Answer: Veronica spicata Royal Candles, or Dwarf Spike Speedwell, grows best in full sunshine and can develop a fungal disease called powdery mildew if it is crowded, in too much shade, or otherwise stressed. The white patches may be powdery mildew. Try pinching off the affected foliage to stop the spread of the disease. You can use a fungicide to help protect the healthy foliage. Lots of sun, good air circulation around each plant and adequate watering (of the soil, not the leaves) will help your royal candles avoid becoming diseased.
Best wishes with your garden!
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