70 Blaedow et al.: Effect of Paclobutrazol on Apple Scab In 2003, the year after application of PBZ, disease inci- Figure 3. Average incidence of apple scab (%) on shoot leaves of ‘Snow Drift’ crabapple. Treatments consisted of a control [basal drench with 500 mL (15 fl oz) of water], 1 × PBZ [a basal drench with PBZ, 0.79 g a.i. diameter cm−1 , in 500 mL (15 fl oz) of water], or 2 × PBZ [a basal drench with PBZ, 1.58 g a.i. diameter cm−1 , in 500 mL (15 fl oz) of water]. Observations were made 75 days (A), 428 days (B), and 800 days (C) after treatment application (1 April 2002). Lower-case letters indicate significant differences between means for each evaluation date ( = 0.05). during the hot and dry summer months when secondary in- fections did not occur. Trees treated with PBZ produced new leaves at a lower rate, accounting for the higher disease in- cidence in these treatments. dence was significantly lower in PBZ-treated ‘Hopa’ com- pared to the control trees (Figure 4B). A similar response was not observed for the ‘Snow Drift’ cultivar (Figure 3B). In 2004, although the 1 × PBZ rate had no significant effect on disease incidence in ‘Snow Drift,’ trees treated at twice the recommended rate had a significantly reduced incidence of scab (Figure 3C). No difference was observed between treat- ments in the ‘Hopa’ cultivar in 2004 (Figure 4C). Although some significant differences in disease incidence between treatments were seen during the study, the reduction in apple scab incidence was insufficient to prevent near-total defolia- tion of all trees by the end of July, suggesting that PBZ, when applied as a basal drench, could not be used to control apple scab in these cultivars. Growth assessment of the ‘Indian Magic’ whips 64 days after planting showed that of the six treatments, only the foliar applications (treatments 4 and 5) of PBZ reduced shoot growth (Figure 5). The whips that had received treatments 1, 2, and 6 (control, PBZ basal drench, and one-time treatment with propiconizole after infection, respectively) had a higher disease incidence 53 days after planting than those that had received treatments 3, 4, and 5 (the standard spray program with propiconizole and the foliar applications of PBZ 4 days after budbreak or after heavy infection, respectively) (Figure Figure 5. Average annual terminal shoot growth (cm) of ‘Indian Magic’ crabapple saplings 64 days after planting (22 June 2002). Treatments consisted of (1) control; (2) soil drench with PBZ (0.79 g a.i. diameter cm−1 ) at time of Figure 4. Average incidence of apple scab (%) on shoot leaves of ‘Hopa’ crabapple. Treatments consisted of a control [basal drench with 500 mL (15 fl oz) of water], 1× PBZ [a basal drench with PBZ, 0.79 g a.i. diameter cm−1 , in 500 mL (15 fl oz) of of water], or 2 × PBZ [a basal drench with PBZ, 1.58 g a.i. diameter cm−1 , in 500 mL (15 fl oz) of water]. Observations were made 75 days (A), 428 days (B), and 800 days (C) after treatment application (1 April 2002). Lower-case letters indicate significant differences between means for each evaluation date ( = 0.05). ©2006 International Society of Arboriculture planting; (3) foliar application of propiconizole (0.016% by volume) every 2 weeks beginning 4 days after bud- break (2 May 2002); (4) foliar application of PBZ (0.02 g mL−1 plication of PBZ (0.02 g mL−1 ) 4 days after budbreak (2 May 2002); (5) foliar ap- ) 4 days after the first heavy infection period (15 May 2002); and (6) foliar application of propiconizole (0.016% by volume) 4 days after the first heavy infection period (15 May 2002). Lower-case letters indicate significant differences between means ( = 0.05).
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