Arboriculture & Urban Forestry 36(3): May 2010 ease. Twenty-four pin oaks with an average DBH of 75 cm were selected for this experiment. Using the micro-injection tech- nique previously described, oxytetracycline hydrochloride was applied to twelve of the trees the week of June 8–15, 2007, at the rate of 0.14 grams a.i. in 7.5 ml water/2.5 cm tree diam- eter (label rate). Based on data from the previous year (2006), average leaf scorch was 60% for each treatment group. The two treatments (oxytetracycline hydrochloride-treated versus untreated) were replicated 12 times and the experiment was established in a randomized complete block design. Beginning with first symptom appearance, trees were evaluated for per- cent scorch symptoms at about two-week intervals on August 9 and 23; September 6 and 20; and October 4, 20, and 31, 2007. RESULTS PBZ Drench Treatment Experiment 1 In the golf course environment, from 2003 to 2006, average bac- terial leaf scorch symptoms of PBZ-treated trees decreased grad- ually from 18% to 13.7%, while average scorch of water-treated trees increased from 18.1% to 28% over the same three years. These results were not statistically significant (P = 0.05). On the golf course, annual increase in trunk diameter of water treated- trees was 0.4 cm, compared to 0.13 cm for PBZ-treated trees. Experiment 2 Along the city streets from 2003 to 2006, average bacterial leaf scorch symptoms of PBZ-treated trees increased gradually from 41% to 79.5%, while average scorch of water-treated trees increased from 42.1% to 72.5% over the same three years. Street-side pin oaks in the experiment were generally more severely diseased than those on the golf course and PBZ treatments made little impact on scorch levels. These results were not statistically significant (P = 0.05). Surfactant-Assisted Treatment Bacterial leaf scorch symptoms increased for all treatments from 2005 to 2006. If there had been no treatments, it would be expected that leaf scorch symptoms would increase gradu- ally from one year to the next in any case. Average bacterial leaf scorch ranged from 13% to 40% in 2005, and from 37% to 67% in 2006, depending on the treatment. None of the treat- ments reduced or moderated scorch levels in 2005 and in 2006. Table 3. Root flare injection treatments (Spring and Summer 2007). Treatment Date Oxytetracycline hydrochloridez Oxytetracycline hydrochloridez Oxytetracycline hydrochloridez Calcium complex oxytetracycliney Untreated control May 8 or May 9 May 29 or May 30 June 18 or June 20 June 1 - Rate 0.14 grams a.i. in 7.5 ml water/ 2.5 cm tree diameter (label rate) 0.14 grams a.i. in 7.5 ml water/ 2.5 cm tree diameter (label rate) 0.14 grams a.i. in 7.5 ml water/ 2.5 cm tree diameter (label rate) 0.13 grams a.i. in 3 ml suspension/ 2.5 cm tree diameter (label rate) - zBacastat OSC, Rainbow Treecare Scientific Advancements, Minneapolis, MN, U.S. yMycoject, Mauget Inc., Arcadia, CA, U.S. ©2010 International Society of Arboriculture 143 Where surfactant was used, grayish blue-green colored crustose lichens growing on the bark surface developed a light brown col- or and any plant foliage exposed to the chemical turned brown. Antibiotic Root Flare Injections Experiment 1 Results are presented in Figure 1. The graph represents scorch levels as a percent of the previous year’s leaf scorch, but statis- tics are based on differences in absolute scorch values. Treat- ments using antibiotics generally delayed the onset of symp- toms by approximately two to three weeks. By the end of the 2006 growing season, these same treatments resulted in less disease than was present in the trees the year before, while untreated trees were more diseased than the previous year. Experiment 2 Results are presented in Figure 2. The graph represents scorch levels as a percent of the previous year’s leaf scorch, but sta- tistics are based on differences in season-long area under the disease progress curve (AUDPC) values. For all treatments, bacterial leaf scorch symptoms gradually increased from Au- gust through October. Antibiotic treatments generally delayed the onset of symptoms by approximately two to three weeks. It appears that the late May oxytetracycline hydrochloride treatment (three weeks after first leaves were fully expanded) provided slightly better results than the early May or mid- June treatments. Calcium complex oxytetracycline treatments were not as effective as oxytetracycline hydrochloride treat- ments made on similar application dates. By mid-October, the antibiotic-treated trees showed less scorch than the same trees showed at about the same time in October 2006. At the same time, untreated trees were more diseased than the previous year. Experiment 3 Results are presented in Figure 3. The graph represents scorch levels as a percent of the previous year’s leaf scorch. Bacterial leaf scorch symptoms gradually increased from August through Octo- ber for both treatments. Oxytetracycline hydrochloride treatment generally delayed the onset of symptoms by approximately one to two weeks but these differences were not statistically significant. By mid-October, untreated trees were more diseased than the pre- vious year, while the antibiotic-treated trees showed less scorch than the same trees showed at about the same time in October 2006. Application method pressurized microinjection pressurized microinjection pressurized microinjection pressurized microinjection -
May 2010
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