By: Amy Grant
Hydrangeas are flowering shrubs that produce big, showy blooms in the summer, making them a much sought-after addition to the landscape. Gorgeous they may be, unless you have a hydrangea with powdery mildew. The resultant powdery substance on hydrangeas afflicted with the disease renders them less than lovely.
So, what causes hydrangea powdery mildew? Is there a powdery mildew hydrangea treatment? Fear not, read on to learn about treating powdery mildew on hydrangeas.
A powdery substance on the plant is the number one symptom of a hydrangea with powdery mildew. This foliar disease may be caused by several pathogens: Golovinomyces orontii (formerly Erysiphe polygoni), Erysiphe poeltii, Microsphaera friesii, and Oidium hotensiae are most prevalent on the big leaf hydrangeas.
Conditions that hydrangea favors also favor the pathogens – high humidity in heavy shade. The pathogen overwinters and then warm days combined with cool nights signal a growing period.
The powdery substance on hydrangeas begins as small and fuzzy gray lesions on the upper surface of the foliage. These fuzzy lesions are made up of a web of fungal hyphae. The disease is easily spread by air currents and direct contact with other plants. As the disease progresses, plant growth slows or ceases, and flowering is reduced.
Powdery Mildew Hydrangea Treatment
High humidity in greenhouse settings fosters hydrangea powdery mildew so monitor the humidity and keep it low.
In the landscape, allow plenty of space between plantings to provide good air circulation and if need be move plants to a sunnier area. Also, remove plant debris that can harbor this and other foliar diseases. When possible, plant resistant cultivars. The big leaf varieties of hydrangea tend to be more susceptible to hydrangea powdery mildew. Oakleaf varieties show more resistance.
When all else fails, treating powdery mildew on hydrangeas may require chemical control. There are a few options available, but none seem to be entirely capable of completely eradicating the disease.
Another option for treating powdery mildew is by making your own organic treatment. Combine two to three drops of dish soap with ½ tablespoon of baking soda and ½ gallon (about 2 L.) of water. This will change the pH and prevent the mildew from forming or spreading. Mist leaves with the solution.
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Only downy mildew, which affects the hydrangea more often, can be prevented. Keep the leaves dry by always watering the hydrangea from below. Plant strengtheners based on algae, which you can get in specialist shops, can effectively prevent an infestation.
If a plant has been allowed to dry out, the first step to recovery is to thoroughly water the entire root ball. If it is still in a pot, try setting the plant in a tub of water up to the rim until the dry roots have been completely soaked. Then allow the hydrangea to drain freely. At his point, one’s instinct is to continue watering the plant to excess. However, it must be remembered that the roots are damaged, and they will be unable to deal with soil that it is too wet. So, allow time between each watering for the soil to dry out slightly and for the roots to regenerate.
Remove the diseased leaves and apply a fungicide that is recommended for cercospora leaf spot (following the manufacturer’s application directions). Look for products that contain the one of the following active ingredients: azoxystrobin, chlorothalonil, mancozeb, myclobutanil, and thiophanate-methyl. These ingredients are used in curative systemic fungicide products. The fungicide should be applied as soon as you notice the disease. You may need to do several applications of the fungicide.