The zinnias I planted were doing very well but have developed some white dusty leaves and aren’t looking as healthy. What am I doing wrong? Over-watering? Is this mould?
Likely this is powdery mildew, a mold or fungus that can be encouraged by over-enthusiastic watering.
Powdery mildew won’t kill the plant, just makes the leaves look nasty and can weaken the plant. Remove the diseased leaves and dispose of them in the garbage (not the compost, as the fungal spores can survive and spread disease).
Check with your local nursery for a fungicide to combat powdery mildew. This should be applied as soon as you see any sign of the fungus, so using it now that the fungus is well-established likely will not help much.
To discourage powdery mildew in the future, ensure the zinnias are planted in full sun, and are not crowded together so they get lots of air circulation. Keep the area under and around the zinnias clean and free of debris (as the fungus overwinters in plant debris).
Zinnias tolerate hot, dry weather and only need deep watering once a week or so in order to develop strong root systems, which helps the plants fend off disease. Water at the base of the plants, taking care not to wet the leaves, as this can promote fungal growth. If you can, use a soaker hose, which will keep the leaves dry. If you use a sprinkler or hand-water and can’t avoid splashing the leaves, the best time to water is mid-morning, so that the moisture dries quickly from the leaves.
Finally, note that there are zinnia varieties that are resistant to powdery mildew. If this problem continues, you may want to look for these varieties for your garden. Some experts recommend planting “mildew-prone” varieties towards the back of the garden, with the resistant varieties in front.
For more information, see University of California. Integrated Pest Management Program. Powdery mildew on ornamentals.