I appreciate every time I have written in and I have received great advice.
So, what brings me here today? I am attaching a photo of my ten year old J. Maple. I just noticed that, on two areas of the tree, the leaves are looking “bleached”. They are not curled or dropping off, nor are there any web or cocoon-like substances on the leaves. I took a leaf into Sheridan this morning and the tree expert I spoke with (who is new to this location, due to the recent retirement of Amin, I think), told me she thought it was spider mites. But, as I said, I don’t see any cocoon like substances. She advised me to cut off all the affected leaves and to spray with remaining leaves with homemade insecticidal soap and water (I was going to use Dr. Bonner’s Castor-oil soap) and to clean up all debris under the tree. I started to cut off the leaves, and then looked at the “healthy” leaves. I am not sure what I am looking for, but it looks like even the healthy red leaves have teeny tiny beigeish spots on the undersides. Is that spider mites? I notice them on a J. Maple that is further down in the garden but that does not have the bleached leaf look happening. Perhaps this is something that all J. Maples have, I just never looked that closely before!
My question is, at this late time in the season, when the leaves will be falling off in a month, should I even bother to be cutting off the affected leaves, if the spider mites are going to freeze soon anyway (I hope)? What would you do at this time of the year if your J. Maple had these leaves? As always, I appreciate your wise advice.
Your seemingly simple question is not so easy to answer!
You’ve already spoken to an expert who has diagnosed the issue as spider mites – I won’t second-guess them, although it is important to confirm that the problem is indeed mites. You will see from the information below that Japanese maple trees are sensitive to the products typically used to control mites, and while some experts state they can be used, others recommend avoidance, so the issue is not straightforward. Accordingly, I suggest that you speak with an arborist to both confirm that mites are the problem and also to determine the best treatment. They can also help identify the spots on the undersides of the leaves of your 2 trees. Landscape Ontario provides contact information for arborists throughout the province.
If it does turn out to be the start of a mite infestation that could be a lot worse next growing season, cutting off the affected leaves may be a good start, but may not suffice. Unfortunately, mites will return and their numbers can increase very quickly.
I’ll briefly discuss treatment of spider mites and also what might be affecting the leaves on the 2 trees.
Mites on single tree
Stressed tree? As spider mites show up on weak or stressed plants, it is important to ensure that your tree remains healthy. Remove fallen leaves and other debris from around the tree (put it in the garbage, don’t compost it as we’re not sure which pests the debris might contain), make sure that the tree is well-watered (but not over-watered) and that the soil in which it is planted drains well (avoid soggy roots). Japanese maple trees grow well in clay or sandy soils, and other soil types in between. Give the soil a boost by adding nutrients – work compost into the soil around the tree in the spring and early summer. You’ve had the tree for 10 years now, so presumably it’s been happy in its location – I don’t know why it would be particularly stressed this year. See The University of New Hampshire’s How should I plant and care for a Japanese maple? Regular use of a strong stream of water from the hose will help knock the mites off the tree and should decrease the extent of infestation. For details on how to control mites, see Ohio State University Extension’s Spider Mites and Their Control.
For many trees and other plants, in order to control spider mites, the best treatment would be either dormant oil (applied in late winter or early spring, when the plant is dormant) or insecticidal soap, applied during the warm growing season, not now (it’s early November). Caution using dormant oil or insecticidal soap on Japanese maples!
Unfortunately, Japanese maple trees are very sensitive to dormant oil, which can burn them, although advice about its use on these trees varies among experts. Some say that dormant oil should be avoided entirely, while others caution that it’s best to start by testing a small part of the tree with the oil to see if it tolerates the treatment. See University of Nevada, Reno Extension. Horticultural Oils – What a Gardener Needs to Know.
As for insecticidal soaps, again, some experts point out that these soaps will harm Japanese maples, while others recommend their use. For example, see Clemson Cooperative Extension. Insecticidal soaps for garden pest control.
The conflicting information in the literature about using dormant oil or soaps on the Japanese maple is one reason I recommend that you consult with an arborist.
Note too that we do not recommend home remedies in gardens. You mention Dr. Bonner’s Castor-oil soap (I believe you mean Castile oil). This soap is designed for human use, so should not be used on plants. Rather, an insecticidal soap, which is highly refined and designed for use on plants, should be used, as noted in the Clemson Cooperative Extension article above.
Should you treat the tree now? If your tree has mites, it’s important to know if the infestation is (or will be) severe. This does not seem to be the case now, but could be next season. For example, consider the life cycle of the maple spider mite (Oligonychus aceris), which is one of several types of mites that could be affecting your tree and note how quickly mite populations can increase. The critters over-winter on the maple bark and it only takes 4-6 days for the eggs to hatch and become adults – with the females laying hundreds of eggs over a period of up to 3 weeks. There can be several overlapping generations during the summer. Other types of mites may overwinter in the egg stage, or as adults in protected spots on host plants or in the soil. Mites pierce leaves and suck out fluids from the plant cells, resulting in the leaves looking flecked with yellowish or whitish dots. I suggest that you speak with an arborist to determine what you can do now to prevent a mite infestation next year.
Beige spots on leaves of 2 Japanese maple trees
I’m not sure what the beige spots are on the undersides of healthy leaves on your 2 trees, spots like you describe should not be present on leaves. There is a fungal disease called leaf spot, which could cause spots, but these spots should have changed from being light-coloured early in the growing season to much darker, even black, by now. This does not seem to be the case with your trees. In any event, leaf spot is generally not serious enough to merit treatment. You’ve likely seen the tar-coloured spots on maple trees in your neighbourhood – unsightly, but not harmful to the trees. As mentioned above, it’s important to keep the trees healthy. And a chat with an arborist should set your mind at ease. As an arborist may not be available right away, take several photos (lots of closeups) of the spotted leaves and keep a few of them in a ziploc bag.
One of my colleagues provided additional comments about what might be stressing your Japanese maple tree(s): If the quality/amount of sunlight available to the trees has changed, e.g., if the spot now receives more sun due to removal of another tree – this can cause leaf burning/discolouration. Another very common issue is root girdling. Japanese maples are notorious for this when grown in a nursery. Sadly, the trade is not trained sufficiently to correct this problem and this seems to be becoming more of an issue the last 10 years. Stress caused by a girdling root may not be evident for several years after the tree is planted. See Toronto Master Gardeners, Girdling of Japanese maple. Again, an arborist would be able to help.
All the best keeping your Japanese maple trees healthy and pest-free!