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Powdery mildew is a common disease on many plants, and rhododendrons are no exception but there is one big difference. On Rhododendrons, the white mildew usually associated with powdery mildew often doesn’t develop like on other plants making it sometimes harder to identify. However, the leaves usually develop a yellowish-green or purplish-brown spots or blotches on the surface of the leaves. The good news is that it isn’t necessarily fatal to your plant and can be treated. If you have drooping leaves with brown leaves this could be leaf scorch which I have covered here.
A fungal disease
Powdery mildew is a fungal disease that affects the leaves of plants. This mildew spreads through the air so can travel easily throughout your garden. It’s quite easy for you and your plants to live with this disease as it just makes the leaves a bit unsightly.
However, it can spread quite viciously if your rhododendrons are in a really shaded place or are in a relatively damp location. The problem is rhododendrons love shady positions which are often a little damp. That being said, I have found that if you plant rhododendrons in their favoured location of partial shade, sheltered from wind, and well-drained soil, any powdery mildew infections should remain mild.
How to spot powdery mildew
Powdery mildew manifests as a fine white powder on the plant’s leaves, particularly the upper side. If your rhododendron is an evergreen, you may first notice patches of light green, yellow, brown or purple on the leaves. The powdery dust may also be on the stems and flowers of your plants. The discolouration may be more noticeable than the powder itself as explained earlier.
What to do
If you notice just a light coating of the white powder, you can just keep checking to make sure it’s not getting worst. Trying to remove the powder may result in the fungal spores becoming airborne and spreading throughout the plant and beyond. The same is true if you use fungicide on the leaves. Apply the fungicide very carefully so as not to distribute the powder throughout the plant.
It’s up to you whether you remove the affected leaves. Doing so might leave your rhododendron bare in places and looking a bit sad. Powdery mildew is a disease that the plant can live with, though its health may be a little compromised. However, burning the infected leaves which fall to the ground will stop the spores from spreading.
Prevention of powdery mildew
You can reduce the chances of powdery mildew by planting your rhododendron in the appropriate location. Prune the shrubs to give them an open structure for the air to circulate freely through. (See When and how to prune rhododendrons for information as to how to do this.)
Don’t water the rhododendrons from overhead early in the morning or in the evening. However, watering from overhead in the mid-morning in dry weather works to reduce the mildew as it doesn’t like to be wet.