Powdery Mildew
Bane Of The Frustrated Gardener

Of all the diseases that affect our outdoor plants, none is more pervasive and troublesome to as many species as powdery mildew. In particularly wet or humid years, it can be the bane of a gardener’s existence, and threaten an entire season’s efforts. It’s an enemy that northern gardeners need to understand in order to manage it effectively.

Powdery mildew is a fungal infection of many plants, almost always affecting the leaves, although forming flower buds can be affected as well. The general “disease” is actually an assortment of fungi, each of which only affects certain types of plants, albeit with similar symptoms. However, all types of plants can be affected; shrubs, perennials, annuals, roses, fruits, veggies, even houseplants. It is particularly troublesome on annuals, especially vegetables.

A severe powdery mildew infection is, unfortunately, hard for a gardener to miss. It usually manifests as a whitish or grayish powder which appears on the surface of the leaves, although it often starts on the undersides and then spreads to the tops where one is able to see it. The powdery substance rubs off on your hands, and is often mistaken for dust or grime on the leaves. The fungi are parasitic, their tiny roots penetrating the leaves and feeding off the plant’s nutrients, causing the leaves to curl and eventually drop.

The fungi overwinter in fallen plant debris, with the spores carried to the new growth in spring by insects, rain and wind. The spores can also be spread throughout the growing season under right conditions. While it doesn’t require presence of moisture to spread, it is far more troublesome in situations of high humidity and poor air circulation, and may appear much earlier in particularly wet years. It is often a mid to late-season disease, not necessarily fatal to the plant, although it may trigger premature defoliation. New, vigorous, succulent growth is most susceptible to an infection.

As with many plant afflictions, there are two ways to control powdery mildew; cultural and chemical. Cultural controls begin with the selection of resistant species and cultivars, where available. Don’t crowd the plants when planting, to allow for good air circulation. Limit the amount of nitrogen fertilizer you apply to the soil to keep the plants growing slower. Always water susceptible plants at the root level rather than over the foliage, to keep it dry. And be sure to burn affected foliage in fall rather than composting it, as there is no guarantee that composting will kill the winter spores.

Most commercially available fungicides are an effective although expensive solution. Sulfur and neem oil are effective controls, although sulfur can be injurious to many plants. Some gardeners have claimed that a light baking soda solution or an application of skim milk are also effective, but these are not independently verified. Remember to be particularly careful with chemical treatments on vegetable and fruit crops, and always read the labels.