Late Summer Powdery Mildew


When Leaves Turn Powdery White

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Unless you applied some sort of powder insecticide, white powder on plant leaves can mean only one thing: powdery mildew! Photo: Jeff Kubina, Flickr

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.

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Some garden phloxes get powdery mildew every year. Photo:

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

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By the time the leaf is turning powdery white, it’s too late to treat with a fungicide. Photo: Scot Nelson, Flckr

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.


Baking soda spray can help prevent powdery mildew.

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.


Anti-transpirants can help prevent powdery mildew.

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

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Planting bee balm (Monarda) at the back of the border, where its leaves won’t be as noticeable, can be quite an effective treatment. Photo: K. M., Flickr

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!

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Healthy leaves on PM resistant cucumber ‘Marketmore 76’. Photo: caribbeangardenseed

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.

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Phlox paniculata ‘Bright Eyes’ is one of many mildew-resistant phloxes. Photo:

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!”20170831A Jeff Kubina, Flickr

Leaf Diseases Won’t Take Over Your Yard


Think before your spray for diseases: it is really necessary?

I regularly hear gardeners terribly concerned about foliar diseases: powdery mildew, black spot, rust, etc. Their main concern seems to be that the disease will spread from the first plant infected to everything they grow. Hence the belief that they have to react quickly to treat the diseased plant or even to eliminate it before the disease spreads.

This is actually fairly true if you grow other plants of the same species: a disease that affects one potato plant may well affect other potato plants… but it’s very unlikely to infest fuchsias, apple trees, marigolds, and other unrelated plants.

That’s because most foliar diseases are specific: they have a host plant and live only on this plant or perhaps on a few others that are genetically similar.

Powdery mildew

Take powdery mildew as an example.


The powdery mildew that affects this squash (a strain of Podospheara xanthii) will only infest other related plants.

It’s a common disease found on a wide range of plants… but in actual fact, it’s not one disease, but many, a series of fungal diseases in a wide range of genera: Oidium, Erysiphe, Sphaerotheca, Uncinula, Podosphaera, Leveillula, etc. Each of these genera contains several to many species and each of these species is further divided into strains. Each strain (and there are thousands!) has its preferred host. What these diseases have in common is that, at some point in their development, the plant’s foliage will be covered in what appears to be a white powder (actually sporangia, the final stage of the disease), giving them all a shared common name: powdery mildew. But despite this common symptom, in fact, powdery mildews are in fact different diseases.

That means that when your phlox suffers from powdery mildew, you may have to worry that the disease might spread to other phlox plants, but not to bee balms, lilacs, tomatoes, squash, etc. Each of these plants has its own strain of powdery mildew, one it only shares with its closest relatives.

To Treat or Not to Treat?

Knowing this detail can greatly influence your response.


Powdery mildew on a ninebark is certainly visible, but really doesn’t harm its host.

If you cultivate only one ninebark (Physocarpus), for example, and it shows symptoms of powdery mildew, with some leaves turning entirely white… it may not even be worthwhile reacting.

The disease is not going to spread to your other plants and is, in fact, fairly harmless even to the ninebark: it certainly doesn’t undermine its overall health. It’s one of those diseases that can be said to be mostly esthetic. So what if your ninebark turns a bit white at end of the season? Does that keep you from sleeping at night? Is it important enough for you to get up and spray a potentially toxic fungicide to try and control it?

I learned long ago that no garden is perfect and the occasional leaf disease simply isn’t worth fighting. I just turn a blind eye to it and go on with my life.

You still want to try and control powdery mildew? Here are a few home remedies you could try.

Other Diseases

Powdery mildew is probably the most common plant disease, but there are many others. However, the information above applies to almost all foliar diseases. Black spot (Marssonina rosae) attacks only roses, for example; tomato late blight (Phytophthora infestans) only tomatoes (but there is another strain of Phytophthora infestans that infests potatoes), apple scab would be a thing of the past if there were no apple trees… and so on.

Grow Resistant Varieties

At any rate, the single best treatment for leaf diseases is…. to plant resistant varieties. There are powdery mildew-resistant varieties of phlox and bee balm, rust-resistant forms of hollyhock, black spot-resistant roses, late blight-resistant tomatoes, scab-resistant apples… and the list goes on and on. If you’ve had a problem with a plant disease in the past, chances are that there is a disease-resistant variety you could easily substitute.

You’ll be amazed at home many gardening problems you can solve simply by planting the right plants!

Home Remedies for Powdery Mildew


20160122AWhen you see the first symptom of powdery mildew, that is, a white powder covering all of or part of a leaf, as if someone had dusted it with flour, it is already too late to save it. This “powder” isn’t something you can simply rub off. It results from the appearance of sporangia (spore-producing organs) from the fungal disease that has invaded the leaf already. It’s actually the penultimate stage of the disease (in the last step, the leaf blackens and dies!). The disease has already been at work inside the plant’s leaves for days if not weeks by the time the sporangia appear. So any home remedy used on powdery mildew can only prevent it, not cure it.

Of course, the very best “home remedy” against powdery mildew is to plant mildew-resistant varieties and to grow them under optimal conditions. That would include good ventilation and keeping the soil moist during periods of drought (powdery mildew is one of the rare diseases that is more likely to occur when soil is dry rather than moist). On the other hand, if you’re not yet at the point in your evolution as a gardener that you’re ready to yank out plants chronically subject to mildew to replace them by ones that are, here are two home remedies you can try while you’re waiting.

Baking Soda to the Rescue

20160122BMix 5 mL (1 tsp.) of baking soda (found in the pantry) into 1 quart/1 liter of water and spray this solution on plants susceptible to powdery mildew. For the powder to better stick to the leaf, add a few drops of insecticidal soap or dish soap to the solution. I repeat that this treatment only helps prevent the disease and won’t cure it. If you start to the treat the leaves after they already show the typical dusty white appearance indicative of powdery mildew, you may be able to stop the disease from spreading further, but the damaged leaves will not recover.

Milk With That?

You’ve run out of baking soda? Try milk! Yes, milk has also proven to be an effective preventative fungicide against powdery mildew. The recipe? Just add one part milk to nine parts water and spray to saturation. And in case you wondered, skim milk has been found to be just as effective as whole milk.

Using a Weed to Your Advantage


20150718AHorsetail (Equisetum arvense) is a very invasive fern (yes, botanists have recently decided that it is no longer a “fern relative”, but a true fern) that many gardeners must remove regularly from their gardens. But as long as you’re yanking it out, isn’t there something you could do with it?

Of course, it’s not a question of adding living horsetail rhizomes into the compost bin, as they may survive the composting process and then invade your gardens again when you use the compost. However, there is another way you can use horsetail…

It so happens that horsetail is very rich in silica (up to 8% of its dried tissues). In fact, more than any other plant. And silica helps plants to better resist disease. So why not prepare a homemade horsetail spray, useful in helping prevent various fungal diseases such as powdery mildew, black spot of roses, gray mold (Botrytis), mildew and some rusts?

To this end, let 2 ounces (50 grams) of fresh horsetail macerate in 1 quart (1 liter) of water for 24 hours, then boil for 15 to 20 minutes and let cool. Filter to remove the leaves and rhizomes (which can now be safely added to the compost bin).

20150718BAdd 1 cup (250 ml) of the concentrate to 1 quart (1 liter) of water and spray every 2 weeks on plants subject to disease, such as vegetables, phlox, bee balm and roses. Plants are readily able to absorb minerals, including silica, through their foliage.

Alternatively, you can water the plants with the decoction, which will allow them to absorb silica through their roots. This is less efficient than spraying, though, as soil organisms will also take their share of the silica before it reaches the plant roots.

Note that, as with commercial fungicides, horsetail spray can’t cure a disease once the symptoms are visible; at most it can slow its progression. Logically, therefore, it should be applied as a preventative treatment to healthy plants that are prone to fungal diseases.