Hello, my Polemonium caeruleum which has only been planted for a few weeks has developed white spots on its top leaves. Could I be watering it too much?
Thank you for submitting your question to the Toronto Master Gardeners.
You are correct that this powdery mildew/mold spots which is very prevalent on Polemonium caeruleum. Contributing factors can be a combination of too much water or humidity and insufficient air circulation.
It is advisable at this point to take the following actions;
- Remove or cut back the portions of your plants that have powdery mildew on them. If you see it on a few leaves, remove those leaves and do not compost them.
- Increase your air circulation by thinning and pruning.The photograph does not suggest crowding with other plants, but the adjacent fence could prevent optimal air circulation.
- Don’t fertilize the affected plants until the problem is corrected. Powdery mildew favors young, succulent growth.
- Avoid water plants from above and perhaps less often,( possibly use drip lines if needed). Also do not water in the evening hours to avoid dampness going into night time.
- Apply a fungicide. There are many fungicides available. Check the label to be sure they are safe and effective on the type of plant that is infected. Look for ingredients such as potassium bicarbonate, neem oil, sulfur, or copper for continuous protection, most fungicides will need repeat applications every 7 to 14 days. Always follow the label instructions for both application and waiting period before harvest
For further reading : https://horticulture.ahdb.org.uk/download/448/file