Arboriculture & Urban Forestry 35(5): September 2009 Arboriculture & Urban Forestry 2009. 35(5): 263–270 263 The Influence of Calcium Sprays to Reduce Fungicide Inputs Against Apple Scab [Venturia inaequalis (Cooke) G. Wint.] Glynn C. Percival and Ian Haynes Abstract. A goal of pathogen management in the arboriculture industry should be to eliminate or reduce the amount of active ingredients of synthetic fun- gicides used within a growing season. The aim of this research paper was to evaluate a nonfungicidal pathogen management system by investigating the hypothesis that replacing an apple scab fungicide spray program with calcium reduces fruit and leaf scab severity. In addition, a separate study investigat- ed if a relationship existed between calcium concentrations within foliar tissue of resistant, intermediate, and sensitive apple species and varieties toward scab infection. A number of commercially available calcium products were applied to apple cv. Crown Gold trees at four distinct growth stages (bud break, 90% petal fall, early fruitlet, two weeks after early fruitlet). A comparative evaluation of the synthetic fungicide penconazole commercially used for scab control was also conducted. The experiment was performed in 2006 and repeated in 2007 at the University of Reading Experimental Field Site (UK). Ap- plication of calcium sprays significantly reduced the leaf and fruit scab severity of apple cv. Crown Gold however; in a separate study no relationship ex- isted between foliar calcium content and susceptibility to apple scab attack between resistant, intermediate, and sensitive Malus species. Greatest protec- tion in both field trials was provided by the synthetic fungicide penconazole. Within the calcium products evaluated, greatest protection in both field trials was provided by calcium chloride and calcium hydroxide. The integration of calcium foliar sprays into existing scab management practices offers a useful addition to reduce scab severity on ornamental apples that has applicability against other foliar diseases frequently encountered within urban landscapes. Key Words. Fruit; Holistic Approach; Integrated Disease Management; Pathogen Control; Plant Health Care; Urban Landscapes. During their life cycle, urban trees are susceptible to attack by several foliar pathogens. If uncontrolled, these attacks will result in high mortality rates and/or undesirable reductions in tree aes- thetic appearance. For example, apple scab represents a world- wide problem of fruiting and ornamental apples planted into UK, U.S., and European landscapes. Initial infection appears as olive- green to sooty or smudgy spots on the leaf surface or leaf petiole. On older leaves, the infected areas form definite spots, which are slightly raised, black, and velvety in appearance. As the infection develops, the leaves turn yellow and drop prematurely. Premature defoliation makes the tree aesthetically undesirable and greatly weakens it. Infected fruit becomes deformed, scabby, and usu- ally drops before maturity (Cuthbertson and Murchie 2003). As suppliers and growers of apples generally adopt a zero tolerance policy towards scab, any scab infection reduces tree quality and marketable yield (Percival and Boyle 2005). Consequently, the economics of fruit and ornamental tree production require fre- quent application of synthetic fungicides throughout the grow- ing season (Berrie and Xu 2003). As strains of pathogenic fungi resistant to synthetic fungicides develop, coupled with increased public concern over pesticide residues and the fact that arborists manage trees in densely populated areas where potential fun- gicide contact with pedestrians could lead to litigation claims, new approaches to pathogen management are needed (Chris- tiansen et al. 1999; Percival 2001; Percival and Haynes 2008). Calcium is a major macronutrient in trees, important to the structural integrity of cell walls and plasma membranes, as well as maintaining membrane integrity and transport func- tion (Steponkus 1984). Calcium is additionally involved with nitrate uptake and metabolism, enzyme activity and starch me- tabolism. Because calcium is immobile in the phloem, calci- um-based fertilizers are widely used by the orchard industry to reduce calcium-deficient fruit disorders such as bitter pit of apples, cork spot of pears, as well as ensuring optimal soil pH for tree growth (Raese 1997). However, there is also a growing body of evidence noting that enhanced calcium concentrations in leaf, stem, and root tissue by calcium fertilization can aid in reducing pathogen severity caused by several fungi and bacte- ria. Pertinent examples include enhanced resistance against soft rot of potato caused by Erwinia carotovora subsp. amylovora (McGuire and Kelman 1984; McGuire and Kelman 1986; Bain et al. 1996), Phoma exigua (gangrene) and Fusarium solani (dry rot) of potato (Olsson 1998), Botrytis cinerea of apple (Conway et al. 1991), sweet cherries (Ippolito et al. 2005), and brown rot of peach caused by Monilinia fructicola (Elmer et al. 2006). Recent research also indicates the form of calcium is poten- tially important for disease suppression (Elmer et al. 2006). For example, calcium nitrate reduced the incidence of brown rot of cherries in two of a three-year trial while calcium chloride forms had no effect on brown rot incidence (Wojcik 2001). Commer- cially, a wide range of calcium fertilizers exist to include cal- cium nitrate, calcium sulfate, calcium chloride, etc. The efficacy of many of these calcium products has not been evaluated for their potential in foliar pathogen suppression. The objectives of this research were to evaluate the effectiveness and feasibility of commercially available calcium fertilizers on suppressing severity ©2009 International Society of Arboriculture
September 2009
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