I see powdery mildew on maple leaves everywhere on my nature walks. In Oregon, we have bigleaf maple trees that grow wild across the state. Pair abundant maple foliage with wet, humid weather, and you get mildew growth explosions! 

Though the white, powdery growth is unsightly, it rarely kills maple trees. Most maple species lose their leaves in the fall, and they sprout new ones in spring. The new foliage grows mildew-free despite the infection from the previous year. 

The worst the powdery mildew (PM) will do is make your tree look unsightly. It often occurs late in the growing season as summer turns into fall. Because it rarely spreads in spring and summer, you may not have to worry about it too much.

Powdery mildew on maple leaves is easy to prevent but difficult to control. Once it arrives, it’ll spread rapidly. It uses the wind to push its airborne spores onto maple tree leaves. Once there, they germinate and grow, creating large patches that work like spore factories.

Mercury 1L Trigger Sprayer

Mercury 1L Trigger Sprayer

Trim Infected Leaves

Cut back affected leaves to prevent spread.

Trimming the infected leaves is a great way to start tackling powdery mildew on maple trees. Any patches will act as spore banks, spreading the spores throughout the tree. Continuous upkeep helps control the outbreaks when they’re small in size.

Obviously, you can’t remove every single leaf. Trim the infected ones when the patches arrive, and watch diligently to see if more appear. Remove no more than a third of the tree’s total leaves.

Use care when trimming young saplings and fresh transplants. They can’t tolerate excessive pruning, and they need the energy from their leaves to survive through the winter. Trim them sparingly, and ensure they’re pruned properly so they receive good airflow during the year.

Wet the Leaves (Or Don’t)

Water sometimes helps, but sometimes it can be more harmful.

Water works in an interesting way with PM. It washes the spores off patches to render them useless, but moves spores to the ground where they proliferate. It also boosts the humidity levels around the mildew. This fungal condition thrives with warmth and humidity, and excess water evaporates to help the disease spread. 

In areas like the Pacific Northwest, where PM is abundant, consider watering with a different method than overhead irrigation. Overhead sprinklers are easy to set up and use, but they wet the leaves in the process. 

Instead of overhead sprinklers, use drip irrigation, hand watering, or olla pots. Get creative! Olla pots are unique tools for watering, as they release moisture slowly when the plants need it. Bury the pots in the ground, fill them with water, and cover them with a lid. They’ll wet the soil instead of the maple leaves.

Water in the Morning

The time you water has an impact on garden health.

Watering the leaves during midday can wash the spores off and prevent them from spreading to the rest of the tree, but it will wash spores onto the soil below. Wetting the leaves in the early morning, afternoon, or at night can speed up the fungus’s spread. The best time to water is mid-morning, before the sun rises high in the sky. 

Water in the morning, and the moisture will seep into the soil to reach the plants’ roots. The rising sun will cause the moisture to evaporate quickly. The quick evaporation is key in controlling the mildew’s spread. 

As the fall season approaches, set your sprinklers to water less often. Cool temperatures allow the water to sit in the soil for longer, and returning rains naturally wet the ground. You’ll need to irrigate way less in the fall than during spring or summer.

Stay Away from Fungicides

Mildew can quickly become resistant to fungicides.

Your first impulse may be to grab a fungicide spray that’ll kill powdery mildew on maple trees. You’ll be surprised to hear there aren’t many options that work, at least for us home gardeners. Farmers may use toxic synthetic fungicides to protect their crops, but those products are incredibly harmful and, in some states, illegal to use at home.

Fungicides also lead to PM resistance. The fungi will evolve to resist the spray, and you’ll need a different, more potent one in the future. Continuous usage creates super-strains that spread out of control. 

There are some organic products listed for home garden usage. However, these products don’t kill PM. They help control its spread. You’re better off not using them at all if the disease is already widespread. 

If the patches are small, you may use potassium bicarbonate, neem oil, or milk spray on them. These treatment solutions kill the spores, though they don’t kill the mildew once it’s growing on the maple leaves. Consider using preventative measures instead of spraying fungicides annually. 

Clean Up Fall Debris

Spores hide in debris and spread around the garden.

This powdery fungus overwinters in fallen maple debris. The leaves house the overwintering fungal structures, and they release new spores the next year. These spores waft in the air and land on healthy new maple tree leaves. 

Cleaning up the fallen leaves is the best way to control powdery mildew on maple leaves in your garden. It’ll prevent widespread outbreaks next year, and you’ll see infections much later in the growing season. 

Don’t throw away those leaves! Use them instead. Hot compost them to at least 140°F (60°C) to kill the mildew, or bury them deep underground away from the maple. Consider giving them to a city composting program if you lack the space to let them decay at home.

Don’t Overfertilize

Overfeeding makes maple more vulnerable to powdery mildew.

Fertilizers, especially those rich in nitrogen, lead to rapid root and shoot development. Too much nitrogen causes leaf development at the expense of roots, flowers, and fruits. This isn’t good when trying to get rid of PM, as cramped conditions lead to its rapid spread.  

Avoid overfertilizing at all costs, and consider using compost instead of fertilizer. Repeat additions of compost feed the soil, which in turn feeds your plants. Compost decays slowly and feeds the plants at a controlled rate. 

Or, use a soil test to see how fertile or poor the dirt is. Once you know the nutrient levels, you’ll be able to fertilize the proper amounts and avoid excessive leafy growth. 

Space Out Plants

Airflow is crucial to preventing the spread of disease.

To reduce cramped conditions, space out your maple trees properly when transplanting them. Consider their mature size, not how big they are now. Many maples reach impressive heights, some over 100 feet tall! 

If it’s fall, it’s the perfect time for transplanting. Move young maple saplings now if they’re too close together. Dig them up, transplant them, and water them well. 

If the trees are mature, try pruning them to improve air circulation. Remove dead, damaged, or diseased wood, and take out criss-crossing or awkwardly formed branches. 

Deal With It

Powdery mildew on maple leaves is not the end of the world.

In my area, we deal with powdery mildew on maple trees. Preventative measures prevent the fungus from spreading, but there is little to be done once it arrives. It is a fact of gardening life, especially if you live in a wet, humid area that’s warm in late summer.

Mitigate the damage, plant PM-resistant maples, and consider living with the disease. It’s inevitable, and it’s less harmful to maples than it is to crops like squashes and grapes. Your tree won’t die. It’ll rebound in the spring!