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.
Description of the Tree
Cornus florida ‘Appalachian Joy’ is ≈4–5 m tall and broadly columnar after 10–15 years of growth that has white (Green-White Group 157D; Royal Horticulture Society, 1990) bracts with orange-red clefts (35B) (Fig. 1A and B). Bracts are stiff and overlap, and the number of bracts subtending inflorescences ranged from 4 to 8, but averaged 5.6 (n = 44). Primary bract length is 3.8 cm and width is 4.4 cm (n = 44). Inner (extra) bracts are generally smaller and are of variable size (Fig. 1A and B). Petals are yellow orange (14B), sepals are red (42D), and the number of flowers average 23 per inflorescence (n = 44). Leaves are yellow green (144A), bark is gray green (198A), fall foliage color is primarily red (45B), but displays a variety of other colors (Fig. 1C and D). ‘Appalachian Joy’ is highly resistant to powdery mildew, but has not been evaluated for other diseases such as dogwood anthracnose.
Origin and Disease Resistance Evaluation
A search for flowering dogwood germplasm resistant to powdery mildew was started in 1994 as an alternative strategy to use of fungicides for disease prevention/control. Seedlings (n = 20,922) in an abandoned nursery field near Decherd, TN, and donated to the University of Tennessee were examined for signs and symptoms of powdery mildew in Aug. 1994 and 1995. From these examinations, we selected 84 plants that appeared to be free of powdery mildew. Disease free, dormant seedlings were harvested in the fall of 1995 and then subjected to high powdery mildew disease pressure in greenhouse studies for 2 years. Twenty seedlings were highly resistant to powdery mildew and the frequency for resistance in dogwood populations was estimated to be ≈0.1% (Windham and Witte, 1998). For replicated studies, the 20 powdery mildew resistant seedlings (PMRS) were grown and propagated vegetatively at the University of Tennessee Institute of Agriculture nursery greenhouses using a protocol that ensured optimum growth of the trees (Witte et al., 2000).
In 1998 and 1999, the level of resistance in the 20 PMRS was compared with the level of resistance found in C. florida ‘Cherokee Brave’, which has red bracts, and was the only cultivar at the time to have demonstrated resistant to powdery mildew, and ‘Cloud Nine’, a white-bracted, very susceptible cultivar (Hagan et al., 1998; Windham, 1996). Resistance levels for the group of 20 PMRS was statistically similar to level of resistance found in ‘Cherokee Brave’ (Windham and Witte, 1998). Statistical analysis comparing individual lines within the group of PMRS was not possible because only a single tree of each of the 20 genotypes was available during the 2 years. In this trial, the seedling that would be eventually named ‘Appalachian Joy’ was rated as 0 (no visible signs or symptoms of powdery mildew), whereas susceptible control trees averaged a score of 5.0 with greater than 50% of the foliage exhibiting disease symptoms and signs (see Table 2 for complete description of disease assessment scale values). In a replicated trial conducted from 2003 to 2009, disease resistance of ‘Appalachian Joy’ was compared with resistance levels in ‘Jeans Appalachian Snow’ PP 13099, ‘Karen’s Appalachian Blush’ PP13165, and ‘Kay’s Appalachian Mist’ PP13098, all white cultivars that are highly resistant to powdery mildew (Windham et al., 2003), and ‘Cherokee Daybreak’, a white variegated cultivar that is highly susceptible to powdery mildew (Table 2). Powdery mildew was not observed on ‘Appalachian Joy’, whereas a low incidence (<1% foliage affected) of powdery mildew was seen on ‘Karen’s Appalachian Blush’ and ‘Kay’s Appalachian Mist’ in some years. ‘Cherokee Daybreak’ consistently had over 50% coverage of foliage with powdery mildew, and during most years of the trial, almost 100% of the foliage showed signs and symptoms of powdery mildew. The disease rating for this cultivar was statistically (P = 0.011) different according to Fisher’s exact test than all the powdery mildew resistant cultivars included in the trial. Powdery mildew on the leaves of ‘Appalachian Joy’ has occasionally been observed on trees planted in the landscape and stock blocks over the last 10 years, but only as small lesions affecting less than 2% of the foliage. Despite this very low level of disease incidence, ‘Appalachian Joy’ is the only documented white, multibracted flowering dogwood cultivar with strong resistant to powdery mildew.
Comparison of flowering dogwood (Cornus florida) cultivars for resistance to powdery mildew caused by Erispyhe pulchra.z Numbers represent the mean disease rating from 2003 to 2009.
Genomic DNA from ‘Appalachian Joy’ and eight other cultivars of flowering dogwood, including powdery mildew susceptible and resistant, and a multibracted line, was isolated and compared with 17 polymorphic microsatellite loci using the method of Wadl et al. (2008). A similarity index was generated by using Nei’s coefficient (Table 3) and a dendrogram constructed using unweighted pair group method with arithmetic mean cluster analysis (Fig. 2). The DNA analyses demonstrated that ‘Appalachian Joy’ was most similar to ‘Appalachian Blush’, but was distinctly differentiated from all cultivars included in the study.
Genetic similarity indices for nine Cornus florida cultivars using 17 C. florida microsatellite loci. Higher values indicate less genetic similarity and lower values greater genetic similarity.z
The University of Tennessee was issued a United States plant patent PP 18238 for ‘Appalachian Joy’ in 2007 based on disease resistance to powdery mildew, supernumerary bracts, and genetic differentiation from other cultivars. The patent lists A.S. Windham, M.T. Windham, and R.N. Trigiano coinventors. A dozen trees are planted at the Oak Ridge Arboretum (Forest Resources Research and Education Center, Oak Ridge, TN) and Ellington Agricultural Center (5201 Marchant Drive, Nashville, TN), and are available for propagation materials. Additional bulking of this cultivar via budding has been accomplished by selected nurseries. Please contact R.N. Trigiano (email@example.com) for additional information.
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. & 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