James A. Chatfield, Ohio State University Extension, Northeast District/Horticulture and Crop Science; Nancy A. Taylor, Plant Pathology, C. Wayne Ellett Plant and Pest Diagnostic Clinic, Ohio State University; Erik A. Draper, Ohio State University Extension, Geauga County; and Joseph F. Boggs, Ohio State University Extension, Hamilton County/Southwest District.
In an article in the January 2002 issue of the Journal of Arboriculture titled "Variation in Field Susceptibility of Native and Exotic Ash Species to Anthracnose," Karel Jacobs and Donna Danielson, of the Morton Arboretum near Chicago, reported their 1997 to 1999 studies of ash anthracnose (pathogen: Gnomoniella fraxini). The article is quite interesting because of its relevance to the interplay of host resistance and environmental factors in the disease triangle.
Eight ash species were tested, and greatest susceptibility to ash anthracnose was demonstrated by Fraxinus chinensis, followed by F. angustifolia (Syrian ash), F. pennsylvanica (green ash), and to a lesser degree, F. mandshurica (Manchurian ash) which was similar to F. excelsior (European ash).
Pumpkin ash (F. tomentosa) and white ash (F.americana) were comparatively tolerant and blue ash (F. quadrangulata) showed virtually no symptoms. A general trend was noted that "trees that leafed out earlier developed more disease than those that leafed out later."
Popular white ash cultivars such as 'Autumn Purple' and 'Autumn Applause' had quite low anthracnose ratings; in fact, let's give a hand to 'Autumn Applause' which had no anthracnose symptoms for any of the three years tested.
In conclusion, to quote from the article:
"Green ash is typically considered more resistant to anthracnose and white ash more susceptible." [Conversely] "Our findings… green ash appears more susceptible than white ash, although differences may be minimized in years of low disease pressure…When data from all trees were analyzed collectively, individuals that leafed out early also tended to develop more disease than those that leafed out later. These findings suggest that ontogenic resistance may operate in ash anthracnose whereby young, succulent leaves are more susceptible to infection than fully expanded leaves…This data should assist in targeting ash species for breeding programs, and in particular, suggest that the North American native blue ash would be especially promising as a source of resistance."