Anthracnose Caused by Discula fraxinea on the New Host Chinese Fringetree and White Ash in Delaware
Authors: Fisher Gregory N. , Mulrooney R. P. , Rossman A. Y. , Castlebury L. A. 
Published: May 2007

Leaf blight or anthracnose symptoms have been noted on the current year's growth of Chinese fringetree (Chionanthus retusis L.) since the early 1990s in Newark, DE. Symptoms begin as dark, greenish brown, water-soaked lesions on the edges of young leaves. With time, lesions enlarge, turn darker brown, and coalesce with necrotic areas turning dry and light brown. Isolations from infected leaves consistently yielded a golden brown fungal culture on acidified potato dextrose agar. Similar symptoms were observed on white ash (Fraxinus americana L.), from which a similar fungus was isolated. C. retusis and F. americana belong to the Oleaceae, the olive family. Acervuli were inconspicuous on leaves, but conidia were easily observed. Conidia were small, non-septate, ellipsoidal, hyaline, and averaged 5.5 × 3.5 μm. In culture, the fungus formed conspicuous, concentric zones of flocculent mycelium and spherical conidiomata when exposed to diurnal light. Isolates from C. retusis and ash were identified as Discula fraxinea (Peck) Redlin & Stack, the anamorph of Gnomoniella fraxini Redlin & Stack (Gnomoniaceae, Diaporthales) (3). Sequences of the internal transcribed spacer region (ITS) of rDNA indicated that the isolates from C. retusis and white ash (GenBank Accession No. AY455810-AY455818) were conspecific with D. fraxinea isolates from Maryland and Oregon, with six or fewer base pair substitutions or insertion/deletions (indels) across all isolates. Ash anthracnose has been reported from the central and eastern United States and California, the prairie provinces in Canada, and recently from British Columbia, usually under the synonyms of Gloeosporium aridum and G. fraxineum (1,2). Koch's postulates were completed when isolates of D. fraxinea from C. retusis, green ash (F.pennsylvanica Marsh.), and white ash were inoculated onto 3- to 4-year-old trees of C. retusis and F. americana in 2000 and 2001. Seven replicate branches with emerging leaves were sprayed to runoff with a conidial suspension (60,000 conidia per ml) and covered with plastic bags for 24 h. After 20 days, 85% of C. retusis branches inoculated with a C. retusis isolate developed symptoms, and D. fraxinea was isolated from 78.6% of symptomatic leaves. Isolates from green ash in Maryland and white ash in Delaware produced symptoms on 57% of C. retusis branches. Only 37% of white ash branches inoculated with isolates from C. retusis and white ash developed symptoms, but D. fraxinea was isolated from 100% of symptomatic leaves. No symptoms developed on control branches. To our knowledge, this is the first report of D. fraxinea on C. retusis or on any member of that plant genus (1,2). In addition, D. fraxinea has not previously been reported on F. americana in Delaware. Specimens and cultures of D. fraxinea from C. retusis (BPI 746408, CBS 114058) and F. americana (BPI 746411, CBS 114051) were deposited at the U.S. National Fungus Collection and the Centraalbureau voor Schimmelcultures, The Netherlands.

References: (1) D. F. Farr et al. Fungi on Plants and Plant Products in the United States. The American Phytopathological Society, St. Paul, MN, 1989. (2) D. F. Farr et al. Fungal Databases. Systematic Botany and Mycology Laboratory, On-line publication. ARS, USDA, 2003. (3) S. C. Redlin and R. W. Stack. Mycotaxon 32:175, 1988.