way to controlling this problem is proactive prevention. Here are some tips to prevent mildew:

• Choose plants that are resistant to powdery mildew. Many mildew-resistant varieties have been developed and can be bought from major seed suppliers.
• Examine plants with a history of severe powdery mildew once a week. As soon as you spot mildew starting to develop, pinch off the worst of the leaves.
• Do not compost any infected plant, as the disease can still be spread by the wind and persist in the composted materials.
• Selectively prune overcrowded areas to increase air circulation around your plants. This helps to reduce relative humidity.
• After pruning infected parts, do not allow pruning shears to touch healthy leaves. First sterilize your pruners with rubbing alcohol.
• Do not allow soil to dry for too long, especially around susceptible plants. A regular watering schedule will help to keep plants healthy.
• Fungicides should only be used to protect high-value plants with a history of disease. Fungicides will not cure or remove existing powdery mildew infections. Once the majority of leaves have leaf spots, it is too late to treat. If using a fungicide earlier in the season as a preventive or to help kill any overwintering spores, use something organic such as lime sulphur.

• Make sure you clean up any infected leaves in the fall and don’t leave them at the base of the plants where the spores can re-infect in the spring.

A pinch of prevention goes a long way.

Joanne Young is a Niagara-on-the-Lake garden expert and coach. See her website at joanneyoung.ca