My newly planted groundcover rose “The Fairy” has been attacked by powdery mildew. I’m finding a few newly planted shrubs having the same problem. The likely cause is the very hot and dry weather we had in May, just as the shrubs were getting settled and expanding their root area. In general, powdery mildew attacks plants that are under stress and can be caused by lack of water as easily as by too much water. The treatment that seems to be most effective is spraying with sulphur, either as a water spray (1 tbsp sulphur/litre water) well shaken, since it doesn’t dissolve, or sprinkling sulphur powder directly on the leaves. And trying to keep the plants as happy as possible otherwise. In general, when a plant is under stress or fighting some disease it is not a good idea to fertilize. If however the plant is actively growing, but showing signs of deficiency, like pale leaves with darker veining, a mild fertilizer like seaweed feed can be beneficial. With any plant problem, the first step is to examine the plant and soil carefully to see what the cause is, and then to remedy that. With the mildew, the sulphur treatment is remarkably effective, although it will need to be re-applied after rain, or if the symptoms return. Powdery mildew appears as a white or greyish blotching on the leaves; new leaves will appear twisted. After sulphur treatment the affected areas can turn brown because of the damage done by the mildew to the leaf. Generally a plant will outgrow this.
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