Fungal diseases are one of the most common leaf problems in hydrangeas. Among them, powdery mildew often makes an appearance in high humidity or when warm days are followed by cool nights.
While treatment options are many, prevention is where you should try to focus most of your energy in combating this fungal disease.
Creating an environment that’s inhospitable for fungal diseases is the best line of defence against this foliar disease.
To better understand the mechanism of action of this fungal ailment, I’m going to cover the most important things you need to know about powdery mildew including how it gets on your hydrangeas, how to treat the disease, and how to keep it from coming back.
What is Powdery Mildew?
Powdery mildew causes a white-grey, powdery substance to form on the surface of the leaves. You may also notice a cottony substance growing on the bottom of the leaves.
Small, gray lesions caused by the web of fungus can also appear on the upper part of the leaves. Yellowish-green blotches or leaves turning purple are also signs of this foliage disease.
There are several fungi that are responsible for the emergence of powdery mildew including Microsphaera friesii, Oidium hotensiae, Golovinomyces orontii, and Erysiphe poeltii.
Of all the hydrangea varieties grown in the United States, powdery mildew affects big leaf hydrangeas the most.
Left untreated, powdery mildew can cause any or all of the following issues:
- Stunted growth of buds and leaves
- Premature defoliation
- Reduced leaf production or smaller leaves
- Reduced bloom production or smaller blooms
- Bloom malformation
- Elongated shoots
Although the fungus seems to grow primarily on the surface of the leaves, causing a white powder coating, it does sink root-like structures into the leaves to feed off its nutrients, causing lesions and further damage.
What may at first seem like an aesthetical issue that affects the leaves, left untreated, powdery mildew can take over your hydrangea and cause any of the above problems, including complete premature foliage loss.
How Powdery Mildew Gets on Hydrangeas?
Fungal diseases are easily spread via spores from one plant to the other, and from one branch to the other within the same shrub or plant.
The reason why it’s difficult to deal with powdery mildew is that it can easily get on your hydrangeas by way of winds. They can overwinter on your plants and emerge when conditions are favorable.
Unfortunately, environmental conditions that favor the host plant also favor the pathogen too. High humidity, lack of air circulation and heavy shade are especially hospitable for powdery mildew.
Your hydrangeas can come with powdery mildew spores on their leaves from the nursery or greenhouse, you can transfer the spores when pruning the shrubs without cleaning the blades first.
Even raindrops can spread the spores from one branch to the other. Therefore, powdery mildew is widespread, and it can easily get on your hydrangeas.
The key is to create an inhospitable ground for them so that they cannot survive on your hydrangeas.
Before I cover some of the best prevention methods for powdery mildew, I’m going to discuss some of the treatment options available in case your hydrangeas are already affected by this foliar problem.
Dealing with Powdery Mildew on Hydrangeas
Once you notice the symptoms of powdery mildew on your hydrangeas (white powder coating, or small, gray lesions), there are a few treatment options that you can try.
Because of the resistance of some fungal strains to fungicides, there may be no guarantee that only one treatment method will eradicate the problem.
It’s also the reason why there’s a long list of chemicals designed to treat powdery mildew on hydrangeas.
But there are also organic methods, and more recently, biological control methods as well, which use various strains of Bacillus subtilis to fight fungal problems.
Organic treatment options for powdery mildew include neem oil spray, which is an organic fungicide that you can use to control and prevent the spread of powdery mildew.
One semi-organic method is to use a mixture of dish soap (a few drops) combined with baking soda (half a tablespoon), and half a gallon of water. Thoroughly coat the leaves with the mixture.
Biological methods include Bayer Advanced Natria Disease Control RTU and the Cease or Rhapsody fungicides.
Be advised that regardless of the method you choose, repeat applications are needed. Use chemical fungicides only as a last resort as they can have adverse effects on pollinators and even the foliage of the hydrangea shrub itself.
Naturally, you can cut away leaves and stems to prevent further stems or even entire sections of the plant if the problem is severe enough.
Can Powdery Mildew Kill Hydrangea Plant?
Powdery mildew doesn’t kill hydrangeas as a rule. But it can weaken its defenses to the point where the plant may be unable to battle other diseases or other fungal attacks.
Not to mention that it can destroy foliage, buds and blooms, causing the plant to grow smaller and weaker.
Therefore, while it may not kill your hydrangea, powdery mildew can certainly weaken the resilience of hydrangeas.
Prevent Powdery Mildew on Hydrangeas
I mentioned that prevention is where you should focus your efforts to combat powdery mildew on hydrangeas.
Here are some of the best ways to make it difficult for fungal diseases to take a hold of your shrubs:
- If you’re growing hydrangeas in a greenhouse, increase aeration and reduce humidity
- Don’t plant hydrangeas in heavy shade; plant them in a location with a couple of hours of full sun per day
- Don’t water hydrangeas from above, only at the base
- Don’t prune hydrangeas with unclean blades; clean the blades even between snips on the same shrub
- Allow hydrangeas a reasonable distance from one another when planting, so they’re not overcrowded
- Clean debris at the base of the plant, spent blooms, and dead or diseased canes to improve the aeration of the shrub
Your aim should be to reduce moisture on the leaves, keep the plant away from heavy shade, and increase aeration around the shrub.
Powdery mildew is a tricky fungal disease that can be caused by multiple fungal strains, some of which are resistant to chemical fungicides.
Prevention works best, but if you’re faced with the disease, removing affected sections of the plant and applying organic and biological treatment methods religiously should clear up the problem.