Japanese Maple with powdery mildew

Question: The half of the tree that is not in direct sun is affected by this powdery mildew. The sunny side looks very healthy (the tree is about 15' tall). Would you recommend treating with a fungicide? Can I hope to control it without removing the affected leaves (i.e. wait for the fall leaf drop). We have lilacs and crepe myrtles in he yard, though they are not (yet) affected.

responder: Answered by the Home and Garden Information Center (HGIC), University of Maryland Extension. For timely gardening information and tips, subscribe to our Maryland Grows blog and quarterly newsletter. Follow us on Facebook, Instagram, and YouTube. Replied October 25, 2024, 10:50 AM EDT
response: Powdery mildew is a very common fungal disease that is usually minor and only an eyesore, not a threat to long-term plant health. This late in the growing season, when leaves are about to shed, treatment is both not needed and not practical. Fungicides work best as preventatives, and have limited (if any) ability to cure existing infections. Usually, treatment needs to begin while leaves are still healthy, just before the typical infection season starts, and continue for the length of time that infection is likely. For powdery mildew, this is too long of a period (much of spring, cooler summer spells, and autumn) to expose other organisms to fungicide applications that can harm them.  When the leaves fall off, you can rake them up and dispose of them if you want, or compost them if a home compost pile is hot (not passive "cold" composting, which won't kill the spores). While powdery mildew looks essentially the same on a variety of plants that contract it, often it's different mildew species causing the infections, and the fungus won't move from one plant species to another different species. Still, because environmental conditions are what contributes to infection (warm days, cooler nights, and some humidity), unrelated mildews can appear on different plants at the same time. Therefore, the maple mildew won't infect the lilac or crapemyrtle, and vice versa, but each could develop its own infection around the same time because of the weather conditions.  Raking away fallen mildew-riddled leaves won't hurt, but isn't foolproof, as spores could always blow back into an area in a future season (how they presumably arrived in the first place if the tree didn't have mildew originally), and sometimes because spores overwinter on the twigs or tucked into the leaf buds themselves. The shadier side of this tree was likely affected more visibly with mildew because the sunnier side probably had leaf surfaces dry off faster after rain or dew, reducing the humidity levels hugging the leaf surface that would have benefitted the fungal spores attempting to infect the leaf.  Miri

responder: The Question Asker Replied October 25, 2024, 11:33 AM EDT
response: Thank you for taking the effort and time to provide such  a thorough answer! BTW I never realized Powdery Mildew had plant-specific species.   Quentin

responder: Answered by the Home and Garden Information Center (HGIC), University of Maryland Extension. For timely gardening information and tips, subscribe to our Maryland Grows blog and quarterly newsletter. Follow us on Facebook, Instagram, and YouTube. Replied October 25, 2024, 11:37 AM EDT
response: You're welcome! We don't know the specifics of what the host plant range is for each species of powdery mildew, but in general, there could be more separate species than it might seem, given how commonly the fungus appears on a variety of unrelated plants. Other fungi are much more cosmopolitan in their host plant range, so that trait can vary from one disease to another.  Miri