The fungus, Guignardia aesculi, produces the disease called Guignardia Leaf Blotch of Aesculus. It’s common on buckeyes and horsechestnuts in Ohio with early symptoms appearing around this time of the year.
The disease symptoms are generally described as zonate lesions on the leaflets that are often surrounded by a yellow, chlorotic halo. The lesions are initially small, reddish brown, and often bounded by the leaf veins.
As the summer progresses, the lesions expand to become large, dark brown blotches. Heavily infected trees may appear scorched as the brown leaf blotches envelop entire leaflets.
In 2020, I came across obvious reddish-brown lesions on a yellow buckeye (Aesculus flava) in southwest Ohio. My quick-draw diagnosis was the lesions were symptoms of Guignardia. However, as I zoomed in with my camera to take some pictures, some of the blotches “suddenly morphed” into something that didn’t fit with Guignardia.
Question #6 in our “20 Questions on Plant Diagnostics” is “What Exactly Do You See?” What exactly I saw through my camera viewfinder was that the upper leaflet surface on some of the necrotic areas appeared wrinkled like the tissue was pulling away from the leaf, as if it was delaminating.
My observation produced a complete metamorphosis of my diagnosis. Leaf tissue delamination is the calling card of a leafminer. This was not Guignardia.
Of course, the images below show that Guignardia should never be eliminated without a second look to separate the blotch-like symptoms produced by a leafminer from the leaf blotch symptoms of Guignardia, and vice versa. The mix-messaging with having two look-a-like problems occurring on the same leaflets at the same time raises the importance of answering the question, “What exactly do you see?”
I’ve been revisiting the yellow buckeye annually since my diagnostic U-turn. The leafminer feeds just beneath the upper leaf epidermis. It’s why I observed the wrinkling of the necrotic tissue in 2020. Also, unlike Guignardia leaf blotch, there’s little evidence on the underside of the leaflet of the blotch on the upper leaf surface created by leafminer activity.
Thanks to Charlie Eisman, author of “Leafminers of North America, Second Edition,” [ https://charleyeiseman.com/publications/ ], the leafminer moth is identified as Cameraria aesculisella, family Gracillariidae. This leafminer is known to target the “Buckeye side” of Aesculus including Ohio Buckeye (A. glabra) and Red Buckeye (A. pavia) as well as yellow buckeye. The moth has no common name approved by the Entomological Society of America (ESA); however, I’m calling it the “Buckeye Leafmining Moth” in this Alert.
Mining just beneath the upper leaf epidermis is commonly associated with moths belonging to the family Gracillariidae. These and other small moths are called “microlepidoptera” which is not a taxonomic group but a nod to their diminutive size.
The leafmining activity of the buckeye leafmining moth is very different from some of our more common leafminers like the Boxwood Leafminer midge fly (Monarthropalpus flavus, family Cecidomyiidae), Hawthorn Leafminer sawfly (Profenusa canadensis, family Tenthredinidae), and Yellow Poplar Weevil (Odontopus calceatus, family Curculionidae). These leafminers produce “blotch mines” by tunneling between both the upper and lower leaf surfaces causing both leaf surfaces to delaminate.
Carefully removing the paper-thin upper epidermis covering the buckeye leafmining moth’s blotch mines will reveal the unusual-looking moth caterpillar. The leafmines commonly contain silk filaments that point to a caterpillar rather than a beetle or midge fly.
Another common feature of leaf mines produced by microlepidopteran leafminers is the occurrence of circular structures within the leafmines. The structures may be pupation chambers or used for other purposes. These structures are very apparent in the mines of the Poison Ivy Leafmining moth (Cameraria guttifinitella).
The image below was taken this past Tuesday and shows that the buckeye leafmining moth caterpillars on yellow buckeye are pupating. The pupa was originally located in an aforementioned circular structure and bounded by silk; however, the wind rolled the pupa to one side as I was taking the picture.
I’ve been observing the activity of the buckeye leafmining moth since 2020. I also photographed a blotch mine last year on a wild horsechesnut tree in a park in southwest Ohio and thought it was produced by the same moth. However, given that the buckeye leafmining moth hasn’t been seen on horsechesnut, I now have some doubts.
Although the leafmines produced by the buckeye leafmining moth caterpillars are readily apparent, I’ve never seen the damage rise to a level that would threaten the overall health of the tree. So, I consider the moth more of an oddity rather than a serious pest.
Selected Reference
Boggs, J., E. Draper, and J. Chatfield. 2017. 20 Questions on Plant Diagnosis, Ohio State University Extension Fact Sheet, PLPATH-GEN-3. Digital Access: https://ohioline.osu.edu/factsheet/plpath-gen-3
