314 Garbelotto et al.: Controlling Sudden Oak Death the three isolates in treated trees. Results of the comparative analyses among isolates for Expts. 1 through 4 were, respec- tively, as follows (df2 for all four trials): Fratio1.7 and P0.19; Fratio0.46 and P0.64; Fratio0.47 and P 0.62; and Fratio 0.49 and P 0.61. Figure 3 indicates that a delay in treatment resulted in a decrease of average efficacy in slowing the growth of the pathogen. An inverse correlation was found (Y 112.44– 0.43X, R2 0.94, P 0.0013) between efficacy of treatment and time of treatment since inoculation. Average efficacy of treatment was 95%, 56%, 44%, 30%, 35%, and 5% for injections administered at 60, 120, 156, 168, 192, and 264 hr after inoculation. No phytotoxicity was ever ob- served with injection treatments at the rates of active ingre- dient used for this study. Therapeutic foliar sprays slowed growth of the pathogen as evidenced by the smaller lesion size in trees whose crowns were sprayed with phosphites, but the reduction was limited. The effect on trees was inconsis- tent as indicated by the large variation in response among trees, and there was no strong statistical difference between lesion size of treated versus untreated trees. Mild to severe phytotoxicity with abundant leaf scorching was noticed in all but one experiment. Preventive injection treatments were al- ways successful in consistently and durably controlling dis- ease. Preventive foliar treatments resulted in a temporary re- duction in pathogen growth as evidenced by smaller lesion sizes. In Expt. 6 after 2 weeks, lesion size was 0% and 31% of untreated controls for preventive injection and foliar treatments, respectively. Four weeks after treatment, lesion size was 6% and 62% of untreated controls for preventive injections and foliar treatments, respectively. In Expt. 6, all positive effects of foliar treatments disappeared after 8 weeks, whereas injections were still effectively reducing growth of the pathogen (Table 1). Eight months after treat- ment (Expt. 5), there remained a significant reduction in le- sion size of trees that had been injected with phosphites. Phytotoxicity on coast live oak foliage was observed after preventive foliar treatments with phosphite + surfactant so- lutions. Topical applications of phosphites and phosphites + sur- factant did not cause any significant reduction in lesion size associated with growth of the pathogen. Expt. 2 shows that lesion size in trees whose bark had been treated therapeuti- cally was indistinguishable from lesion size of untreated trees. However, when an organosilicate surfactant (Pentra- bark™) was mixed with the phosphite solution, topical ap- plication on the bark was as successful as preventive injec- tions, and lesion sizes in treated trees were indistinguishable from the negative control. Soil drenches failed to produce any significant results as shown by results of Expts. 1 and 4 (Table 1; Figure 1). ©2007 International Society of Arboriculture DISCUSSION Administration of phosphites to coast live oak potted trees resulted in significant reduction, or even arrest, of lesion growth caused by P. ramorum in planta. It has been observed that trees affected by the typical terminal sudden oak death symptom, e.g., sudden necrosis of the entire crown, are girdled or almost completely girdled along the entire circum- ference of the trunk. In most cases, the girdling occurs ≈1m (≈3.3 ft) above the root collar, but girdling cankers have been reported at all heights on the main stem. We believe that plants affected by smaller lesions may survive for longer periods of time. When trees’ growth rate surpasses growth rate of the pathogen, healing of the lesion should occur. Phosphite treatments may lead to an increase of percentage of healing cankers attributable to their dual ability to slow the growth of the pathogen while enhancing growth of the plant host and compartmentalization of lesions caused by P. ramorum. Not all application methods were successful. Soil drenches and topical bark applications of phosphites without additives yielded no significant reduction of disease levels. Foliar ap- plications resulted in a trend toward smaller lesions, but re- sults were inconsistent among trials, and mild to extreme leaf scorching and twig dieback was noted, indicating phytotox- icity was a side effect of such treatment. Furthermore, data from Expt. 6 indicated the beneficial effects of foliar treat- ments are short-lived and are lost after only 8 weeks (Table 1). Preventive injections and bark application of phosphites amended with the organosilicate Pentrabark™were the most effective and consistent treatments. Injections were shown to be effective up to 8 months after application (Table 1). In all trials, lesions in trees treated according to either one of the two successful methods described were indistinguishable from lesions of negative controls, i.e., trees that were wounded but only mock-inoculated with a plug of sterile agar. No phytotoxicity was ever observed associated with either of the two effective treatment approaches, but it should be highlighted that if the solution for bark application is applied on plant leaves, it will completely burn the vegetative tissue. If valuable plants are around the tree, we suggest protecting them with a nylon sheet or a plastic tarp. Injections on adult trees should be done at a frequency of 15 cm (6 in) across the circumference. Colored plastic screw- cap plugs can be inserted in the hole drilled for injection. Because no product is dispersed in the environment, injec- tions are extremely appropriate for public areas. On the other hand, injections require trained professionals and cannot be performed on all trees. Injections can be easily performed on young cylindrical trees, but as trees grow older, imperfections such as knots, irregular grain, cavitation, and embedded branch stubs may result in failed injection. Injections were
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