Plant Doctor Archive

Powdery mildew on rhodo

The leaves of our rhododendron are not looking good. They have gone white, as if they have powdery mildew. What should I do?

SOME varieties of rhododendrons and azaleas are more susceptible to powdery mildew than others. I have heard yellow ones tend to be worse than others, but how badly any variety is affected will depend on your local weather conditions.

The disease is usually at its worst during warm, humid, summer weather. You can't control the weather, so the next best thing is to spray your plant with a fungicide, such as Super Sulphur, Bravo or Fungus & Mildew Spray.

Ideally spray in spring to protect new growth before disease symptoms appear, then again later in the season when conditions favour the disease, and in autumn to reduce the number of disease spores which can overwinter to cause problems again the following season. Mix the spray according to the label recommendations.

Some gardeners have had success reducing mildew problems by thoroughly spraying the foliage with a solution of liquid seaweed fertiliser, so that could be worth a try, too.

Reproduced with permission from the former Weekend Gardener magazine. The views expressed here are not necessarily those of the RNZIH.