Symptoms of Guignardia Leaf Blotch are beginning to develop on Aesculus spp. in southwest Ohio. The disease is produced by the fungus, Guignardia aesculi. It’s common on buckeyes and horsechestnuts with early symptoms appearing around this time of the year.

In 2020, I came across obvious reddish-brown lesions on a yellow buckeye (Aesculus flava) in southwest Ohio. My quick-draw diagnosis was that the lesions were symptoms of Guignardia Leaf Blotch. However, as I zoomed in with my camera to take some close-up pictures, some of the leaf blotches “suddenly morphed” into something that didn’t fit with Guignardia.

As I peered through my camera viewfinder, I saw that some of the necrotic areas on the upper leaflet surfaces appeared wrinkled like the tissue was pulling away from the leaf, as if it were delaminating. Leaf tissue delamination is the calling card of a leafminer. This was not Guignardia Leaf Blotch.

The true culprit was the native leafmining moth, Cameraria aesculisella, family Gracillariidae. 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.

Since my diagnostic enlightenment in 2020, I have observed the same Guignardia look-alike leafmining activity on Red Buckeye (A. pavia) and horsechestnut (A. hippocastanum). The moth has no common name approved by the Entomological Society of America (ESA). In previous BYGL Alerts, I referred to the moth as the “Buckeye Leafmining Moth; however, I’m calling it the “Aesculus Leafmining Moth” in this Alert to reflect the occurrence on both buckeyes and horsechestnuts.

Readers should be aware that using the made-up common name to seek online information may yield references to the Horsechestnut Leafmining Moth (Cameraria ohridella) currently ripping through its namesake host in Europe. This leafminer has also been found on other members of the Aesculus genus in Europe as well as on Silver Maple (Acer saccharinum) in Belarus.

The origin of the Horsechestnut Leafmining Moth continues to be investigated. However, it appears to remain confined to Europe and has not yet been found in North America. Conversely, the Aesculus Leafmining Moth remains restricted to North America.

Of course, as shown in the image below, leaf blotch symptoms produced by leafming caterpillars as well as Guignardia may occur on the same leaflet. The mix-messaging of 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?”

What Exactly Do You See?

Question #6 in our “20 Questions on Plant Diagnostics” is “What Exactly Do You See?” Guignardia Leaf Blotch symptoms are described as zonate lesions on the leaflets that are often surrounded by a yellow, chlorotic halo. The fungal pathogen affects all layers of the leaflets from the upper epidermis to the lower epidermis, so the necrosis is evident on both sides of the leaflets.

The Guignardia 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.

The Aesculus Leafmining Moth caterpillars feed just beneath the upper leaf epidermis. It’s why I observed the wrinkling of the necrotic tissue in 2020. Unlike Guignardia Leaf Blotch, there’s little evidence on the underside of the leaflets of the leafmining activity.

Carefully removing the paper-thin upper epidermis covering the blotch mines will reveal the unusual-looking leafmining 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 this and other 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 images below show Aesculus Leafming Moth pupae. The pupa in the second image 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.

Although the leafmines produced by the moth caterpillars on Aesculus spp. 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.

Final Note: Is it a Buckeye or Horsechestnut?

Question #1 in our “20 Questions on Plant Diagnostics” is “What is the Plant?” Ann Chanon (OSU Extension, Lake County), who earned her Ph.D. researching Aesculus, shared a helpful tip shown in the images below. Buckeye leaflets are joined to the compound leaf petiole with a short, impossible-to-pronounce stem called a petiolule. Horsechestnuts lack petiolules.

Ron Wilson

In The Garden With Ron Wilson