68 Blaedow et al.: Effect of Paclobutrazol on Apple Scab spot count per leaf was 14.4 on control plants and 2.4 on plants soil drenched with PBZ. PBZ and several other triazole growth regulators significantly reduced the severity of pow- dery mildew on Phlox paniculata and Rudbeckia hirta (Hill and Latimer 2004). Mycelial growth and spore germination of eight woody plant pathogens were inhibited by PBZ in vitro (Jacobs and Berg 2000). Fungi tested included the oak wilt fungus Ceratocystis fagacearum and Sphaeropsis sap- inea, which causes shoot and tip blight of red pine. Both C. fagacearum and the butternut pathogen Sirococcus clavigig- nenti-juglandacearum were found to be particularly sensitive to the compound (Jacobs and Berg 2000), whereas vesicular- arbuscular mychorrizal fungi were not adversely affected (Michelini et al. 1989). Morphological or physiological changes in host plants in- duced by tree growth retardants, rather than fungicidal activ- ity, have also been suggested as the cause of the reduced incidence of fungal disease (Davis and Dimond 1952; Beck- man 1957; Pennypacker et al. 1982; B. Fraedrich, Bartlett Tree Research Laboratories, pers. comm.). The potential for PBZ to act as a systemic fungicide, along with its persistence in plants (ranging from 2 to 5 years), would make it a unique tool in controlling a wide variety of woody plant diseases. The purported benefits of protection from fungal disease and other environmental stresses, along with tree growth regulating activity, make PBZ a potentially valuable tool in the management and maintenance of trees in urban areas (Davis and Curry 1991; Fletcher et al. 1999; Chaney 2001). We recently investigated the efficacy of PBZ for the control of apple scab, a primarily foliar disease caus- ing leaf lesions and premature defoliation on susceptible Ma- lus species. MATERIALS AND METHODS Mature crabapples (Malus spp.) of two different varieties, ‘Hopa’ and ‘Snow Drift,’ growing at the Purdue University Horticulture Farm in West Lafayette, Indiana, U.S. were treated with PBZ using the basal drench method, and the natural occurrence of apple scab was monitored. The trees were uniformly spaced approximately 4.5 m (14.9 ft) apart in an orchard-like plantation. All treatments were made on 1 April 2002. Soil was removed around the base of trees to expose root flares, creating a shallow reservoir into which the PBZ suspension was applied. Treatments consisted of (1) basal drench with 500 mL (15 fl oz) of water to serve as the control; (2) a basal drench with PBZ at the recommended rate (0.79 g a.i. diameter cm−1) in 500 mL (15 fl oz) of water (1 × PBZ); or (3) a basal drench with PBZ at twice the recom- mended rate (1.58 g a.i. diameter cm−1) in 500 mL (15 fl oz) of water (2 × PBZ). All trees with a stem caliper of 11 to 15 cm (4.4 to 6 in) were assigned to one of the three treatments in a randomized complete block design consisting of 23 and ©2006 International Society of Arboriculture 10 replications of the ‘Hopa’ (69 trees) and ‘Snow Drift’ (30 trees) varieties, respectively. Annual terminal shoot growth was measured to verify the presence and activity of PBZ in the crown. Average annual terminal shoot growth was calculated for each cultivar by measuring the length of the five longest terminal shoots from five randomly selected trees in each treatment. Shoot growth measurements were taken at the end of the growing season in 2002 (210 days after treatment) and during the disease as- sessments in 2003 and 2004 (428 and 800 days after treat- ment, respectively). The occurrence of apple scab was monitored on the leaves of terminal shoots for 26 months after the application of PBZ, allowing adequate time for uptake and translocation of the triazole growth retardant to the foliage. Observations were made periodically during the year of treatment (50, 57, and 75 days after treatment). However, by early July near-total de- foliation of all trees in the orchard prevented further assess- ments in 2002. Disease incidence was assessed again in June 2003 (428 days after treatment) and for a final time in June 2004 (800 days after treatment). Wet and cool conditions early in the spring of 2002 through 2004 resulted in high disease incidence for both cultivars each year. Disease inci- dence (measured as the percentage of infected leaves) was calculated for each treatment by assessing the 10 youngest leaves on each of 12 terminal shoots for each tree. Three shoots in each of the four cardinal directions were selected near eye level for assessment. On 18 April 2002, 150 ‘Indian Magic’ crabapple saplings (whips) were planted between the mature crabapples in the orchard described above. At the time of planting, all whips were topped 1 m (3.3 ft) above the ground and all lateral shoots were removed. The whips were randomly assigned to one of six treatment groups in a randomized complete block design to determine the efficacy of PBZ, applied as a basal drench method or by foliar application, for apple scab control. The fungicide propiconazole, commonly used for apple scab control, was also used as a means to judge PBZ’s effective- ness in reducing the incidence of apple scab. There were 18 replications for each treatment with the exception of the con- trol, for which 54 replications were used. Treatments for ‘In- dian Magic’ whips included: 1. Control. 2. Basal drench with PBZ at the recommended rate (0.79 g a.i. cm−1 diameter) at the time of planting. 3. Standard spray program: foliar application of propi- conazole every 2 weeks 4 days after budbreak. 4. One-time foliar application of PBZ (0.02 g a.i. mL−1) sprayed until leaf drip, 4 days after budbreak. 5. One-time foliar application of PBZ (0.02 g a.i. mL−1) sprayed until leaf drip, 4 days after the first heavy in- fection period, to assess eradicative abilities.
March 2006
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