4 Mmbaga et al.: Multiple Disease Resistance in Lilacs (S. vulgaris). Resistance to both powdery mildew and bacterial blight would allow for attractive foliage following flowering. Alternaria blight can have a devastating effect on the foliage (Figure 1c); if this disease occurs in conjunction with powdery mildew and/or bacterial blight, plants become much more unsightly by midsummer. Powdery mildew and Alternaria blight were favored by the commonly warm and humid weather conditions of Tennessee. Throughout the study, Alternaria blight symp- toms initially appeared on the upper or middle parts of the plants, sometimes on one side of the plant and subsequently spreading over the entire plant. Powdery mildew symptoms began on the lower leaves, progressing toward the upper parts. The mode of infection is typical of plant pathogens that persist between seasons on infected plants. Pschedt and Moorman (2001) reported that the bacterial blight patho- gen in lilacs can overseason on infected twigs or as epi- phytes on healthy-appearing wood. It has not been demonstrated that powdery mildew can overseason, but it is believed to survive on infected leaf debris. Powdery mildew of lilacs forms abundant ascocarps beginning in July and germinates the following spring by liberating the ascospores. These ascospores apparently serve as the primary inoculum to initiate the disease cycle under favorable weather conditions. Variation in disease severity during a growing season is dependent on the prevailing temperature and relative humidity conditions and may change from season to season because temperature and humidity will affect the production and dispersal of secondary inoculum. Many species of Alternaria are routinely found on decomposing plant material and on the leaf surfaces of most trees and shrubs (Sinclair et al. 1987). Most are nonpatho- genic (Rotem 1994). The Alternaria isolate that we obtained from lilac was aggressive and highly pathogenic on suscep- tible lilacs (Mmbaga and Sheng 1987). Based on its morphol- ogy and cultural characteristics, this isolate was identified as A. alternata (Anderson and Thrane 1996). This species has a wide host range, causing leaf spots, blights, and blossom and fruit rot on many plant parts (Rotem 1994). More than 380 hosts have been recorded in the USDA Systematic Botany and Mycology Fungus–Host Distribution Database (http:// nt.ars-grin.gov). This genus is also considered to be one of the most important allergenic molds in the United States (Pharmacia Diagnostics 1992). Additional studies using RAPD-PCR analysis are ongoing to confirm our identifica- tion of this pathogen (Weir et al. 1998). We observed a wide difference in susceptibility among the lilac taxa in the study. In many cultivars, Alternaria blight was more damaging than bacterial blight or powdery mildew, and the combination as a disease complex with powdery mildew was synergistic. In those taxa, the pathological reaction to the two diseases as a complex was more severe than expected. ©2005 International Society of Arboriculture This study provides information on multiple disease resistance to the disease complex of powdery mildew, bacterial blight, and Alternaria blight and provides informa- tion on the performance of 56 commercial cultivars (Table 4). A few taxa had multiple disease resistance, while others were susceptible to all diseases (Table 4). Because multiple disease resistance occurs in this genus, new cultivars with multiple resistances could be developed by conventional breeding methods. LITERATURE CITED Anderson, B. and U. Thrane. 1996. Differentiation of Alternaria infectoria and Alternaria alternata based on morphology, metabolite profiles, and cultural characteristics. Can. J. Microbiol. 42:685–689. Barnett, H.L., and B.B. Hunter. 1998. Illustrated Genera of Imperfect Fungi. APS Press, St. Paul, MN. 217 pp. Clement, D.L., S.A. Gill, and W. Potts. 1994. Alternatives for powdery mildew control on lilacs. J. Arboric. 20:227–230. Gomez, K.A., and A.A. Gomez. 1984. Statistical Procedures for Agricultural Research. John Wiley and Sons, New York, NY. 679 pp. Hibben, C.R., J.T. Walker, and J.R. Allison 1977. Powdery mildew ratings of lilac species and cultivars. Plant Dis. Rep. 55:475–478. Hildebrand, D.C., M.N. Schoroth, and D.C. Sands. 1988. Psudomonads, pp 60–80. In Schaad, N.W. (Ed.). Laboratory Guide for the Identification of Plant Pathogenic Bacteria. APS Press, St. Paul, MN. 158 pp. Horsfall, J.G., and R.W. Barratt. 1945. An improved grading system for measuring plant disease. Phytopathology 35:655. Jones, R.K. 1986. Powdery mildew, pp 24–25. In Jones, R.K. and R.C. Lambe. (Eds.). Diseases of Woody Ornamental Plants and Their Control in Nurseries. North Carolina Agricultural Extension Service, Raleigh, NC. 130 pp. Koch, R. 1882. Über die Midzbrandimpfung: Eine Entgegnung auf den von Pasteur in Genf Gehaltenen Vortrag. Theodor Fischer, Kassel and Berlin, Germany. Mmbaga, M.T., and H. Sheng. 1997. Evaluation of lilac (Syringa spp.) for multiple disease resistance to powdery mildew and bacterial blight in McMinnville, TN. Proc. South. Nurserymen’s Assoc. Res. Conf. 42:512–518. Mmbaga, M.T., E. Nnodu, and R.J. Sauvé. 2003. Alternaria blight in lilac. Proc. South. Nurserymen’s Assoc. Res. Conf. 48:240–242. Pharmacia Diagnostics. 1992. Alternaria alternata (A. tenius). www.unicapinvitrosight.com/templates/Allergens.asp?id= 2204. Pschedt, J.W., and G.W. Moorman. 2001. Lilac diseases. In Jones, R.K., and D.M. Benson (Eds.). Diseases of Woody Ornamentals and Trees in Nurseries. APS Press, St. Paul, MN. 482 pp.
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