112 Watson and Jacobs: Control of Apple Scab and Cytospora Canker with Paclobutrazol Arboriculture & Urban Forestry 2012. 38(3): 112–116 Control of Apple Scab and Cytospora Canker with Paclobutrazol Gary Watson and Karel Jacobs Abstract. Forty Malus ‘Radiant’ crabapples, a variety known to be highly susceptible to apple scab disease, were field-planted then treated three months later with paclobutrazol (PBZ) as a basal drench, at rates of 0, 0.8, 1.6, and 2.4 g a.i./cm caliper. Apple scab was significantly reduced for two years post treatment by all rates of PBZ applied. However, significant growth regulation occurred through the third and final year of the study. Thirty Picea pungens (Colorado spruce) trees in containers were treated with PBZ, applied as a basal drench, at rates of 0, 1.6, and 3.2 g a.i./cm caliper. Cytospora canker development from subsequent branch inoculations was significantly reduced by both PBZ treatment rates and persisted through the end of the two year monitoring period. Cy- tospora canker disease control with only moderate growth regulation indicates that a PBZ basal drench could be developed into a viable landscape treatment. Key Words. Colorado Spruce; Crabapple; Disease Control; Growth Regulation; Paclobutrazol; Picea pungens. Paclobutrazol (PBZ) is a triazole-based compound with noted plant growth regulator activity. PBZ inhibits biosynthesis of gib- berellins in plants, and sterols in fungi, by blocking steps in the isoprenoid pathway (Rademacher 2000). Gibberellins are plant growth hormones that mediate stem elongation and leaf size. Plants treated with PBZ develop a shorter, more compact form with reduced length between internodes as well as smaller leaves (Davis et al. 1985). The growth-regulating attributes have result- ed in PBZ being used in utility arboriculture to extend line clear- ance trim cycles and reduce outages (Mann et al. 1995; Burch et al. 1996; Asín et al. 2007). Sterols are required for fungal cell membrane function (Baldwin and Rathmell 1988). Sterol inhibi- tion is the mode of action of closely related triazole fungicides. Root systems of trees treated with PBZ can have increased extension (Watson 2004) and greater fine root density (Watson 1996). PBZ has been shown to improve cold, flood, and salt toler- ance in several plant taxa (Lin et al. 2006a; Lin et al. 2006b; Haji- hashemi et al. 2007). Higher plant water potential (Ashokan et al. 1995; Watson 2001), and reduced stomatal conductance (Asho- kan et al. 1995) and water use (Wample and Culver 1983; Ruter and Martin 1994) have also been documented. These combined effects of PBZ on reducing plant stress may contribute to reduc- ing the severity of plant diseases. PBZ applied to mature trees as a basal drench produced slight control of bacterial leaf scorch on pin and red oak when the severity of disease prior to treatment was moderate (causal agent Xylella fastidiosa) (Hartman et al. 2010). PBZ has fungitoxic properties (Jacobs and Berg 2000; Blae- dow et al. 2006), owing to its inhibition of fungal sterol biosyn- thesis (Baldwin and Rathmell 1988), and is effective in control- ling some diseases. For example, on mature crabapples, basal drench applications of PBZ suppressed apple scab disease (causal agent Venturia inaequalis), but not when applied to smaller trees at planting (Blaedow et al. 2006; Percival 2008). McDonald et al. ©2012 International Society of Arboriculture (2006) found that PBZ suppressed dollar spot foliar disease of creeping bentgrass (causal agent Sclerotinia homoeocarpa) but had no effect on gray leaf spot (causal agent Pyricularia grisea). The intention of this study was to determine if a PBZ soil drench treatment could control two diseases: apple scab (causal agent Venturia inaequalis) and cytospora canker (causal agent teliomorph Leucostoma kunzeii var. piceae; ana- morph Cytospora kunzei) when applied to young, suscep- tible host trees. The PBZ formulation used in this research study is currently not labeled for control of fungal diseases. METHODS Crabapple: Apple Scab Forty, 3.0 cm caliper bare root Malus ‘Radiant’ crabapples, a va- riety known to be highly susceptible to apple scab (Beckerman et al. 2009), were purchased (Bailey Nursery, St. Paul, Minnesota, U.S.) and planted on 1.5 m centers in a research field nursery at The Morton Arboretum (Lisle, Illinois, U.S.) in June 2001. Drip irriga- tion was provided as needed throughout the study. PBZ (Cambi- stat 2SC, Rainbow Treecare Scientific Advancements, Minneapo- lis, Minnesota, U.S.) was applied as a basal drench on September 5, 2001, at rates of 0, 0.8, 1.6, and 2.4 g a.i./cm caliper. The rec- ommended label rate for crabapple is 1.6 g a.i./cm. The four treat- ments were applied to trees using a completely randomized design. Terminal twig growth and leaf area measurements were re- corded annually in August beginning in 2002. Terminal twig growth was measured on three vigorous, non-apical branch- es, from the upper half of the crown. When growth was too small to measure accurately, it was recorded as 1 cm. Four undamaged fully expanded leaves from the same branch- es were collected to measure leaf area on a Delta-T (video)
May 2012
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