Plant Doctor Archive

Powdery mildew on oaks

CAN you tell me what's wrong with my young oak trees? They started off healthy in spring, but now all the leaves have a white coating and they've stopped growing.

POWDERY mildew is the culprit. There are different powdery mildews - some attack a range of plants, while others only affect one or two particular species.

The problem usually starts in warm humid weather in spring or early summer and continues to get worse through the rest of summer. When the autumn rains arrive it's usually less of a problem, but by then the damage is done, with the affected leaves often falling off early.

You can't do much to change the weather or stop the fungal spores drifting in on the wind, but the disease won't kill your young trees - although it might slow them down a bit.

If you do want to take some action, you could spray with a fungicide such as Fungus Fighter or Fungus & Mildew Spray. Or you could try a weekly spray of one teaspoonful of baking soda dissolved in one litre of water with a couple of drops of dishwashing liquid added, or organic sprays such as Nature's Way Fungus Spray or Super Sulphur.

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