Susceptibility of Catalpa, Chilopsis, and Hybrids to Powdery Mildew and Catalpa Sphinx Larvae

A diverse collection of germplasm representing 24 taxa from Catalpa sect. Catalpa Paclt and sect.MacrocatalpaGrisebach, Chilopsis D. Don, and ·Chitalpa Elias & Wisura were screened for susceptibility to powdery mildew (PM) incited by Erysiphe elevata (Burr.) U. Braun & S. Takam and feeding by catalpa sphinx larvae (CSL), Ceratomia catalpae (Boisduval). PM screening was conducted on plants grown in a lathhouse (50% shade) in 2004 and a gravel pad (100% sun) in 2005. The PM causal organism was identified as E. elevata both years. Disease incidence and severity were recorded at 2-week intervals for 6 weeks and used to calculate area under the disease progress curves (AUDPC) for each year. North American Catalpa in sect. Catalpa, Chilopsis, and ·Chitalpa taxa were all moderately to highly susceptible to PM. Chinese Catalpa in sect. Catalpa and West Indian species in sect. Macrocatalpa were resistant to PM. Hybrids between North American and Chinese Catalpa in sect. Catalpa varied in susceptibility, indicating transmission of partial resistance to PM. No differences in survival or growth were found in a no-choice feeding study with CSL reared on taxa from Chilopsis, ·Chitalpa, or either section of Catalpa. Future breeding of ·Chitalpa can use sources of PM resistance identified in this study, but a source of resistance to CSLwas not found. The genus Catalpa Scop. (Bignoniaceae Juss.) is composed of 11 species in two welldefined sections, Catalpa andMacrocatalpa, differentiated by leaf morphology and seed characteristics as well as geographic distribution (Paclt, 1952). Section Catalpa contains six species of deciduous trees with a disjunct distribution between East Asia (four species) and eastern North America (two species). All species from this section are in cultivation except C. tibetica Forrest, but only the two North American species are commonly cultivated as ornamental trees. Southern catalpa, C. bignonioides Walt., is native to the southeastern United States (Alabama to western Florida) and northern catalpa, C. speciosa (Ward. ex Barn.) Ward. ex Engelm., is native to the south-central United States. Both are cultivated as well as naturalized inmany urban areas of the eastern United States (Rehder, 1940). Section Macrocatalpa is comprised of five species of semievergreen trees restricted to the West Indies. These species are poorly represented in cultivation, with the exception of C. longissima (Jacq.) Dum.-Cours. (Haitian yokewood), which is cultivated throughout the West Indies as an important landscape and timber species and in Florida and Hawaii for landscapes (Francis, 1990). Desertwillow or desertcatalpa, Chilopsis D. Don., is a monotypic genus related to Catalpa. Chilopsis linearis (Cav.) Sweet is a small to medium-sized tree with willow-like leaves and attractive flowers in summer found in washes and arroyos in desert regions of the southwestern United States, from southern California to Texas, and south-central Mexico (Henrickson, 1985). The species and its cultivars are grown throughout its native range and adjacent regions and are valued for drought tolerance and attractive flowers (Dirr, 1998; Henrickson, 1985; Tipton, 1987). Catalpa and Chilopsis are very similar but are differentiated by number of stamens, two in Catalpa vs. four in Chilopsis, and leaf morphology, large ovate to cordate leaves in Catalpa vs. linear to lanceolate leaves in Chilopsis. Traditional classifications have placed them in the large, pan-tropical tribe Tecomeae Endl. (Henrickson, 1985). However, this tribe was shown recently to be paraphyletic, suggesting that the tribe be divided with Catalpa (both sections) and Chilopsis forming a new tribe sister to the Oroxyleae Gentry (Olmstead, unpublished data; Spangler and Olmstead, 1999). Intergeneric hybrids between Chilopsis linearis and Catalpa bignonioides bred byRusanov (1964) were introduced into the United States in 1977 and formally described by Elias and Wisura (1991) as ·Chitalpa tashkentensis. ·Chitalpa has performed well in arid climates, but suffers in more humid climates from severe powdery mildew (PM) infections (Dirr, 1998) and in eastern North America from herbivory by catalpa sphinx moth larvae, Ceratomia catalpae (Lepidoptera: Sphingidae) (T.G. Ranney, personal observation). Powdery mildews (PM) are obligate fungal parasites (Ascomycetes) characterized by epiphytic white mycelium that often cause distortions of new growth, chlorosis, necrosis, and premature leaf fall (Braun, 1987), limiting both growth and aesthetic qualities of infected plants. Seven different species of PM have been identified onCatalpa spp. with two host-specific to Catalpa: Erysiphe catalpae Simonyan and E. elevata (syn. Microsphaera elevata Burr.) (Ale-Agha et al., 2004; Braun, 1987; Braun et al., 2002; Farr et al., 1989). Of the two Catalpa-specific PMs, only E. elevata has been reported in North America, with E. catalpae currently restricted to Europe and Asia (Braun, 1987; Sinclair et al., 1987). Received for publication 24 Apr. 2006. Accepted for publication 22 July 2006. This research was funded, in part, by the North Carolina Agricultural Research Service (NCARS), Raleigh, N.C.; North Carolina Association of Nurserymen, Inc., Raleigh, N.C.; and J. Frank Schmidt Family Charitable Foundation, Boring, Ore. Use of trade names in this publication does not imply endorsement by the NCARS of products named nor criticism of similar ones not mentioned. We gratefully acknowledge Tom Ward, Arnold Arboretum, Harvard University, Jamaica Plain, Mass.; Alessandro Chiari, Brooklyn Botanic Garden, Brooklyn, N.Y.; Mary Hirshfeld, Cornell Plantations, Cornell University, Ithaca, N.Y.; Benoit Jonckheere and Don Evans, Fairchild Tropical Botanic Garden, Coral Gables, Fla.; Mark Krautman, Heritage Seedlings, Salem, Ore.; the JC Raulston Arboretum, Raleigh, N.C.; Veta Bonnewell, Morton Arboretum, Lisle, Ill.; Bill Carson, Native Texas Nursery, Austin, Texas; Andrew Bunting, Scott Arboretum, Swarthmore, Pa.; the Sir Harold Hillier Gardens, Hampshire, UK; and Dawn Stover and David Creech, Mast Arboretum, Stephen F. Austin University, Nacogdoches, Texas, for providing germplasm used in our studies. Technical assistance of Thomas Eaker, Joel Mowrey, Nathan Lynch, Irene Palmer, and staff of the Mountain Horticultural Crops Research and Extension Center is greatly appreciated. We thank William Swallow for statistical and James F. Walgenbach for entomologic advice. From a dissertation submitted by R.T.O. in partial fulfillment of the requirements for the PhD degree. Current address: Floral and Nursery Plants Research Unit, USDA-ARS, US National Arboretum, 3501 New York Ave. NE, Washington, DC. Professor. Professor emeritus. To whom reprint requests should be addressed; e-mail [email protected]. HORTSCIENCE VOL. 41(7) DECEMBER 2006 1629 JOBNAME: horts 41#7 2006 PAGE: 1 OUTPUT: October 25 00:32:17 2006 tsp/horts/127877/01585