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Horsechestnut leaf blotch

Written by J.B. Tanney Revision 2025

General information and importance

Phyllosticta paviae (= Guignardia aesculi) causes leaf blotch in horsechestnut and buckeye trees (Aesculus). Horsechestnut leaf blotch is commonly observed in urban environments, particularly following wet spring conditions. The leaf blotch the pathogen causes is primarily a cosmetic concern in urban trees, but severe infections can spread throughout the entire crown, leading to premature defoliation later in the growing season. The disease does not typically affect tree longevity but can lead to reduced aesthetic value and increased maintenance costs in affected urban areas. Phyllosticta paviae is commonly found in and presumed to be native to North America. It has been introduced to Europe, where it may cause more severe infections.

Distribution and hosts

Horsechestnut leaf blotch is widespread throughout eastern Canada, affecting various Aesculus species, including Ohio buckeye (A. glabra) and horsechestnut (A. hippocastanum). The disease is also prevalent across the eastern United States, affecting several species of Aesculus. Phyllosticta paviae was likely introduced from North America to Europe, where it has become widespread. The disease has also been reported in South Korea. Further studies are needed to assess the full extent of the distribution of P. paviae outside of these regions, particularly in areas with increasing global trade in plant materials.

Tree parts affected

Leaves, petioles

Symptoms and signs

Infected leaves initially develop large, water-soaked, irregularly shaped spots, which soon turn into reddish-brown lesions with a surrounding yellow margin. As the spots coalesce, they may cover large areas of the leaf surface, causing them to curl, turn brown, dry, and become brittle, ultimately leading to premature leaf drop. Small black specks (pycnidia) may appear scattered over the lesions, distinguishing this disease from symptoms caused by abiotic stresses such as drought, air pollution, salt, and hot, dry, sunny weather. In some cases, small, reddish-brown, elongated spots may also develop on the petioles. Leaf blotch symptoms may intensify in years with prolonged wet weather, particularly in spring, leading to more extensive leaf damage and defoliation.

Phyllosticta paviae pseudothecia are solitary, dark brown, and ostiolate, measuring 90 to 180 micrometres in diameter. They contain clavate to cylindrical asci, which are stalked, thickened, and rounded at the apex. These asci are bitunicate, eight-spored, and measure 50 to 80 micrometres × 14 to 18 micrometres. Ascospores are hyaline, smooth, and either straight or slightly curved. They vary in shape (ovoid, ellipsoidal, or rhomboidal) and are aseptate, with granular or occasionally guttulate contents. Ascospores measure 12 to 18 micrometres × 7 to 9 micrometres, with a gelatinous appendage at one or both ends that disappears at maturity. Pycnidia are solitary, globose to pyriform in shape, dark brown, and ostiolate, measuring 80 to 165 micrometres in diameter. They contain cylindrical or conical hyaline conidiogenous cells, which measure 5 to 10 micrometres × 2 to 4 micrometres. Conidia are hyaline, smooth, and can be globose, ellipsoidal, clavate, or obpyriform. They are aseptate and thin walled, with coarse granular contents or rarely guttulate, measuring 9 to 18 micrometres × 6 to 12 micrometres. Conidia are surrounded by a gelatinous sheath and feature an apical appendage. Conidia are exuded from the ostiole in the form of a milky white tendril. Spermagonia, morphologically similar to pycnidia but smaller (40–95 micrometres), contain filamentous to cylindrical spermatiogenous cells. These cells produce spermatia that are hyaline, smooth, and vary from oblong-cylindrical to dumbbell-shaped. Spermatia are straight or slightly curved, guttulate, and measure 3 to 10 micrometres × 0.5 to 2.5 micrometres.

Symptoms of horsechestnut leaf blotch may be confused with leaf scorch or other abiotic damage, but the presence of P. paviae pycnidia and specific lesion patterns can help distinguish this disease.

Disease cycle

In the spring, P. paviae pseudothecia mature on dead overwintered leaves and release airborne ascospores during wet conditions. These ascospores then infect young Aesculus leaves. Leaf blotches typically appear within two to three weeks after infection, and reinfections may occur from pycnidia that develop in early June. Wet weather exacerbates P. paviae infections, promoting ascospores in spring and conidia throughout the summer. Conidia are likely spread through rain splash and the movement of water through the crown. Fallen leaves can harbour inoculum that overwinters, initiating new infections the following spring.

Damage

Horsechestnut leaf blotch is unsightly, but it is not considered a serious disease in Canada. Severe P. paviae infections may affect the entire crown, causing premature defoliation, but this typically occurs late in the growing season and therefore does not appear to significantly affect the health of the tree. Although the disease rarely affects tree health, its effect on the appearance of trees in urban areas can result in increased management and maintenance efforts.

Prevention and management

Monitoring and early identification of horsechestnut leaf blotch symptoms are crucial for effective management of the disease, especially in urban settings where aesthetic value is a primary concern. In urban settings, horsechestnut leaf blotch can be reduced through basic sanitation measures such as raking and disposing of (e.g., offsite disposal or burning) fallen leaves in autumn to reduce overwintering inoculum. Pruning the tree canopy may improve air circulation, which helps leaves dry faster and reduces the risk of infection. Avoid planting Aesculus in high-humidity sites to reduce the likelihood of infection.

Pest management strategies for a particular pest vary depending on several factors. These include:

Decisions about pest management strategies require information about each of these factors for informed decision-making. These various factors should then be weighed carefully in terms of costs and benefits before action is taken against any particular pest.

Selected references

Hudson, H.J. 1987. Guignardia leaf blotch of horsechestnut. Transactions of the British Mycological Society 89(3): 400–401. https://doi.org/10.1016/S0007-1536(87)80129-8

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://doi.org/10.3390/f12070952

Myren, D.T.; Laflamme, G.; Singh, P.; Magasi, L.P.; Lachance, D., editors. 1994. Tree diseases of eastern Canada. Natural Resources Canada, Canadian Forest Service, Science and Sustainable Development Directorate. Ottawa, Ontario. 159 p. https://ostrnrcan-dostrncan.canada.ca/entities/publication/123e1f42-369f-4c95-aded-82d95f675a52?fromSearchPage=true

Pastiráková, K.; Pastirák, M.; Celar, F.; Shin, H.-D. 2009. Guignardia aesculi on species of Aesculus: new records from Europe and Asia. Mycotaxon 108(1): 287–296. https://doi.org/10.5248/108.287

Punithalingam, E. 1993. Guignardia aesculi. IMI Descriptions of Fungi and Bacteria, Set 117, No. 1165. Mycopathologia 123: 53–54.

Wikee, S.; Udayanga, D.; Crous, P.W.; Chukeatirote, E.; McKenzie, E.H.C.; Bahkali, A.H.; Dai, D.; Hyde, K.D. 2011. Phyllosticta—an overview of current status of species recognition. Fungal Diversity 51: 43–61. https://doi.org/10.1007/s13225-011-0146-5

Cite this fact sheet

Tanney, J.B. 2025. Horsechestnut leaf blotch. In J.P. Brandt, B.I. Daigle, J.-L. St-Germain, A.C. Skinner, B.C. Callan, and V.G. Nealis, editors. Trees, insects, mites, and diseases of Canada’s forests. Natural Resources Canada, Canadian Forest Service, Headquarters. Ottawa, Ontario.