River Birch leaves yellowing; have tiny brown dots on them

Question: We have a river birch tree that has leaves that are turning yellow.  The veins are still green.  Under a hand lens (magnifier), I can see tiny brown spots (like bugs?).  Do you know what they are and how I might treat the tree, please?  Attached/uploaded is a picture I took of a leaf through my hand lens.  Thank you!

responder: An Ask Extension Expert Replied July 12, 2024, 10:51 AM EDT
response: I'm sorry to hear about your tree! I can't tell from the photo if the brown spots are insects/mites or if they are just browning leaf features; it's possible that they might be something like spider mites but I would need a better photo or a physical sample to be sure (you could bring a sample to the Extension office in Castle Rock if you like).   Are the leaves all uniformly turning yellow on the tree, or are the symptoms restricted to older/younger leaves, or one area of the tree?  This sort of yellowing with green veins can indicate nutrient deficiency and even herbicide damage.   Would you also please let me know where the tree is sited (mulch, a lawn, garden bed, etc) and how it is watered and otherwise cared for? Cordially, John

responder: The Question Asker Replied July 12, 2024, 11:35 AM EDT
response: Dear John,   Thank you for the quick response!     The yellowing leaves are mostly on new growth, at the lowest levels of the tree.  I’ve attached several pictures that I hope will help.  I’ve just found that the infection is on older leaves, too; they just haven’t begun to turn color.   We had the birch planted about 25 years ago.  The photo “Birch 2004-06-00” is sometime early in the tree’s history.  We put in a sprinkler system so it gets watered regularly (M-W-Sat, 15 mins, then 5 mins each day).  The “Birch 2024-07-12” I took just this morning.   It started having this issue last spring, so I contacted a tree company who said it was sawfly.  They treated it last spring (a spray) and last fall, where last fall it was a big injection of imidacloprid (I’m not crazy about that…). I have to say that I can’t find any images of sawfly (larvae or other) that looks like what’s on the leaves.   The other two pictures are of a leaf I picked this morning – a more mature leaf that hasn’t really started showing yellow.  The 50x (I have a Dino-Lite) shows a lot of these things.  The 80x shows the joint of a vein (kind of hairy) with a lot of these things.  I just noticed white specks, too, that I remember covered the leaves last year, so I attached a leaf photo from then, too (River Birch Leaf 2023-05-06).   Thank you!   Shellie     From: ask=<personal data hidden> <ask=<personal data hidden>> On Behalf Of Ask ExtensionSent: Friday, July 12, 2024 8:51 AMTo: shellieluallin <<personal data hidden>>Subject: Re: River Birch leaves yellowing; have tiny brown dots on them (#0146801)

responder: An Ask Extension Expert Replied July 12, 2024, 3:28 PM EDT
response: Thank you. As a quick note, we also received your Extension Contact Form submission from the state office, we will continue to address this problem here if that works for you. The brown spots may be eggs of some kind, but do not appear to have the features of adult arthropods.  Mites are known to take advantage of leaf hairs for shelter, so this remains a possibility.  Mite populations can also be exacerbated by certain insecticide treatments. It is tough to say from the photo--it is still possible that they are a natural leaf feature or innocuous.   It is definitely not sawflies; these would be causing defoliation of the tree rather than yellowing, and you would see the crowds of caterpillar-like insects perpetrating the deed.   The lawn in your photo looks a bit dry, it's possible that the tree is experiencing drought stress now that things are heating up; this could be leading to symptoms that otherwise wouldn't be noticeable.  Consider checking the soil around the tree to make sure your watering times are adequate and that the soil is amply moist.  The yellowing may also be the result of a nutrient deficiency. It would be helpful if you were able to bring or send samples to the Extension office of a few leaves, both affected and unaffected.   In the mean time, make sure that the tree is adequately watered, since a drought-stressed tree is more likely to be susceptible to other issues.  Consider also spraying the foliage with water, particularly in the hottest parts of the day.  This will both cool the tree and will help to discourage mite activity, if spider mites are active.  Spider mites do not appreciate humidity, but thrive when conditions are hot and dry. Sorry that we don't (yet) have a good answer for you! Cordially, John

responder: The Question Asker Replied July 15, 2024, 5:46 PM EDT
response: Dear John,   This avenue works great, thanks!  I’m sorry I duplicated my submission – I just wasn’t sure the first was right once I saw the second place to ask.   I’m glad to know it’s not sawflies.  That did confuse me.    I have increased watering time on the sprinkler system.  Also, I have been hand watering the yard around the tree and spraying water for cooling.  I will continue to do this while it’s so hot.   I’m not sure when I can get samples to the Extension office.  Can I mail them?  Please, if I can mail leaves, to what address should I send them?  I would bag and label samples, and include this ticket number (#0146801).   Thank you!   Shellie   From: ask=<personal data hidden> <ask=<personal data hidden>> On Behalf Of Ask ExtensionSent: Friday, July 12, 2024 1:29 PMTo: shellieluallin <<personal data hidden>>Subject: Re: River Birch leaves yellowing; have tiny brown dots on them (#0146801)

responder: An Ask Extension Expert Replied July 15, 2024, 5:56 PM EDT
response: Excellent, the extra water is a great idea for birches. You could mail or bring samples to 410 Fairgrounds Road, Castle Rock, CO 80104. Cordially, John

responder: An Ask Extension Expert Replied August 07, 2024, 4:10 PM EDT
response: Hi Shellie, Thank you for sending in your leaf samples, and sorry for the delay in getting back to you!  From what I can see, I do not think you are dealing with insects or a disease.  From discussion with my Extension colleagues along the Front Range, the most likely suspect is heat--particularly rather abrupt heat the followed a reasonably moderate spring and early summer.  Many trees seem to have abandoned interior leaves as the summer heated up this year, leading to an unsightly mid-summer preview of fall.   Be sure that the tree continues to receive ample water, and of course if the symptoms change or progress further, be sure to let us know! Cordially, John

responder: The Question Asker Replied August 07, 2024, 7:36 PM EDT
response: Hi John,   Thank you for taking a look at it.  We’ll make sure it continues to get water.  Maybe I’m optimistic but I think it’s looking better.   Did you have a chance to look at the leaves under magnification to see those little brown dots on them?   I am enjoying this discussion!   Shellie   From: ask=<personal data hidden> <ask=<personal data hidden>> On Behalf Of Ask ExtensionSent: Wednesday, August 7, 2024 2:10 PMTo: shellieluallin <<personal data hidden>>Subject: Re: River Birch leaves yellowing; have tiny brown dots on them (#0146801)

responder: An Ask Extension Expert Replied August 19, 2024, 11:04 AM EDT
response: Hi Shellie,  Yes, from what I can tell, I think those dots are normal.  They look too large to be stomates, but it's possible that it is anatomy associated with guard cells; they could also be glands.  The even, predictable distribution and presence across both symptomatic and asymptomatic leaves suggests that it is not part of a disease process. John

responder: The Question Asker Replied August 20, 2024, 10:31 AM EDT
response: Hi John, Thank you so much for all of your attention to this and your helpful responses.  My concerns about the tree are greatly alleviated. Be Well! Shellie From: ask=<personal data hidden> On Behalf Of Ask ExtensionSent: Monday, August 19, 2024 9:05 AMTo: shellieluallin Subject: Re: River Birch leaves yellowing; have tiny brown dots on them (#0146801)