Aureobasidium apocryptum (Anthracnose on maple)

Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Ascomycota
Subphylum	Pezizomycotina
Class	Dothideomycetes
Subclass	Dothideomycetidae
Order	Dothideales
Family	Dothioraceae
Genus	Aureobasidium

Scientific Name

Aureobasidium apocryptum

Scientific Name Synonyms

Kabatiella apocrypta

Gloeosporium apocryptum

Common Name

anthracnose

Author: Garrett Ridge and Mike Munster, North Carolina State University

Synonyms in Older Literature

Gloeosporium apocryptum Ellis & Everh., 1888

Kabatiella apocrypta (Ellis & Everh.) Arx, 1957

Pathogen

Aureobasidium apocryptum is an asexual fungus that causes a common non-fatal leaf blight on many species of maple. A. apocryptum forms small acervuli (40-70 µm in diameter) that erupt through the epidermis of the maple leaf (Figures 1-2). The acervuli contain small, hyaline, non-septate conidia (7-12 x 2.5-4.5 µm) that are borne on short, broad conidiophores (14-20 x 5.5-7.5 µm) (Figure 3).

There is no known teliomorph of A. apocryptum.

Hosts, Signs, and Symptoms

Aureobasidium apocryptum is one of several foliar pathogens causing anthracnose on maple. It infects many species of maple (Acer), including: Japanese, Norwegian, mountain, red, silver, sugar, and Tatarian Maples.

Signs of infection include inconspicuous light brown acervuli located on the lower surfaces of lesions, especially along veins.

Symptoms are most visible in late spring or in summer as scattered, sharply defined reddish-brown dead areas formed on or near leaf veins (Figure 4). These lesions may enlarge rapidly and coalesce, killing large areas of affected leaves (Figure 5). The disease also may cause twisting, crinkling, or other leaf malformations. On Japanese maples, discrete white spots form on leaves. Severe infection leads to defoliation.

Ecology

A. apocryptum overwinters in infected leaf litter. Primary inoculum is produced during or shortly after wet weather in early spring. Spores formed in the acervuli are spread by splashing water to newly emerging leaves. Infection occurs during these same wet periods, and necrotic spots develop on the leaves. Spores produced on infected leaves can lead to secondary cycles of infection within the tree canopy throughout the growing season. Since wetness is critical, the amount of disease will vary from year to year, depending on the weather conditions during leaf expansion.

Geographic Distribution

A. apocryptum occurs on many species of Acer across North America, Europe, and Japan. It was first reported on Acer negundo by Ellis and Everhart in 1888.

Management

Cultural

Removing and destroying leaves from trees infected with maple anthracnose prevents A. apocryptum from over-wintering in fallen leaves on the ground. Avoid overhead irrigation and splashing water to prevent further spread of the disease. Occasionally, maple anthracnose may defoliate trees early in the growing season, but these trees typically are able to produce new leaves within a few weeks. Defoliated trees should be watered and properly fertilized. Ultimately, damage caused by A. apocryptum does not jeopardize the long-term health of the tree.

Chemical

Fungicides are rarely needed to manage maple anthracnose. Fungicide sprays may be used on trees with a history of the disease or on nursery stock, where aesthetic quality is key. The first fungicide application should occur at bud break and scheduled spraying should be continued through the spring, which is when the fungus is active and the leaves susceptible.

Diagnostic procedures

Symptoms of maple anthracnose caused by A. apocryptum may be confused with leaf scorch caused by drought and heat injury. The presence of conidiophores and conidia distinguish this fungal disease from symptoms of environmental stress. A. apocryptum can be distinguished from other pathogens of maple by its formation of one-celled, colorless conidia in groups of four to eight on the tips of short, broad conidiophores. Other fungi that cause anthracnose on maple, Discula campestris, Discula umbrinella, and Monostichella histeroidea, also produces acervuli, but the condia are quite distinct. Two kinds of conidia are formed by M. histeroidea: larger condia are nearly globose (15-21 x 13-15 µm), while smaller conidia are rod-shaped. Yet another pathogen of maple, Phyllosticta minima, produces pycnidia. This fungus is treated in its own fact sheet.

If diagnosis is uncertain, incubate affected tissue in a moist chamber at room temperature for three days. Keep leaf tissue elevated on a screen to prevent the sample from getting wet and overgrown with secondary organisms. Mount portions of the fungal growth on a microscope slide using sticky tape or by cutting a thin-section of leaf tissue. Isolation from leaf tissue is usually not necessary, but can be done using water agar. In culture, A. apocryptum does not form fruiting bodies but instead produces conidia on hyphal cells and lacks distinct conidiophores.

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)