Participants in this week’s OGIA/OSU Extension Diagnostic Walkabout examined the characteristic symptoms of Guignardia Leaf Blotch on Aesculus spp. in southwest Ohio. The disease is produced by the fungus, Guignardia aesculi (family Botryosphaeriaceae). It’s a common fungal disease on buckeyes and horsechestnuts, with symptoms appearing around this time of the year.
It’s also common at this time of the year for the handiwork of the so-called Buckeye / Horsechestnut Leafmining Moth, or Aesculus Leafmining Moth (Cameraria aesculisella, family Gracillariidae) to appear on its namesake host. Mining just beneath the upper leaf epidermis is commonly associated with moths belonging to the family. These and other small moths are called “microlepidoptera,” which is not a taxonomic group but a nod to their diminutive size.
Diagnostic Tips
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 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.
Aesculus Leafmining Moth caterpillars feed just beneath the upper leaf epidermis. Consequently, it’s common to observe wrinkling of the necrotic tissue. 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 (C. guttifinitella).
The images below show Aesculus Leafmining 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.
Why Worry?
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.
However, readers should be aware that seeking online information about the leafmining moth found in Ohio may yield references to the Horsechestnut Leafmining Moth (C. 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 European leafmining moth on horsechestnut 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. On the other hand, the European moth seems to be much more aggressive, so we need to pay close attention to leafmining activity on Aesculus in North America.
Guignardia Leaf Blotch has a host range across the Aesculus spectrum. However, some members of the genus are more susceptible than others. For example, the leaf blotch is seldom observed on Bottlebrush Buckeye (A. parviflora). However, it’s common on the Ohio Buckeye (A. glabra), where symptoms include an unusual reddening of the leaflets. The leaf blotch can run rampant on Red Horsechestnut (Aesculus × carnea). Mid-season impacts include browned canopies and heavy leaf loss.
Some Taxing Taxonomy
Although I’ve noted that the leaf blotch disease on Aesculus is caused by the fungus, G. aesculi, readers who dig deeper will sometimes find that the fungal causal agent behind the disease is listed as Phyllosticta sphaeropsoidea. In the strange world of fungal duel naming taxonomy, P. sphaeropsoidea is the sexual stage (teleomorph) and G. aesculi is the asexual stage (anamorph) of the same fungus.
In the more comprehensible world of entomology taxonomy, a paper published in 1975 reported that through a process called paedogenesis, larvae (maggots) of the Common Drone Fly (Eristalis tenax, family Syrphidae) can reproduce asexually to produce more maggots. However, the sexual adults and asexual maggots aren’t given two different scientific names. That’s why the duel naming by mycologists is a bit confusing to entomologists.
Selected References
Ibrahim, I. A., & Gad, A. M. (1975). The occurrence of paedogenesis in Eristalis larvae (Diptera: Syrphidae). Journal of Medical Entomology, 12(2), 268-268.
https://doi.org/10.1093/jmedent/12.2.268
Kopačka, M., Nachman, G., & Zemek, R. (2021). Seasonal changes and the interaction between the horse chestnut leaf miner Cameraria ohridella and horse chestnut leaf blotch disease caused by Guignardia aesculi. Forests, 12(7), 952.
https://www.mdpi.com/1999-4907/12/7/952
Lees, D., Lopez-Vaamonde, C., Augustin, S., & Garden, M. B. (2009). Cameraria ohridella Deschka & Dimic 1986. Encyclopedia of life. 2009, 19 p.
https://hal.inrae.fr/hal-02824036v1
Liao, C., Ohshima, I. and Huang, G., 2019. A new leaf-mining moth, Caloptilia aesculi, sp. nov.(Lepidoptera: Gracillariidae: Gracillariinae) feeding on Aesculus chinensis Bunge (Hippocastanaceae) from China. Zootaxa, 4586(3), pp.586-600.
https://www.mapress.com/zt/article/view/zootaxa.4586.3.13
Sinchuk, A. V., Sinchuk, N. V., Baryshnikova, S. V., & Kolbas, A. P. (2023). A new host plant for Cameraria ohridella Deschka & Dimić, 1986 (Lepidoptera: Gracilleriidae). SHILAP Revista de lepidopterología, 51(203), 539-547.
https://doi.org/10.57065/shilap.539
Tomiczek, C., & Krehan, H. (1998). The horsechesnut leafmining moth (Cameraria ohridella): a new pest in Central Europe. Arboriculture & Urban Forestry (AUF), 24(3), 144-148.
https://doi.org/10.48044/jauf.1998.018
Zimmermannová-Pastirčáková, K. (2002). Occurrence of horsechestnut leaf blotch and cultural characteristics of its causal agent–fungus Phyllosticta sphaeropsoidea, an anamorph of Guignardia aesculi. Folia oecologica, 30, 245-250.
https://go.osu.edu/phyllosticta
