(June 2004)

Much as gardeners believe otherwise, there are easy explanations why lilac trees and shrubs flower poorly in the spring. For optimum flowering the next spring, powdery mildew disease must be controlled during the summer months so lilac foliage produces substantial amounts of carbohydrates, this sugar used to develop dormant flower buds from mid-August through late October. Powdery mildew stops lilac leaves from making sugar, therefore the plant flowers poorly or not at all the following spring.

Most gardeners have run encountered powdery mildew before, but there’s still good reason now to look at the disease and why it endangers plants.

Powdery mildew happens at night when tropical temperatures and humidity cause outbreaks of the disease. When nighttime temperatures and humidity add up to 150 or more, disease happens. You find powdery mildew attacking a multitude of plants, but the species found on lilac also attacks alder, hazelnut, honeysuckle, oak and plane trees. Other species of powdery mildew are found on azalea, blueberry, catalpa, crapemyrtle, euonymus, hydrangea, locust, magnolia, poplar, privet, rhododendron, tulip poplar, viburnum and walnut. A host of annuals and perennials also experience the disease.

On lilacs, powdery mildew first appears on older leaves as white, irregular deposits. In extreme cases, all leaves on the lilac are often covered with thin, white fungal deposits that don’t wash off with rain or sprinkling. When the disease attacks new foliage, these leaves usually curl and twist out of shape.

Now we come to fungicides registered for powdery mildew control.

We don’t want you spending all sorts of dollars to stop the disease on your lilacs, so in spite of heavy advertising by Bayer for its powdered Bayleton fungicide, you’re better off relying on Daconil 2787 to save your shrubs. You’ll find Daconil at your local nursery under Ortho Garden Disease Control, the fungicide being Daconil. A quart container will help you stop powdery mildew for the next 5 years.

Using a hand-pump sprayer, try to spray in the evening when no overnight rain is forecast. Adjust the nozzle so you have a misting spray on lilac leaves. Repeat every 2 weeks until mid-September. If a leaf shows disease, pick it off when you see it.

Here is a photograph of the Ortho product, but not the same one you’ll find at the nursery. We called Scotts (2 people) in early June to ask them to e-mail us a photo of their Daconil container, but they turned a deaf ear to our request. Shame on Scotts!