How to Prevent and Control Powdery Mildew on Apple Trees

While powdery mildews don’t kill their hosts, they sure do a number on them. And that is particularly true for powdery mildew of apples and crabapples, Podosphaera leucotricha.

The apple powdery mildew attacks both cultivated and wild apples and crabapples. And it occurs in the all regions of the world that produce apples!

Close up a limb and leaves of an apple tree covered in powdery mildew.

Powdery mildews get their name from the white spores that are produced by the mycelia (fungal threads). The disease on apples attacks virtually every stage of the plant – buds, blossoms, new shoots, leaves, and fruit.

The disease can be severe enough that no fruit form.

While fungicides are the standard treatments, there are some cultural practices that can help control this disease.

Unlike most fungal infections, apple powdery mildew spores do not require moisture to germinate. Therefore, this infection is known as the “dry weather disease.”

Read on as we at Gardener’s Path walk you through the steps you take to diagnose and treat powdery mildew infection on your apple or crabapple tree.

Symptoms of Primary Infections

If your tree is infected, the first thing you will notice is a delay of up to four days in the opening of the infected buds in the spring. These buds are covered with spores.

Next, the leaves and blossoms become covered with the fungal spores as they emerge from their buds. The spores look like a light gray or white powder, and the infected leaves curl upward.

Close up of an apple leaf with a white coating consistent with a Podosphaera leucotricha fungal infection.

Both sides of the leaves and tree shoots will be covered with this powder.

The flowers develop abnormally, are usually greenish-white, and don’t produce fruit.

An Infection That Won’t Stop

These spores are easily blown by the wind and cause secondary infections on new shoots, leaves, and fruit.

As long as the shoots continue growing, the leaves and shoots can continue to become infected.

The infections typically occur at night at 65 to 80 F when the relative humidity is greater than 70%. While this sounds really high, it is common on the lower leaf surface.

An apple tree branch complete coated in a Podosphaera leucotricha fungal infection showing the characteristic white powder-like coating on the leaves.
A white fuzzy coating on your apple tree blooms, leaves, and branches can mean one thing – the Podosphaera leucotricha fungs, or apple powdery mildew.

The disease on the leaves occurs first on the bottoms and may appear like chlorotic spots on the top of the leaves.

As time passes, the tissues that are infected develop the classic silver-gray powdery mildew appearance.

Fruit that is infected will come down with discoloration and netlike reddish brown colors. It may also be distorted and/or dwarfed.

High levels of powdery mildew at the end of the growing season can damage the tree in two ways. First, it can increase the number of infected buds, so you will have a high level of infection next spring. And second, it can inhibit the formation of flower buds, so that there will be fewer or no fruit produced the following season.

Close up of a human hand holding an apple tree leaf shoot infected powdery mildew (Podosphaera leucotricha) fungus.

And if that isn’t bad enough, a tree that is heavily infected with powdery mildew can become susceptible to additional types of infections.

Cultural Controls

You should prune any shoots that appear white in the early spring, so they won’t spread spores.

A pair of human hands uses a pruning tool to remove branches of an apple tree infected with Podosphaera leucotricha.
Pruning and destroying infected areas is one basic method of control. Don’t forget to disinfect your pruning shears between trees and after you are done for the day.

Be sure and disinfect your pruning shears afterwards, and destroy all the infected plant parts, so the disease won’t spread.

Avoid the excessive use of fertilizer, especially in the late summer. This will prevent succulent new tissue from growing, which is easily infected by the fungus.

Another thing you can do to protect your tree is to plant it in a sunny area, since excessive shade, high humidity, and poor circulation all increase the chances of infection.

Fungicides

Since this is such a difficult disease to control, and it is critical to eliminate the spores that will keep infecting your tree, you may choose to apply fungicides. Be sure and choose ones that are labeled specifically for fruit trees.

You have a choice of low toxicity fungicides like horticultural oils. These include jojoba oil, neem oil, and brand name spray oils designed for fruit trees.

Classic fungicides that are used against apple scab, such as sterol inhibitors, are highly effective at controlling powdery mildew. These include myclobutanil and fenbuconazole.
Since the fungus overwinters inside buds, you should start treating your tree early in the season before the blossoms start to show a pinkish color.

The failure to spray before the blooms opens is one of the most common mistakes made in controlling this disease according to the American Phytopathological Society.

Be sure and repeat the sprays at 2-3 week intervals until the new shoots stop growing. That could mean as many as 18 sprays if your cultivar is highly susceptible!

Also be sure to continue spraying even if the weather becomes dry. Unlike most other foliar pathogens, apple powdery mildew continues its growth and spore production in dry weather.

Faithfully applying the fungicides will reduce the need for future applications.

Prevention

You can also spray sulfur fungicides before the symptoms appear.

Be careful with sulfur. You can harm the plant if you apply it within two weeks of a fungicide or if the temperatures are greater than 90 F.

This class of fungicides includes the classic Bordeaux mixture of copper sulfate and lime. This combination is highly effective at preventing powdery mildew and is certified organic. You can buy a pre-packaged mixture designed for small gardens.

Rows of an apple tree orchard with trunks painted with a Bordeaux mixture.
These apple trees have had their trunk coated with Bordeaux mixture, a copper sulfate and calcium oxide in water. This helps to prevent a powdery mildew infection. Though fairly toxic, this method of prevention is considered organic.

The ultimate prevention technique is to plant resistant varieties! Some of the most popular cultivars are the most susceptible – Granny Smith, Jonathan, and Rome for example.

Some of the more common resistant cultivars include:

  • Braeburn
  • Britegold
  • Delicious
  • Enterprise
  • Fuji
  • Gala
  • Jonafree
  • Nittany
  • Winesap

Apple Crop Risk

Although apple powdery mildew will not technically kill your crop, it can debilitate the tree to such an extent that it could be unable to produce any fruit.

This disease is widespread on wild and cultivated apples and crabapples in every part of the world in which they are grown.

Therefore, it is critical to know the symptoms of this mildew, so you can be ready to take action as soon as you see infected tissue.

You will need to spray with some sort of fungicide – sulfur, horticultural oils, or sterol inhibiting fungicides. A strict spray schedule may help save your tree from this aggressive pathogen.

Have you successfully fought off powdery mildew on your apple or crabapple tree? Let us know how your battle went in the comments.

And if you’re still trying to identify a specific disease on your apple tree(s) then some of these guides may be of assistance:

© Ask the Experts, LLC. ALL RIGHTS RESERVED. See our TOS for more details. Uncredited photos: Shutterstock.

About Helga George, PhD

One of Helga George’s greatest childhood joys was reading about rare and greenhouse plants that would not grow in Delaware. Now that she lives near Santa Barbara, California, she is delighted that many of these grow right outside! Fascinated by the childhood discovery that plants make chemicals to defend themselves, Helga embarked on further academic study and obtained two degrees, studying plant diseases as a plant pathology major. She holds a BS in agriculture from Cornell University, and an MS from the University of Massachusetts Amherst. Helga then returned to Cornell to obtain a PhD, studying one of the model systems of plant defense. She transitioned to full-time writing in 2009.

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Jaye Finn
Jaye Finn (@guest_6600)
4 months ago

Seems my tree has this, it’s approx 5 year old, I’m unsure of the variety, aldi bought. I’m going to see I can create a natural fungicide to tree with. I’m going to prune the end of September and I will give the first treatment then. In spring when it starts budding, I will start treatment again. Hopefully it works, I will try a different method in the next year if the natural one fails.

dbts808
dbts808 (@guest_6641)
Reply to  Helga George, PhD
4 months ago

Hi,
I saw that you posted this just 16 hours ago.
I’m presently treating my one apple tree with neem oil.
You say this works?
Just spay-mist it, even the top branches?
any advice?
Thanks;
Dave

Sue
Sue (@guest_6995)
4 months ago

My dwarf apple tree… The lower leaves I’ve noticed have a tiny pattern of white spots. It’s not powder, it’s in the leaf.

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MLT
MLT (@guest_7086)
4 months ago

Hi Helga, I just saw my apple tree leaves and little fruits have the white fungus today. The leaves and little apples look dred out and not healthy. How can I use the neem oil? Do I have to mix it with water? How much water and oil? Please let me know ASAO, so I can start tomorrow to spray it. Tomorrow will be raining. Is it okay to spray? I cut up lots of curly leaves today. Some little fruits are still good. It is a 4-variety apple tree in the pot. I have had it for 10 years.… Read more »

Ann lee
Ann lee (@guest_7154)
4 months ago

Hi, same problem with my apple tree. I cut away loads of leaves, looks like I have to denude the tree to remove all the mildew. Could I avoid this by just using neem oil? Thanking you in advance. I live in Ireland.

Ann lee
Ann lee (@guest_7194)
Reply to  Helga George, PhD
4 months ago

Thank you so much. Garden centres opening on Monday, after lockdown. Take care.

Ann lee
Ann lee (@guest_7196)
Reply to  Ann lee
4 months ago

I’m just in from cutting more leaves off the tree. Should I be gowned up like for corona? We are all so stressed now 😒😒

Ann lee
Ann lee (@guest_7220)
Reply to  Helga George, PhD
4 months ago

Thank you so much Helga, have a lovely Sunday

Aroush
Aroush (@guest_7913)
Reply to  Ann lee
3 months ago

Hi,
My apple tree has same problem White powdery fungus, I am so upset for my tree. I cant go outside in my garden bcoz of hayfever, Please guide me

Sophia
Sophia (@guest_7387)
4 months ago

Hi Helga… please can you help? My little apple is showing signs of mildew. If I spray with neem oil, will it affect the fruit in any way?

Ammon Bundy
Ammon Bundy (@guest_7923)
3 months ago

I just identified that my orchard of 240 apple trees has Powdery Mildew. Many of the trees have fruit on them and the fruit looks okay. Every tree however has the mildew. Many of the new shoots from this season seem to be dead or dying even though there is still good growth and the trees over all look okay. The question I have is, what should I do now? It is early June and the weather is in the 80-90s and will soon be consitently in the 90s. We are in the drier part of Idaho. What treatment should… Read more »

Jeff
Jeff (@guest_8024)
3 months ago

I have a rose bush with powdery mildew about eight feet away from two paradise apple trees. Is there any chance the rose bush mildew will infect my apple trees? Thank you.

Allison Sidhu
Allison Sidhu (@allison-sidhu)
Admin
Noble Member
Reply to  Jeff
3 months ago

Unfortunately this disease can spread easily in ideal conditions, especially if left untreated. See our article on rose diseases for more info.

Melissa
Melissa (@guest_8546)
2 months ago

Hello, my apple tree is 2 years old and started showing signs of powdery mildew a few months ago. I didn’t realize it until a month ago and started spraying with neem. It doesn’t have any flowers or buds yet, just leaves. Should I just spray it, or should I pick off infested leaves and trim the stem/super baby trunk down below where I can see the mildew starts? I’m not sure how much would be too aggressive for such a young tree. Thank you!

Ailwyn Gregory
Ailwyn Gregory (@guest_9048)
1 month ago

Hi, My 11-year-old daughter planted an apple seed as an experiment during lockdown, and it is doing really well. Started to notice this powdery stuff on the leaves… how can I treat this please? It’s approx. 2-3 months old and has been kept in the house. Regards

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Rachel
Rachel (@guest_9931)
4 days ago

Hi Helga, Thank you for this great article. My two apple trees have powdery mildew. I have 4 questions. 1) If all the leaves appear to have the mildew, will it be okay for me to remove all of the leaves, so that only bare stems are left? 2) Should I spray the neem oil all over the branches, stems and open cuts where I’ve trimmed the trees down for autumn? 3) The article says copper sulfate and lime works to ‘prevent’ powdery mildew. Would it still help to apply even though I already have powdery mildew? And should I… Read more »

Rachel
Rachel (@guest_10000)
Reply to  Helga George, PhD
2 days ago

Thank you so so much for your great reply. V much appreciate it. Have a lovely evening. Rachel