Arboriculture & Urban Forestry 38(3): May 2012 Area Meter. Relative chlorophyll content was measured on four leaves from each tree with a Minolta SPAD-502 meter. Apple scab symptoms were rated three times each year, from June through mid-August, when premature leaf fall, due to the infection, had begun. Each crabapple was visually rated by two people, and the average disease severity rating (DSR) rating was used. The DSR scale used was: 0 (<5% of leaves in the canopy showing symptoms), 1 (5%–25%), 2 (26%–50%), 3 (51%–75%), 4 (76%–100%), or 5 (near 100% and substantial leaf loss). Colorado Spruce: Cytospora Canker Sixty Colorado spruce (Picea pungens) trees in #7 (22–29 L) con- tainers were purchased (Bailey Nursery, St. Paul, Minnesota, U.S.) in July 2004 and maintained in the containers at The Morton Arboretum throughout the experiment. PBZ was applied as a basal drench on July 24, 2004, at rates of 0, 1.6, and 3.2 g a.i./cm caliper. The recommend- ed label rate for Colorado spruce is 1.6 g a.i./cm. The three treatments were applied to trees using a randomized complete block design. In June 2005, half of the 12 trees in each PBZ rate group were inoculated with Leucostona kunzei, the causal agent of cytospora canker disease on blue spruce. The fungus was isolated from infected trees and maintained on potato dextrose agar (PDA) at 25°C until inoculation. Inoculation consisted of aseptically collecting a 5 mm mycelial agar plug and inserting it into a small incision made on the abaxial surface of a 1–2 cm diameter branch on each tree. A sterile PDA agar plug was in- serted into a different branch to act as a non-inoculated control. Twig elongation measurements were taken in 2005 and 2006 by averaging apical growth of lateral branch- es from the upper and lower halves of each quadrant (eight total). Growth of the leaders was also measured. Disease severity was evaluated monthly throughout the grow- ing season in 2005 and 2006 using a DSR scale of 0 to 3: 0 (healthy, no symptoms on branch), 1 (some resin and/or slight browning or purpling of needles around inoculation point), 2 (substantial browning of needles beyond inoculation point, resin still vis- ible and more pronounced), or 3 (branch completely browned). Statistical Analyses Twig growth, leaf area, and SPAD data were compared with one-way ANOVA (P ≤ 0.05) with separation of means by the Tukey method (SigmaStat 2.0, SPSS Science, Chicago, Il- linois, U.S.). DSR data was analyzed using a generalized mixed model (SAS procedure PROC MIXED, SAS 9.2), and Hochberg-adjusted P-values (SAS procedure PROC MULT- TEST, SAS 9.2). Significance is reported when P < 0.05. RESULTS AND DISCUSSION Apple Scab Apple scab severity was significantly suppressed for two years by all applied rates of PBZ (Table 1; Figure 1). Disease sup- pression improved with increasing PBZ rate (Table 1). The DSR data indicated that the disease progression was slowed in both 2002 and 2003 by all rates of PBZ applied. Less than 50% of the canopies of treated trees were symptomatic in June and July, compared with 75% to 100% of the canopies of untreated con- trol trees. Treated trees remained less symptomatic than controls 113 through the final rating in each of the first two years. By the third year, there was no disease suppression by any PBZ treatment. All rates of PBZ significantly reduced shoot growth and in- creased SPAD values (greener leaves) for all three years. Leaf surface area was reduced only in the first year. There was no dif- ference in twig growth regulation among the three PBZ rates in any year (Table 2). All of the trees in the 1.6 g and 2.4 g treatment groups, and half of the trees in the 0.8 g treatment group, had twig growth of only about 1 cm total in three years. This is compared to over 50 cm of total twig growth in the untreated controls. The growth regulation amounted to a 95% reduction compared to un- treated trees. This greater than anticipated level of regulation may have been due, at least in part, to the use of small trees. The small- est diameter tree the chemical included on the label is 10 cm DBH. Two years of apple scab disease suppression was attained on a highly susceptible cultivar of crabapple after one basal drench treatment. However, the PBZ label recommends reapplication every third year to maintain tree growth regulation. If apple scab is only suppressed for two years, there will be a third year with no disease suppression between PBZ treatments. Though scab was reduced when disease pressure was especially high (2002), 75% of the canopy still exhibited symptoms by August. Figure 1. August mean DSR for Malus ‘Radiant’ trees treated with 3 rates of paclobutrazol or control. Values represent means of 10 trees. Different letters above the bars within each group indicate a significant difference between rates. Table 1. Monthly mean DSR for Malus ‘Radiant’ trees treated with three rates of Paclobutrazol or control. Values represent means of 10 trees. Different letters after values across each row (month) indicate a significant difference between rates. June 2002 July 2002 August 2002 June 2003 July 2003 August 2003 June 2004 July 2004 August 2004 Control 3.4a 4.0a 5.0a 3.5a 3.7a 3.8a 4.0a 4.4a 4.9a 0.8 g PBZ 1.5b 2.6b 4.7ab 1.6b 2.0b 2.8b 4.0a 4.2a 4.6a 1.6 g PBZ 1.3bc 1.5c 4.1b 1.1b 1.4bc 2.6bc 3.9a 4.1a 4.5a 2.4 g PBZ 0.5c 1.1c 3.8b 0.9b 1.0c 1.7c 3.7a 4.0a 4.0a ©2012 International Society of Arboriculture
May 2012
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