Hi – We have a 4 year old Bloodgood Japanese Maple that started suffering from some die-back last season. This spring, it did not fully leaf out and the leaves that did are mostly wilting and covered with a white almost hairy substance. We have had trouble with powdery mildew in the garden the last few seasons on nearby ninebark, but it looks very different than the white substance seen here. We aren’t sure if we should prune out the dead limbs. If it is a fungus, should we treat the white leaves with either a baking soda or milk solution spray? Thanks so much!
From your photo, this appears to be powdery mildew, which is a fungal disease well known to affect Japanese maples, particularly in the spring and fall when the days are warm and nights cool.
This link provides useful organic strategies for dealing with powdery mildew on your tree, which include pruning and keeping your plant healthy:
In addition, it is wise to avoid applications of nitrogen fertilizer during the summer season because new growth is more susceptible to diseases like powdery mildew. As always, cleaning up affected leaves around the base of the tree is a good practice. Dispose of these leaves rather than putting them into your compost.
As Master Gardeners, our mandate doesn’t allow us to recommend home remedies, but there are several research studies reporting the effectiveness of a milk and water solution. In addition, if you would like to consider a fungicide, there are a few commercial biofungicides approved for use in Ontario which are available at larger nurseries.