The wholesale nursery industry in Tennessee contributes more than $200 million to the annual economy of the state. Flowering dogwood, Cornus florida, is a major income species of this industry and annual sales in Tennessee are in excess of $50 million (Cappiello and Shadow, 2005). Two new fungal diseases that threatened the commercial production of dogwoods were reported in the last decade of the 20th century. During the early 1990s, dogwoods grown in Tennessee and other regions of the eastern United States (Daughtrey et al., 1996) were ravaged by an epidemic of dogwood anthracnose caused by Discula destructiva (Redlin, 1991), an introduced pathogen (Trigiano et al., 1995). This disease can cause the death of seedlings in as little as one growing season, whereas older trees may survive for several years. In 1998, the University of Tennessee introduced C. florida Appalachian Spring, the only documented cultivar with strong resistance to dogwood anthracnose (Windham et al., 1998). Chemical control of this disease is very costly. Since the mid-1990s, flowering dogwoods have been also afflicted with powdery mildew. Although more than one species of a powdery mildew pathogen was initially implicated in these epidemics (McRitchie, 1994; Ranney et al., 1994), the fungus Erysiphe (sect. Microsphaera) pulchra (syn. Microsphaera pulchra) is now recognized as the cause of annual epidemics in Tennessee (Klein et al., 1998) and throughout the United States (Li et al., 2009). Powdery mildew typically does not kill dogwoods, but renders the plants unsalable and also reduces flowering and growth (Li et al., 2009).
The only option that nursery operators had to avert complete crop failure due to powdery mildew before 2005 was a comprehensive fungicide spray program that began in May and extended through September each year. Disease control costs for producing an acre of dogwoods soared to $1975 annually (Li et al., 2009). Many small, one or two person-operated nurseries could not afford increased fungicide control costs as well as the additional costs for labor associated with a mandatory expanded disease management scheme. Because of unfavorable economics, many of the smaller dogwood nurseries subsequently ceased producing flowering dogwood.
There have been only a few controlled studies of powdery mildew resistance in flowering dogwood (Hagan et al., 1998; Windham et al., 2003) and most claims of resistance have been made antidotally in various publications, including a book by Cappiello and Shadow (2005) or in nursery catalogs. Similarly, there are many trees that are purported to have supernumerary bracts, but few are common in the nursery trade. Cultivars that combine resistance to powdery mildew and supernumerary bracts have not been reported (Table 1). To our knowledge, Appalachian Joy PP 18238 is the only cultivar of flowering dogwood in the nursery trade that is highly resistant to powdery mildew and has supernumerary bracts.
Flowering dogwood (Cornus florida) cultivars and selections with either supernumerary bracts, resistance to powdery mildew, caused by Erisyphe pulchra or both.
Cappiello, P. & Shadow, D. 2005 Dogwoods. Timber Press, Portland, OR
Daughtrey, M.L., Hibben, C.R., Britton, K.O., Windham, M.T. & Redlin, S.C. 1996 Dogwood anthracnose: Understanding a disease new to North America Plant Dis. 80 349 358
Hagan, A.K., Hardin, B., Gilliam, C.H., Keever, G.J., Williams, J.D. & Eakes, J. 1998 Susceptibility of cultivars of several dogwood taxa to powdery mildew and spot anthracnose J. Environ. Hort. 16 147 151
Klein, L.A., Windham, M.T. & Trigiano, R.N. 1998 Natural occurrence of Microsphaera pulchra and Phyllactinia guttata on two Cornus species Plant Dis. 82 383 385
Li, Y., Mmbaga, M.T., Windham, A.S., Windham, M.T. & Trigiano, R.N. 2009 Powdery mildew of dogwoods: Current status and future prospects Plant Dis. 93 1084 1092
McRitchie, J.J. 1994 Powdery mildew of flowering dogwood. Plant Pathol. Circ. No. 368. Gainesville, FL
Ranney, T.G., Grand, L.F. & Knighten, J.L. 1994 Resistance of Cornus kousa taxa to dogwood anthracnose and powdery mildew. Proc. S. Nurserymen’s Assoc. Res. Conf. 41:197–199
Royal Horticulture Society 1990 R.H.S. Colour Chart. Royal Horticultural Society, London, UK
Trigiano, R.N., Caetano-Anollés, G., Bassam, B.J. & Windham, M.T. 1995 DNA amplification fingerprinting provides evidence that Discula destructiva, the cause of dogwood anthracnose in North America, is an introduced pathogen Mycologia 87 4 490 500
Wadl, P.A., Wang, X., Trigiano, A.N., Skinner, J.A., Windham, M.T., Rinehart, T.A., Reed, S.M., Pantalone, V.R. & Trigiano, R.N. 2008 Molecular identification key for cultivars and lines of Cornus florida and C. kousa based on microsatellite loci J. Amer. Soc. Hort. Sci. 133 6 783 793
Windham, M.T. 1996 Resistance to powdery mildew in flowering dogwood. Proc. S. Nurserymen’s Assoc. Res. Conf. 41:197–199
Windham, M.T., Graham, E.T., Witte, W.T., Knighten, J.L. & Trigiano, R.N. 1998 Cornus florida ‘Appalachian Spring’: A white flowering dogwood resistant to dogwood anthracnose HortScience 33 1265 1267
Windham, M.T. & Witte, W.T. 1998 Naturally occurring resistance to powdery mildew in seedlings of Cornus florida J. Environ. Hort. 16 173 175
Windham, M.T., Witte, W.T. & Trigiano, R.N. 2003 Three white-bracted cultivars of Cornus florida that are resistant to powdery mildew HortScience 38 1253 1255
Witte, W.T., Windham, M.T., Windham, A.S., Hale, F.A., Fare, D.C. & Clatterbuck, W.K. 2000 American dogwoods. Dogwoods for American gardens. Agricultural Extension Service, The Univ. of Tennessee. PB1679-30-12/00