144 Hartman et al.: Therapeutic Treatments to Manage Oak Bacterial Leaf Scorch Figure 1. Effect of therapeutic treatments on 2006 bacterial leaf scorch disease progress in horse farm oaks expressed as per- cent of October 18, 2005, disease levels. Actual October 18, 2005, scorch levels for each treatment group ranged from 30% to 36% (all trees previously untreated). When a treatment line equals 100%, the scorch level of the previous year (October 18, 2005) has been reached. Actual scorch levels on October 18, 2006, ranged from 24% (Oxytetracycline hydrochloride microinjection plus PBZ) to 57% (untreated control). Graph lines followed by the same lowercase letters are not significantly different ANOVA LSD mean separation (P = 0.05). Figure 3. Effect of oxytetracycline hydrochloride treatment on 2007 bacterial leaf scorch disease progress in street-side oaks expressed as percent of October 19, 2006, disease levels. Ac- tual October 19, 2006, scorch levels for each treatment group ranged from 62% to 63% (all trees untreated). When a treatment line equals 100%, the scorch level of the previous year (October 19, 2006), has been reached. Actual scorch levels on October 20, 2007, ranged from 65% (oxytetracycline hydrochloride microin- jection) to 80% (untreated control). Results were not statistically different, ANOVA LSD mean separation (P = 0.05). DISCUSSION PBZ Drench Treatment Figure 2. Effect of timing of oxytetracycline hydrochloride treat- ments on 2007 bacterial leaf scorch disease progress in horse farm oaks expressed as percent of October 18, 2006, disease levels. Actual October 18, 2006, scorch levels for each treatment group ranged from 50% to 54% (all trees previously untreated). When a treatment line equals 100%, the scorch level of the pre- vious year (October 18, 2006) has been reached. Actual scorch levels on October 16 and October 30, 2007, ranged from 37% and 51%, respectively (Oxytetracycline hydrochloride May 29 micro- injection), to 64% and 82% (untreated control). Graph lines fol- lowed by the same lowercase letters are not significantly different ANOVA LSD mean separation (P = 0.0024). On the golf course, within each group of 10 trees, scorch levels in some individual trees decreased over the three years, while others showed more scorch, but there were more increases in the water treated oaks. Thus, although PBZ application appears to benefit some individual bacterial leaf scorch-infected pin oaks, other individuals were not similarly affected; as a result, statisti- cally significant differences could not be calculated. Neverthe- less, a trend may be occurring. PBZ reduced oak trunk diameter growth and appeared to reduce terminal shoot growth. Also, on the golf course, six of 10 PBZ-treated trees appeared to have darker green foliage by the end of the experiment while only one of 10 water-treated trees showed this trait. In the street-tree infected oaks, scorch symptoms were much more advanced to begin with and treatment effects were not apparent. The bacte- riostatic effects of PBZ on X. fastidiosa noted in the laborato- ry (DeStefano et al. 2007), were not apparent in the field. Golf course pin oaks were favored with better growing conditions than street-side trees in this experiment. Growing along the edge of fairways, these trees had much more rooting space and were wa- tered regularly along with the turfgrass during periods of drought. Surfactant-Assisted Treatment Application of antibiotics or other scorch-disease-suppressing agents directly through the bark using the surfactant did not have a beneficial effect for bacterial leaf scorch disease man- agement in the present study. Although phosphite suppressed fire blight when applied to foliage of apple trees, application of phosphite to the trunk with surfactant did not suppress fire blight (Brannen and Garner 2008). Surfactant is used success- ©2010 International Society of Arboriculture
May 2010
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