208 Miller and Danielson: Relative Susceptibility Among Arborvitae Cultivars to Arborvitae Leafminer Statistical Analysis Measures of defoliation as a function of mean percent infested branch tips (MIBTs) for tree height, growth habit, cardinal direction, and canopy density, were subjected to analysis of variance (ANOVA) using cultivar as the main effect. Percent infested branch tips (IBTs) were arcsine transformed before analysis to correct for non-normality. Where significance was found, means were compared with a Student-Newman- Keuls (SNK) multiple comparison test. Where significant, a Pearson Product Moment correla- tion analysis was performed in some cases. All data are presented as means ± standard error (SEM). All data were analyzed using SigmaStat for Windows® (Jandel Scientific 1992). Data and analyses are shown in Table 2 through Table 8. RESULTS Species/Cultivar Significant differences (F = 18.8; P < 0.001) for MIBTs were found among cultivars (Table 2). Thuja occidentalis ‘Hetz Midget’ and ‘Hoopesii’ showed significantly fewer IBTs (3% ± 0.6) com- pared to all other species/cultivars. Thuja occi- dentalis ‘Holmstrup’, T. plicata ‘Fastigiata’, T. oc- cidentalis ‘Spiralis’, T. occidentalis ‘Gracilis’, and T. occidentalis ‘Techny’ were intermediate in infestation with 10%–20% IBTs. Thuja occidentalis cultivars ‘Aurea’, ‘Douglasii Aurea’, ‘Globosa’, ‘Hetz Wintergreen’, ‘Wagneri’, ‘Wareana’, and T. plicata had between 20% and 29% IBTs. Trees with the greater than 30% IBTs included the straight species T. occidentalis, and T. occidentalis cultivars ‘Um- braculifera’ and ‘Waxen’. The cultivar T. occiden- talis ‘Smaragd’ was highly infested with 42% IBTs. Tree Height There was a highly significant difference (F = 5.8; P < 0.001) between IBTs on short trees (<4 m) versus tall trees (>4 m) (Table 3). Short trees had significantly less damage (16% infested tips) compared to tall trees (24% infested tips). In ad- dition, there was a weak but significant direct correlation [R2 = 0.36, P = 0.009, IBT = 4.82 + (12.91 × tree height)] between tree height and IBTs. The Morton Arboretum’s plant records ©2017 International Society of Arboriculture indicated that all of the plants in the study were well-established, having been planted between 30 and 80 years prior to this study, and that plants within a given cultivar and/or spe- cies were of approximately the same height. Table 2. Mean percentage infested branch tips (IBTs) for all species/cultivar, 2000–2001 ± SEM. Taxa T. occidentalis T. occidentalis ‘Aurea’ T. occidentalis ‘Douglasii Aurea’ T. occidentalis ‘Globosa’ T. occidentalis ‘Gracilis’ T. occidentalis ‘Hetz Midget’ T. occidentalis ‘Hetz Wintergreen’ T. occidentalis ‘Holmstrup’ T. occidentalis ‘Hoopesii’ T. occidentalis ‘Smaragd’ T. occidentalis ‘Spiralis’ T. occidentalis ‘Techny’ T. occidentalis ‘Umbraculifera’ T. occidentalis ‘Wagneri’ T. occidentalis ‘Wareana’ T. occidentalis ‘Waxen’ T. plicata T. plicata ‘Fastigiata’ Mean % infested branch tipsz 36d ± 2.75 23c ± 1.76 29c ± 3.05 22c ± 2.36 16b ± 1.96 3a ± 0.60 21c ± 1.93 11b ± 1.71 6a ± 0.76 42d ± 2.42 14b ± 1.07 19b ± 2.14 31cd ± 3.07 20c ± 1.78 21c ± 1.13 31cd ± 1.90 29c ± 2.70 15b ± 1.47 z Values within columns followed by the same letter are not significantly dif- ferent (P < 0.05, Student-Newman-Keuls multiple comparison test). Table 3. Mean percent infested tips (IBTs) for all species/ cultivars by plant height, 2000–2001 ± SEM. Tree height Short Tall % infested tipsz 16a ± 1.24 24b ± 0.67 z Values within columns followed by the same letter are not significantly different (P < 0.05, t-test). Canopy Zone There was only a slight but statistically significant (F = 4.0; P = 0.003) difference in IBTs between the upper canopy zone (21% infested tips) and the lower canopy zone (25% infested tips). The middle zone was intermediate (24%) in IBTs (Table 4). Height and Zone of Tree When comparing infestation levels among tall and short trees among three canopy zones, short trees had a mean of 8% fewer IBTs compared to taller trees with significant differences (F = 14.3; P < 0.001) among height and zone classes. Short trees (<4 m) had significantly more damage in the up- per zone (24%) and tall trees had greater damage, 30% and 26%, in the lower and middle zones, re-
November 2017
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