2 Mmbaga et al.: Multiple Disease Resistance in Lilacs 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 assess- ment was performed by the same research scientist through- out the experiment. Although several readings were taken during the growing season, readings taken in May targeted bacterial blight incidence. In August, powdery mildew and Alternaria 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. Each accession was evaluated for susceptibility based on mean disease readings collected during each growing season. Plants were categorized as resistant (R), moderately resistant (MR), moderately susceptible (MS), or susceptible (S), where 0 to 1.0 (R), 1.1 to 2.0 (MR), 2.1 to 2.9 (MS), and 3.0 to 5.0 (S). Due to variations in climactic condition among the seasons, the final rating used for each accession was the highest one obtained. Because Alternaria blight was first observed in middle spores/mL suspensions in distilled water to inoculate the four different taxa. Control plants were sprayed with distilled water. The following lilacs were assayed: S. prestoniae ‘Isabella’, S. prestoniae ‘James McFarlane’, S. meyeri ‘Dwarf Korean’, and S. vulgaris (un- named common lilac). Tennessee during summer 1996, proof of pathogenicity tests were performed using Koch’s rules (Koch 1882; Smith 1905). Once performed, 7-day-old cultures on potato dextrose agar (Barnett and Hunter 1998) were used to prepare 106 in PROC ANOVA (Gomez and Gomez 1984; SAS/STAT 1990). The least significant differences (LSD) were calcu- lated according to Fisher’s protected LSD test at P ≤ 0.05. Means followed by different letters demonstrated a statisti- cal significant difference. RESULTS Disease Development and Evaluation The first symptoms of bacterial blight caused by P. syringae pv. syringae were observed in April, soon after budbreak, and developed in severity until early to mid-May. Symptoms consisted of dark brown lesions on leaves and young stems. Actively growing plant parts were girdled, resulting in the death of terminal twigs. Infected leaves shriveled, as did most shoots and flower clusters (Figure 1a). New symptoms appeared throughout June, after which the infection rate began to decline. Blighted plants later recovered by produc- ing new leaves, masking the initial damage. Throughout the study, bacterial blight symptoms were most prominent during the month of May. To confirm the causal agent of this disease, P. syringae pv. syringae was routinely isolated from blighted twigs and leaves. During this study, a total of 12 cultivars were found to be highly resistant and 15 to be susceptible to moderately susceptible (Table 1*). Although the overall mean disease reading for S. vulgaris ‘Avalanche’, S. vulgaris ‘Marie Legraye’, and S. reticulata ‘Ivory Silk’ were low (0.0 to 2.0), they were categorized as moder- ately susceptible because they displayed high infection in one of the years of the study. During that year, they received scores higher than 2.5. Similarly, S. vulgaris ‘Harry Bickle’, S. vulgaris ‘Silver King’, S. hyacinthiflora ‘Excel’, and S. reticulata ‘Summer Snow’ were categorized as susceptible because they had disease readings of higher than 3 during one year (Table 1). Inoculated plants were incubated in a moist chamber at 28°C (82°F) and 100% relative humidity for 24 hours and subsequently moved to a greenhouse maintained at 25°C ± 4°C (77°F ±7°F). Disease symptoms were recorded 10 weeks later. A randomized complete block design with four replications of single-tree per treatment was used. Statistical Analyses All statistical analyses were performed as appropriate to the experimental design using the SAS (Statistical Analysis Systems, Inc., Cary, NC) general linear models procedure (Schlotzauer and Littell 1987). Multiple comparisons between pairs of mean disease severity from different treatments using a series of t-tests followed SAS procedures ©2005 International Society of Arboriculture Symptoms of powdery mildew began to appear in July (1997, 1998, and 2002) and in June (1996). In response to humidity gradients, disease symptoms first appeared on the lower leaves then progressed to upper plant parts. Powdery mildew symptoms persisted throughout the growing season and increased in severity over time to reach their highest level in August (Figure 1b). A total of 29 cultivars (S. meyeri, S. prestoniae, S. josiflexa, S. hyacinthiflora, S. reticulata, S. microphylla, S. patula, S. henryi, and 13 cultivars of S. vulgaris) were resistant to powdery mildew; 19 cultivars of S. vulgaris and one of S. chinensis were susceptible or moderately susceptible (Table 2). Although some defoliation associated with severe powdery mildew infection (data not shown) was observed, the effect of this disease is mostly aesthetic and caused no significant difference in growth. Beginning in mid-June to early July, severe leaf scorching *Tables for this article appear on pp. 7–9.
January 2005
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