Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Ascomycota
Subphylum	Pezizomycotina
Class	Dothideomycetes
Order	Botryosphaeriales
Family	Botryosphaeriaceae
Genus	Guignardia

Scientific Name

Guignardia aesculi

Common Name

Guignardia blotch of buckeyes and horsechestnuts

Author: Clarissa Balbalian, Mississippi State University

Reviewed by: Matthew T. Kasson, Virginia Tech

Pathogen

Guignardia aesculi is a fungus that causes leaf blotch disease on Aesculus sp. in the United States and Europe. The fungus has three distinct morphological stages on the leaf, a sexual (pseudothecial) stage, and two asexual stages; a macroconidial (pycnidial) stage, and a microconidial stage (spermagonial) stage. Based on molecular data and the end of dual nomenclature for fungi, the generic name Phyllosticta has been chosen over Guignardia and Wikee, et. al. (2013) suggest that the genus should be placed in the family Phyllostictaceae. While Phyllosticta sphaeropsoidea Ellis & Everhart may become the unit name for this fungus, Wikee, et. al. (2011) maintain that the uniqueness of this taxa needs to be established with molecular data, therefore this publication will continue to refer to the fungus by the teleomorph name Guignardia aesculi.

Features of the three stages of *Guignardia aesculi*
Stage	Fruiting Structure	Spore Description	Season of Occurrence	Function
Microconidial anamorph Also known as Leptodothiorella aesculicola (Sacc.) Sivan.	Spermagonium 40-110 μm, ostiolate	Spermatia Hyaline, dumbell-shaped, oblong 3-9μm x 0.75-3.0 μm	Spermagonia begin forming in August or September. Occur on top and bottom leaf surfaces and are interspersed among the pycnidia on the leaf surface.	Undetermined
Macroconidial anamorph Also known as Phyllosticta sphaeropsoidea Ellis & Everhart	Pycnidium Usually globose, distinctly ostiolate, 90-175 μm	Conidia Ovoid 10-16 μm x 6.5-10 μm, hyaline with granular contents, 1-2 guttulae, and 1-2 nuclei. Hyaline apical appendage 6-7.5 μm long.	Form shortly after leaf lesions become visible. Common throughout the lesion, from the necrotic center to the leading edge. Primarily on upper leaf surface.	Secondary inoculum
Sexual	Pseudothecium Globose, subepidermal, ostiolate, 80-150 μm in diameter. Ascus bitunicate, subclavate, 54-70 μm x 15-17 μm.	Ascospores One-celled, 8 per ascus, subelliptical, granular content, 12-18 μm x 7-9 μm	Perithecia develop in winter and ascospores are released in early spring.	Primary inoculum

Cultural Characteristics

Based on descriptions by Pastirčáková, et al. 2009 (Pseudothecia not produced in culture)
Culture Medium	Mycelial Characteristics	Pycnidial Characteristics
Malt Extract Agar	Slow growing, white, floccose aerial mycelium Microscopic features: hyaline, branched, septate, 4-6 μm wide.	Dark pycnidia clustered, aggregated
Rice Agar	Little aerial mycelium	Abundant, dark, globose, solitary, rarely clustered, with conidial and spermatial cavities.

Symptoms and Signs

Leaf lesions are irregular in shape and begin as water-soaked spots that become dark reddish-brown with a yellow margin that merges into the green leaf tissue. Small spots may be vein-delimited. Large spots cause leaf distortion and can cause premature leaf drop. Pycnidia can be seen in the leaf lesions as black specks either scattered over the surface of the leaf, or crowded together in one section of a lesion. Lesions infrequently form on petioles and fruit and are insignificant when they do. Fruit lesions remain small and do not cause fruit rot.

Ecology and Spread

Guignardia aesculi overwinters as immature pseudothecia on fallen leaves. Ascospores are released in the spring (April) and infect newly emerging leaves. Pycnidia form in the late spring (May), shortly after leaf blotch symptoms become visible. Pycnidia serve as secondary inoculum, spreading the disease throughout the growing season. Germinated conidia produce appressoria that actively penetrate the leaf cuticle via an infection peg. Spermagonia develop in late August. The fungus is favored by leaf wetness and the disease can be particularly severe in dense nursery or landscape plantings where air circulation is poor.

Geographic Distribution

Guignardia aesculi is found throughout the range of Aesculus spp. in Canada and the United States. The pathogen is reported on various Aesculus spp. from Europe (Austria, Belgium, Bulgaria, Croatia, Czech Republic, England, Estonia, Hungary, Italy, Lithuania, Norway, Poland, Portugal, Romania, Russia, Switzerland, and Ukraine), and also from Asia (Armenia, China and South Korea).

Management

Management of any pathogen is often dependent upon both cultural and chemical options. Consult your local extension specialist or agent for recommendations relevant to your particular host and state.

Cultural Management

Severity of leaf blotch outbreaks is very dependent on environmental conditions. Wet weather favors the pathogen and infection. Collecting and burning, burying, or otherwise destroying fallen leaves will help reduce fungal inoculum available for spring infections. Improve air circulation to reduce humidity on the leaf surface by pruning landscape trees and using wide plant spacing in nurseries and landscapes. Avoid irrigation practices that wet the leaves. Disease is rare on Aesculus glabra varieties arguta, monticola and sargentii, and Aesculus parviflora and A. parviflora var. serotina.

Chemical Management

The damage from leaf blotch is primarily aesthetic and chemical control is usually not recommended for landscape plantings. Chemical control may be desirable in nursery plantings, where severe infections can slow tree growth, and also in landscape settings that have a history of severe symptoms. Fungicides should be applied beginning at bud break and reapplied at the recommended label interval, until environmental conditions are no longer conducive for disease development. Active ingredients that are labeled for use include: chlorothalonil, chlorothalonil plus thiophanate methyl, mancozeb, mancozeb plus copper, and propiconazole.

Remember: the label is the law.

Diagnostic procedures

The presence of black pycnidia in the characteristic leaf blotches on Aesculus and microscopic examination of asexual spores. Examination of fallen leaves and the microscopic examination of the sexual stage of the fungus. The fungus can be isolated by surface sterilizing symptomatic leaf tissue pieces with 0.5% NaOCl for 1 minute and transferring them to malt extract agar. Incubate for 5 days at 21°C-25°C, then transfer colonies to malt extract agar or rice agar and incubate at least 2 weeks. Colonies start out white and become dark with age, and pycnidia should form in the culture plates.

Resources and References

Hudson, H.J. 1987. Guignardia leaf blotch of horsechestnut. Trans Br Mycol Soc 89: 400-401.
Pastirĉáková, K., M. Pastirĉák, F. Celar, and H-D. Shin. 2009. Guignardia aesculi on species of Aesculus: new records from Europe and Asia. Mycotaxon 108: 287-296.
Sinclair, W.A., and H.H. Lyon. 2005. Diseases of trees and shrubs, second edition. Cornell University Press. Ithaca, NY.
Slippers, B., E. Boissin, A.J.L. Phillips, J.Z. Groenewald, L. Lombard, M.J. Wingfield, A. Postma, T. Burgess, and P.W. Crous. 2013. Phylogenetic lineages in the Botryosphaeriales: a systematic and evolutionary framework. Stud Mycol 76(1): 31-49.
Stewart, V.B. 1916. The leaf blotch disease of horse-chestnut. Phytopathology 6:5-19.
Wikee, S., D. Udayanga, P.W. Crous, E. Chukeatirote, E.H.C. McKenzie, A.H. Bahkali, D.Q. Dai, and K.D. Hyde. 2011. Phyllosticta-an overview of current status of species recognition. Fungal Divers 51:43-61.
Wikee, S., L. Lombard, C. Nakashima, K. Motohashi, E. Chukeatirote, R. Cheewangkoon, E.H.C. McKenzie, K.D. Hyde, and P.W. Crous. 2013. A phylogenetic re-evaluation of Phyllosticta (Botryosphaeriales). Stud Mycol 76(1): 1-29.

Acknowledgments

National Institute of Food and Agriculture, U.S. Department of Agriculture, under Agreement No. 2011-41530-30708 as part of "Diagnostic Image Series Development for Supporting IPM in the Southern Region" (USDA-NIFA-RIPM-003351)

Guignardia aesculi (guignardia leaf spot on buckeye)