MULTIPLE DISEASE RESISTANCE TO POWDERY MILDEW, BACTERIAL BLIGHT, AND ALTERNARIA BLIGHT IN LILACS (SYRINGA SPP.)

Journal of Arboriculture, Jan 2005 by Mmbaga, Margaret T, Sauvé, Roger J, Nnodu, Emmanuel, Zhou, Suping

Abstract. Fifty-six lilac accessions were evaluated in a 4-year study for resistance to powdery mildew caused by Microsphaera syringae, bacterial blight caused by Pseudomonas syringae pv. syringae, and Alternaria blight caused by Alternaria alternata. Accessions included 39 cultivars of Syringa vulgaris, four of S. prestoniae, three of S. hyacinthiflora, two of S. josiflexa, two of S. meyeri, two of S. reticulata, and one each of S. patula, S. chinensis, S. henryi, and S. microphylla. Of these, six accessions were resistant or moderately resistant to Alternaria blight and powdery mildew, four to bacterial blight and Alternaria blight, and 20 to powdery mildew and bacterial blight. Two cultivars of S. meyeri, 'Dwarf Korean' and 'Palibin', were resistant to all three pathogens. During this study, the most serious disease of lilacs in middle Tennessee, U.S., was Alternaria blight. It caused severe leaf scorching and defoliation that began in early July. Powdery mildew and bacterial blight caused mostly superficial damage with little effect on growth.

Key Words. Alternaria alternata; disease complex; Microsphaera syringae; plant health care; Pseudomonas syringae pv. syringae; Syringa spp.; variety selection.

Lilacs (Syringae spp.) have long been favorites landscape plants in North America. These ornamental shrubs and small trees, native of Asia and southeastern Europe, are easy to grow and can display showy, fragrant flowers and attractive foliage. Most are susceptible to powdery mildew caused by Microsphaem syringae (Sinclair et al. 1987). This disease is a nonlethal but aesthetic persistent problem in the southeastern region of the United States. It reduces the salability of lilacs due to the powdery appearance of mycelia, conidiophores, and conidia that develop on the surfaces of leaves and shoots. Although uncommon, severe infection can result in leaf necrosis, leaf distortion, chlorosis, premature leaf drop, and decreased growth (Clement et al. 1994). Although some resistant cultivars are available, they are not widely used (Hibben et al. 1977; Jones 1986; Clement et al. 1994; Widham et al. 1995).

Severe infection of Pseudomonas syringae pv. syringae, the causal agent of bacterial blight, can be a destructive disease in most Chinese, Japanese, Persian, and common lilacs (Sinclair et al. 1987). Bacterial blight commonly occurs during the early spring when the weather is cool and wet. Actively growing tissue is killed, resulting in the dieback of young shoots and flower clusters (Sinclair et al. 1987; Pschedt and Moorman 2001). Infected leaf petioles and succulent stems develop lesions, droop, wither, and turn brown. Maturing stems may remain erect and may develop brown to black streaks, while mature stems usually remain disease free.

During midsummer 1996, a leaf blight disease caused by Alternaria alternata was first observed in middle Tennessee (Mmbaga and Sheng 1997). Since then, it has increased in severity and now causes significant damage on many lilac cultivars (Mmbaga et al. 2003). Symptoms of this disease consist of brown necrotic lesions that olten coalesce to form large blotches. Lesions may develop concentric rings with ash-colored center zones or remain irregular-shaped blotches without rings. Infected leaves of most cultivars evaluated suddenly wilted, died, and dropped off quickly. In others, a yellow halo developed around each lesion before wilting.

The objective of this study was to identify lilac taxa resistant to powdery mildew, bacterial blight, and Alternaria blight and to a disease complex involving a combination of these diseases.

MATERIALS AND METHODS

Germplasm Evaluation for Resistance to Multiple Diseases

A total of 56 cultivars of nine species were evaluated. These included 39 cultivars of S. vulgaris, two of S. meyeri, one of S. patula, four of S. prestoniae, two of S. josiflexa, three of S. hyacinthiflora, two of S. reticulata, one of S. henryi, one of S. chinmsis, and one of S. microphylla. Accessions were obtained from commercial nurseries. Plants were established in May 1994 at the Tennessee State University Nursery Crop Research Center in McMinnville, Tennessee, using 1.83 m (5.9 ft) within-row plant spacing and 2.4m (7.9 ft) between-row spacing. All plants were irrigated using drip irrigation whenever needed and fertilized during the first week of May of each growing season with a controlled-release fertilizer (14 N-14 P-14 K) at the rate of 41.7 g/m^sup 2^ (1.8 oz/yd^sup 2^). The experimental design consisted of a randomized complete block design with five replicates for each cultivar in which each cultivar was represented by one tree in each of the five rows.

Disease Development and Evaluation

Previously infected plant parts and infested leaf debris provided the primary source of inoculum for the study. No additional inoculum was applied. Disease severity of bacterial and fungal plant pathogens was evaluated at monthly intervals. The study was initiated in early May and terminated in October of each growing season. The diseases rating of Horsfall-Barratt (1945) was used, with 1 = 1% to 10%, 2=11% to 25%, 3 = 26% to 50%, 4 = 51% to 75%, and 5 = 76% to 100% of plant foliage showing signs or disease symptoms. For consistent readings, disease assessment was performed by the same research scientist throughout the experiment. Although several readings were taken during the growing season, readings taken in May targeted bacterial blight incidence. In August, powdery mildew and Allernaria blight incidence are reported. Both powdery mildew and Alternaria blight had the greatest visible impact in those months. During the study, a few cultivars in the collection were killed; thus, only cultivars that were rated during each season are reported in this article.