Knowledgebase
What is this mold on our Japanese Maple? #889041
Asked October 30, 2024, 11:59 AM EDT
Baltimore County Maryland
Expert Response
This is powdery mildew. The warmer days and cool nights are perfect weather for it. If these are not in full sun that may be a contributing factor. Also if they are close together or in a confined space, that will also add to the fungal growth.
A fungicide is not really helpful or warranted since it would need to be applied before any signs of the powdery mildew and be applied at regular intervals through the growing season while the leaves are on the plants. This can get costly and prolonged use of fungicide can be harmful to pollinators as well as beneficial soil microbes.
Instead we recommend pruning to open up the plant branches to help with air circulation and/or thinning the canopy of any overhead tree limbs to allow more sun in the area. At this time of year since the leaves are getting ready to fall off, you can rake up any of the fallen leaves and discard them to try to limit the spread of the fungal spores. You can prune this fall before the weather gets too cold as well.
Next year after the trees leaf out, if the infection is sever, you can check horticultural oil products to see if they are listed for control of powdery mildew and apply per the label directions. More information about this is on the link provided above.
Let us know if you have further questions,
Emily