Powdery Mildew on potted indoor Rosemary plant


Good morning.
I live in Toronto and have a 3-4 year old potted rosemary that I have been successfully overwintering indoors. Right now though, it is rapidly getting overcome by powdery mildew- worse at the top- and it needs immediate help. As I use the leaves in cooking I would like a non-toxic solution to the problem. I’ve looked at the info on your site, including Richters and U. of Illinois, and also on the internet in general and there seem to be a lot of different remedies. Because there are so many options I am wondering which one works the best one and would resolve the issue most quickly and in a harm-free way?
The pot is in a west facing window in the kitchen so it probably needs more light, but that’s the best I can do and it’s been managing. It gets some coolness through the poorly sealed window and also air from the furnace because of the vent below the table. I gave it a half-dose of 10-15-10 fertilizer a couple of weeks ago. Other than that, it’s just been regular tap water that has sat out for awhile. I’ve been fairly attentive about watering, as I find rosemary to be quite fussy and at risk, from either overwatering or underwatering. I amazed that I’ve kept this one going for as long as I have.
I love my plant so recommendations for getting rid of the mildew would be welcome, as well as any suggestions for prepping it for the warmer weather and a new season of being outdoors. I have never managed to get it to flower for example.
Thank you for your assistance.


Thank you for contacting Toronto Master Gardeners.  Powdery mildew is a common problem for rosemary plants.  The disease starts off with powdery white spots on the leaves.  The spots increase in size and cover large parts of the plant giving it a dusty, powdery, or silvery appearance.  Rosemary plants are less likely to develop powdery mildew if these steps are followed:

  • Keep the plant in a well ventilated area.  If the plant is dense, prune out some of the branches to improve air circulation.  Turning a fan on for a few hours a day close to the plant will also increase circulation.
  • When watering do not wet the leaves, instead water the soil directly when the soil is dry to touch. The plant can also be misted on a weekly basis.
  • Avoid fertilizing rosemary during the winter especially with high nitrogen fertilizers. Powdery mildew attacks new succulent growth.

To treat a plant infected with powdery mildew:

  • Isolate the plant as soon as the infection is noticed. Remove and dispose of all infected plant parts. Severely infected plants should be thrown out.
  • Apply a commercial fungicide that it is appropriate for indoor use. Make sure to follow application directions carefully.

If the infection is not too serious, it will probably clear up when it is placed outside for the summer.

To increase the likelihood of blooms on the rosemary plant:

  • Give the plant a prune to remove dead wood and any remaining leaves with powdery mildew between March and June. Disinfect the shears before and after pruning the plant.
  • Position the plant in the full sun.
  • Fertilize sparingly if at all. Avoid using a fertilizer with high nitrogen as this will promote foliage growth rather than flowering.
  • Plant the rosemary in a well draining soil. If needed, amend the soil with sand, grit or perlite to replicate the well draining, low to medium nutrient soils of the Mediterranean.