Japanese Maple Disorder

Question: We live in Rockville and have a 35 year-old Japanese Maple with leaf loss issues. For several years it looks wonderful in the spring but by mid-summer begins losing leaves on some branches (usually only on the side facing west which also is the area where water pools after heavy rains) and has significant webbing along with leaf discoloration. It seems to recover the following year without ill-effects. Is this an infestation or issue of too much water in the root system. See attached photos.
Thanks for your help!

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 July 23, 2024, 2:39 PM EDT
response: This appearance and mild aesthetic deterioration happens to be fairly common among many varieties of Japanese Maple grown in our region. It's partly the nature of our climate (hot and humid summers, coupled with some summer droughts) that causes these plants stress. The photos are a bit too small for us to see enough detail; can you please re-send larger versions (at least 1MB file size per photo)?  The bronzing or dulling of the red leaf color is not a health issue for the tree and is quite normal for red-leaf maple varieties once it gets hot. While a full sun exposure helps to keep red-purple leaf colors more intense overall, ironically the increased heat from such an exposure can override this influence, muddying the leaf colors and turning them greener anyway. While planting red-leaf Japanese Maples in a location that receives a bit of afternoon shade or intermittent dappled light can reduce heat stress, allowing the red-purple colors to remain richer longer, ironically the shade itself would also contribute to a greening of the leaf color during summer.  Overlapping this is the frequent development of some leaf scorch (in dry conditions or when plants are under-watered) plus some leaf spot infections that may look messy but which don't really cause the tree significant health impacts. Powdery mildew can also cause a gray coating on leaves or make them more vulnerable to crisping and browning. Raking-up and disposing of the tree's fallen leaves in autumn/winter may help to reduce the chances of reinfection next year, since it can remove some overwintering pathogen spores, but it's not a foolproof approach since spores could always blow into the area from other plantings, and years with wet spring weather (as we had this year) can foster more plant disease outbreaks in general. Use of a fungicide once symptoms have manifested is not going to provide much, if any, benefit.  Poor drainage by way of flooded root zones will stress Japanese Maples, which prefer even soil moisture in hot weather but which need good drainage at the same time. If branches die back on the wetter side of the tree, it might be due, at least in part, to root loss from periodically soggy soil. If a nearby roof downspout outlet is contributing to that wetness during rainy periods, consider redirecting it so it doesn't flood the root area of the tree. Some leaf symptoms between over-watering and under-watering can overlap, since the impact on the canopy is similar.  We can't make out any webbing in the photos, but it might be due to either caterpillars (none of which tend to strip enough foliage to bother treating, though Fall Webworms might warrant intervention) or a high population of spider mites, the latter of which thrive in hot, dry weather. We can't see it well, but there might be some patches of lichen on the bark of branches or twigs, which is normal and harmless.  To determine when the tree would need watering, including during this ongoing drought, feel the soil about six inches deep in the root zone of the tree. If it has become somewhat dry to the touch at that depth, a thorough watering is probably needed. If damp when checked, watering can probably wait. As very rough guidance, you might need to soak the roots about once a week with 15-20 or more gallons of water (for a tree this size) during periods of hot, dry weather. Using buckets with holes in their base as a way to both measure-out water and drip it into the ground gradually enough to soak in well can be a simple way to irrigate. A short video at the bottom of the watering page linked above mentions this option. By keeping the roots as little stressed by drought as possible, the tree should be able to recover well enough each spring that outbreaks like these recur in mid- or late summer. Leaf buds for next year's growth will start forming soon, if they haven't already, so keeping roots hydrated will also help to ensure that those buds, which will lie dormant all autumn and winter, remain viable for next year's new growth.  Miri

responder: The Question Asker Replied July 23, 2024, 4:36 PM EDT
response: thank you so much for your response. Very informative. I have attached larger size photos as per your request.Sent from my iPhone On Jul 23, 2024, at 2:39 PM, Ask Extension <<personal data hidden>> wrote:   ﻿

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 July 23, 2024, 5:02 PM EDT
response: Thank you for the larger photos. The plant overall looks pretty decent, considering how stressed and bedraggled Japanese Maples can be in tough years. There is some lichen on the bark (harmless and not a problem), and some of the leaf spotting could be either scorch, Phyllosticta leaf spot, or anthracnose, all of which are common and none of which are serious long-term issues outside of cosmetic damage. No fungicide needs to be used, nor would one provide much benefit at this point anyway.  Leafless branch tips is most likely a result of premature leaf drop due to any of the above trio of issues, or dieback due to Botryosphaeria infection following drought stress. The latter is not as likely unless the twig bark itself has become dried, brittle, and more shriveled or dark in color, as opposed to more greenish and fairly smooth. (On younger bark, as it's normal for older bark on thicker branches to be grayer. The Botryosphaeria page we link to focuses on Rhododendrons, but it can affect a wide variety of shrubs and trees.) If present, dead twig tips can simply be pruned off. The most practical time of year to do this would be in mid- to late spring, once all the healthy, live branches have leafed-out so you can tell which won't be producing foliage.  We don't see indications of insect or mite webbing, but feel free to send additional close-up photos if you find some that is concerning.  Miri