Gardening Plant diseases

Late-Season Powdery Mildew: Not Such a Big Problem!

Question

My magnificent maple has had a fungus problem each fall for the last 3 years. I’ve asked all sorts of people about it and received all sorts of answers. Do you have any suggestions?

Terry Reeves

Answer

Yes. I suggest just ignoring it.

It’s simply powdery mildew, a common foliage disease that causes little harm overall. Yet it would be nearly impossible to eradicate.

PW is easy enough to recognize, at least once it has taken hold (in its earliest stages, it works pretty much sight unseen). Leaves and stems and possibly fruits become coated with white powder, as someone has sifted flour over them. You can usually rub it off at this point, although it will probably grow back.

Late-season powdery mildew tends to be pretty much innocuous because the plant has largely finished its job for the season. So, what if it stores away a smidgen fewer minerals at a season when it usually has a surplus? I suspect most country folk have seen fields of pumpkins maturing near Halloween with their fall leaves severely damaged by powdery mildew, but their fruits still in perfect shape. That’s just typical fall behavior for so many plants. In such cases, PM is just a disease of senescence: of aging. Nothing too harmful.

More About Powdery Mildews

Yes, I pluralized mildew here because it isn’t just one disease, but many, probably even thousands of diseases. There is a huge range of different powdery mildews in the fungal genera Erysiphe, Microsphaera, Phyllactinia, Podosphaera, Sawadaea, Sphaerotheca, Uncinula and others. Each genus has several to many species and each species has several to many varieties and each variety as several to many strains. It’s certainly not easy to put a proper Latin name on a powdery mildew!

So, the mildew found on your lilac (Syringa vulgaris) is not the same as the one which appears on your squash (Cucurbita pepo) and even less the strain currently attacking your red-leaved Norway maple (Acer platanoides). In fact, the strain found on one squash might not even the same as the one found on another squash!

Wind Is the Culprit in Spreading Powdery Mildew

What these various fungi have in common is wind transmission. Their tiny spores are carried everywhere by gusts of wind, normally in hot and dry weather. Unlike many diseases carried in this way, powdery mildew spores do not need a moist surface to attach to. However, they do need humid air once they are established.

After the fungus has found a compatible host, it directs small suckers (haustoria) into the upper leaf cells to begin feeding, stealing energy from the plant. They’re pretty much invisible at this point, but the final stage is more visible.

Indeed, when the nights become cooler and atmospheric humidity starts to become really high, usually towards the end of summer or in the fall when there is a lot of dew, the famous white powder starts to form on top of the leaf. In fact, it’s not a powder, but rather a filamentous white mycelium (like a fungus root system).

Even later in the season, the fungi also form small black fruiting bodies which will produce overwintering spores. The disease can overwinter on the plant, on fallen leaves, on tools, in the soil or elsewhere.

At the end of the season, the damaged leaf often turns yellow or brown and dries up, at least partly.

Why Powdery Mildew Treatments Fail

Prevention on a large tree is almost impossible. How can you expect to reach the highest branches with a fungal spray? You can still spray smaller plants with fungicides, even homemade fungicides – sulfur, milk, neem, baking soda, etc. – and thus prevent the worst. But you never, ever get them all.

Back Up 15 Paces!

Ignoring a shrub with a problem because it is more than 15 paces away.
Do you need to treat? Apply the 15 pace rule to see!

However, the really important question is whether it was even necessary to react. With minor cases like these, I suggest invoking the 15-pace rule.

Back up 15 paces. Can you see the problem? If so, a treatment might be worthwhile. But if you can’t, you’d probably do just as well to ignore it. In nature, nothing is perfect and a bit of fungus doesn’t really cause much of a problem. And when there’s an entire tree that needs treating, the chances of succeeding are slim indeed.

Save your efforts for edibles, like vegetables, herbs and fruits. It’s worth fighting for the food you eat! In the case of ornamentals, “late-season powdery mildew” is usually just a minor problem, variable in its effect from year to year, but doing no particular harm to its host. You can even, as in your case, see it as creating an ornamental effect! White powder on a deep red leaf: some people would go for that.

Another thought? Why not consider late season powdery mildew as just step one in the process of leaf decomposition? After the leaf has undergone a bit of PM decomposition, it will free up its minerals more rapidly. And the decomposing leaf is likely to “rot on the spot” rather than be carried away by the wind. In such a situation, having a bit of powdery mildew thus would be beneficial to the mother plant.

A Pound of Prevention

To the extent that powdery mildew can be controlled, prevention is far more like to be successful than stopping and reversing the disease.  And the very best prevention is to grow plants that are naturally resistant to powdery mildew.

Garden phlox David, with white flowers, is resistant to powdery mildew.
‘David’ garden phlox (Phlox paniculata ‘David’) was discovered in 1987, the first garden phlox recognized as resistant to powdery mildew. Since then, specialists have crossed it with other phloxes and created new mildew-resistant strains. Now there are over 100 mildew-resistant cultivars available in plant nurseries. Why would you grow anything else? Photo: David J. Stang, Wikimedia Commons

So, go through plant and seed catalogs and look for resistant varieties, especially if you had a problem the previous year. For vegetables especially, but also many annual and perennial flowers, there is now a very good chance there are resistant varieties.

On the other hand, if you have planted a naturally susceptible tree, it’s unlikely that you will want to uproot it and replace it with a more resistant cultivar! So, it’s especially worthwhile doing a little research before buying long-lived plants. You won’t want to start out with a problem plant that will live 85 years!

Tips to Prevent the Worst Powdery Mildew Problems

Once the disease is visible, it will be too late to erase it. But there are things you can do to prevent it in the first place slow its progress … and to slow or stop its progression in future years. Here are a few:

A solution of baking soda and water to treat powdery mildew.
You can help prevent powdery mildew with a solution of baking soda and water. Photo jordache, depositphotos

• To prevent or to treat a PM infestation in progress, try baking soda spray: mix 1 teaspoon (5 ml) of baking soda and 2 or 3 drops of mild soap (insecticidal soap) in 1 quart (1 liter) of water and spray this solution every 2 weeks on the disease-prone foliage and stems.

• Improve air circulation.

• Sterilize pruning tools with 70% alcohol.

• Remove badly affected leaves.

• Water in the morning so the foliage has time to dry out. Above all, avoid watering at the end of the day or in the evening so the air won’t be humid all night.

• Water the soil, not the foliage. For that reason, a soaker hose or drip irrigation system would come in handy.

• Avoid high nitrogen fertilizers. That is, those with the highest first digit, such as a 15-10-10 fertilizer.

• Find and plant powdery mildew resistant varieties.

What to Do with Leaves Contaminated with Powdery Mildew

Leaves showing symptoms of powdery mildew added to compost bin.
Do leaves showing symptoms of powdery mildew have a place in the compost bin? Opinions vary. Photo: depositphotos

It’s fall. So, soon the leaves with powdery mildew will lying be on the ground and starting to decompose. Should you avoid putting them in the compost bin?

That leads to a quite a controversy. You’ll find experts that say you should use them and others that say you shouldn’t. What’s a home gardener to do when even experts don’t agree?

Well, I tell you what I do. I put affected leaves in the compost bin.

My reasoning is simple enough. That’s what Mother Nature does!

Spores of powdery mildew are already abundantly present everywhere in our environment. Not using leaves because they might carry a few spores is like closing the barn door after the horse is gone.

Using the common lilac as an example, the value of the compost made using (in part) lilac leaves and that I’ll be applying mostly on my vegetables and flower beds (and not on my lilacs!) is far greater to me than the risk that a spore of powdery mildew specifically adapted to my lilac accidentally getting into the mix. And possibly contaminating my lilac yet again. And which really does my lilac no great harm.

And my lilac is even more likely to pick up the disease again from another source than my own compost. After all, the spores would somehow have to be picked up by the wind from the compost buried in the vegetable garden and carried to the lilac’s leaves. I find that rather unlikely.

I Just Don’t Care

Graying lilac leaves show symptoms of powdery mildew.
If my lilac leaves turn a bit grayish in the fall, thats no skin off my back! Photo: Famartin, Wikimedia Commons

Plus, I really don’t care if my lilac picks up powdery mildew. It will just look a bit grayer than usual and I can live with that. It’s just not enough of a problem to worry about!

As for powdery mildew carried from generation to generation in my vegetables … well, I’m very careful to choose resistant varieties of whatever vegetables are involved. Any vegetable cultivar that gets PM is automatically banned from my garden for future use. And I do carry out 4-year crop rotation for my vegetables.

To be honest, I never have anything but the slightest whitening of vegetable leaves at the end of the season due to mildew since I became tough on PM. And certainly, there has been no drop in productivity.

Do as You Wish

Of course, if you are very concerned about your homemade compost carrying powdery mildew spores, by all means collect diseased leaves and let your municipality take care of decomposing them properly. That’s your choice!

But I choose to make my own compost.


Powdery mildew: yes, it’s an annoying plant pathogen, but not always anywhere as harmful as people think. At least, not if you put a smidgen of effort into prevention. And remember the 15-pace rule. It’s up to you to decide if it is worth reacting to its presence or not.

3 comments on “Late-Season Powdery Mildew: Not Such a Big Problem!

  1. I have an older variety of phlox and do have PM every year in the beds it is in, although not as severe when I treat it as suggested above. I have dug it out of some of the flower beds, but it is so persistent that random rogue shoots seem to still grow. I have a bed of beautiful 6′ high yellow coneflowers (similar to echinacea). A few phlox managed to survive my purging and after about 5 years of glorious deep green leaves on robust yellow flowers, this year the echinacea have a pretty heavy case of powdery mildew, as do the rogue phlox. They are near the sidewalk and not so lovely to view as they fail the 15 pace test by a long shot. It is disheartening as the summer has been dry and it is a ventilated area. I always enjoy your articles, although I don’t think I can be too laidback about this particular flowerbed :).

  2. Excellent advice.

  3. I have a lot of older variety phlox. They are beautiful, and the pollinators love them. I’ve noticed the plants on the outside get PM and most of the others don’t. I’d prefer none of them get it, but I’m not going to do anything about it. 🙂

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