When Leaves Turn Powdery White

Everything was going so well. Your perennials were blooming, your shrubs and trees had beautiful foliage, your vegetables were producing well, etc. Then all of a sudden, you notice, sometime in August or September, that the leaves of certain plants are covered in what looks like white powder, as if they were sprinkled with icing sugar. This “powder” often forms first on the lower leaves and then spreads upward. Over time, it becomes denser and thicker. Eventually, it’s no longer just an innocuous powder, the affected leaves begin to curl up and even blacken: not a very pretty sight!

Your plants “sprinkled with icing sugar” are actually ill. They’re suffering from powdery mildew (PM), a widespread fungal disease. Or should I say, fungal diseases, as there are hundreds of species and strains of powdery mildew in several different genera: Oidium, Erysiphe, Microsphaera, Phyllactinia, Podosphaera, Sphaerotheca and Uncinula, among others. Each of these strains is specific to a certain plant or group of plants, so at least there isn’t a risk of cross-contamination. In other words, bee balm powdery mildew won’t affect phlox or squash, phlox powdery mildew won’t harm bee balm or squash and squash powdery mildew won’t harm anything but squash and other cucurbits.

Powdery mildew germinates from wind-born spores that have landed on a leaf. It tends to be most common at the end of the gardening season, because most strains develop best under cool conditions and high humidity … and at the end of summer, the nights become cooler and dew coats plant leaves each morning: exactly the conditions PM needs to develop well.

As with most diseases, PM tends to mostly infest plants that are under stress. You’ll notice it’s less common when the summer is rainy and the plants are therefore well watered and in good shape in the fall, but is much more evident when the summer has been dry, as drought is an important cause of plant stress. Still, some plants very subject to PM, such as certain garden phloxes (Phlox paniculata), tend to suffer from the disease regardless of whether they’re stressed or not.

Ugly, But Not Necessarily Deadly

Powdery mildew is often more an aesthetic affliction than a fatal disease. When it occurs late in the season, affected perennial and woody plants have already stored up reserves for the winter and will still grow back in the spring in perfect shape, as if nothing had happened. By the time powdery mildew hits, they really didn’t need their leaves anymore. However, if PM often causes nothing more than unattractive leaves on more permanent plants, it can be deadly to annuals and vegetables…

But again, it usually strikes late in the season, after the plant has already produced the seeds for the following year. You’ll often see cucumbers or tomatoes with distinctly mildewed leaves still producing healthy fruit! That’s why so few plants have evolved efficient defenses against PM: the disease doesn’t prevent the plant from completing its natural cycle and is therefore not a major concern … at least, from a plant’s point of view.

Preventing Powdery Mildew

When you see leaves turning powdery white, it’s already too late to react. The white powdery coating is the last stage of the disease. Already, the fungus has been hard at work inside the leaves for weeks, extracting sugars and minerals for its own purposes. The appearance of a “white powder” on the surface is a sign that the fungus has reached maturity, because this powder actually consists of sporangia, the organs that produce the spores of the upcoming generation.

You can, of course, cut off damaged leaves and stems and this may improve the plant’s appearance. (Or may not, as sometimes you have to cut the plant to the ground and an empty space may look worse than a bit of graying foliage.) Pruning remains essentially an aesthetic treatment: it does nothing to prevent the disease in future years.

If controlling powdery once its symptoms are visible is difficult, it can be prevented. The usual technique is to apply fungicides from early summer to late fall. There are both organic and chemical fungicides you can try: if they’re useful against PM, the label will say so. And there are homemade treatments, such spraying the leaves with a solution of baking soda (1 tsp/5 ml of baking soda mixed into 1 quart/1 liter of water) or milk (one part milk in 9 parts water), that have proven to be effective in some cases.

Probably the most effective preventive treatment, though, is not to allow plants to suffer from drought stress! When the soil remains evenly moist, powdery mildew is rarely seen. A good mulch that reduces evaporation and helps keep the soil evenly moist, or regular watering without moistening the foliage (use a soaker hose) can do a lot to prevent the disease.

Another treatment that seems to work well is to spray the foliage with an anti-transpirant, like Wilt-Pruf, Wilt Stop or Vapor Gard (offered in garden centers). Apply it in spring, when the foliage is fully formed. The product covers the leaf with a thin film of natural wax that keeps disease spores from reaching the leaf surface. Since anti-transpirants give the leaves a certain luster, it’s easy to see when you need make another application: when the foliage loses its gloss, spray the leaves again. During very rainy summers, several treatments may be necessary; if the summer is drier, often only two are required.

It’s also claimed that powdery mildew is less problematic where air circulation is good. That doesn’t always appear to be true (I’ve seen peonies and ninebarks [Physocarpus opuliformis hybrids] that were severely infested in very windy spots), but certainly, planting PM-sensitive plants in a windy location can do no harm and might help with some strains of the disease.

Will cutting off and burning the foliage of diseased plants, a remedy often recommended, help prevent the problem? If you are the only person in your area who grows phlox, bee balm (Monarda), squash or any other disease-prone plant, this may work (remember the disease is very host-specific), but otherwise you’re probably wasting your time, because the disease is transmitted by the wind and you have no control over what the wind brings. Your plant will soon be infested again from some other source. I’ve seen gardeners cut and burn phlox stems and foliage year after year, plus carefully clean the soil and replace the mulch annually, yet the plants were always just as damaged by the end of the following summer.

Laidback Treatments for Powdery Mildew

Since powdery mildew is rarely lethal to its host plant and it doesn’t keep the plant from flowering or bearing fruit, many laidback gardeners have learned not to be too concerned about it. Planting mildew-susceptible plants at the back of the border where you can’t see their whitening leaves, but where its flowers remain visible and its fruit accessible, can be sufficient. Problem solved!

Another easy solution is to plant varieties that are resistant to powdery mildew. Why cultivate phloxes, bee balms, squash, ninebarks, etc. that are subject to PM when there are so many cultivars that aren’t? (And why do garden centers continue to sell varieties that are prone to this disease?) When shopping for vegetable and flower seeds, remember that seed catalogs inevitably point out when a variety they offer is resistant to powdery mildew. If you’ve had problems in the past, simply buy disease-resistant seeds.

Here’s a useful trick when it comes to perennial and woody plants prone to PM. Rather than buying them in the spring, when PM is not yet visible, buy them in August or September, when any plants that are prone to the disease in will be showing symptoms. You have only to choose the cultivars whose foliage is still in great shape: they’ll be naturally resistant plants!

So today, after you finish reading this blog, get up from your couch and head off to a garden center looking for phloxes, bee balms, lilacs, ninebarks, peonies and the like that are free of powdery mildew. Then when you get back home, just tear out your diseased plants and replace them with healthy specimens for years of easy gardening.

Gardening is so easy when you eliminate “problem plants!”