Arboriculture & Urban Forestry 35(5): September 2009 267 Figure 1. Correlation between leaf calcium content (leaves collect- ed in early May ca. 6 weeks after leaf flush) and scab severity at the cessation of the growing season. Intercept (calculated) = -0.0403; Coefficient = - 1.50; r2 (% variation accounted for) = 0.04; P value = 0.179. = M. × purpurea ‘Eleyi’; = M. ‘Red Jade’; = M. huphen- sis; = M. ‘Cox’s Orange Pippin’ = M. ‘Golden Delicious’. cations of foliar applied calcium reduced the susceptibility of apple trees to infection by Venturia inaequalis, thereby provid- ing arborists with an additional practical tool to complement or replace existing fungicide-based pathogen control programs. Applying a range of calcium products at 2.5 g per tree, 10 g total, over the growing season, significantly increased the cal- cium content in the leaf tissue of apple Crown Gold with no evi- dence of phytotoxicity. However, there was no significant posi- tive correlation between calcium content in leaves of a range of resistant, intermediate and sensitive apple species and scab se- verity at the cessation of the growing season. This is important from two points of view. First analysis of leaf calcium content of breeder’s selections for scab resistant apple varieties could not be used for screening for improved resistance. Second, although application of calcium sprays reduced scab severity this does not appear to be due to the increased presence of calcium ions with leaf tissue. Contrary to this, increasing calcium content with plant tissues has been positively correlated with enhanced resistance against a range of fungal (Phoma exigua var foveata, Monilinia fructicola) and bacterial (Erwinia carotovora subsp atroseptica) pathogens (McGuire and Kelman 1984; McGuire and Kelman 1986; Bain et al. 1996). Increased resistance was shown to be due to calcium-induced, improved structural integrity of the cell wall and middle lamella that physically increased cell wall strength making host plants less susceptible to enzymatic deg- radation (Olsson 1998). Biggs et al. (1997) concluded increased calcium concentrations in the fruit skin of cherry enhanced cell wall resistance to enzymatic digestion from extra-cellular en- zymes such as polygalacturonase produced by M. fructicola. Results of this study indicate that mechanical or physical protec- tion induced by calcium has limited effect on apple scab severity. However, application of calcium based fertilizers were useful in suppressing scab indicating that calcium sprays possess some forms of scab protectant properties. Although this investigation did not specifically investigate the mechanistic basis of enhanced resistance previous research suggests that one or several of the following biochemical mechanisms are involved. Calcium is also known to directly affect some fruit pathogens by interfering with spore germination, germ tube elongation, fungal cell wall thick- ness (Chardonnet et al. 1999; Miceli et al. 1999; Chardonnet et al. 2000) and the activity of pectolytic enzymes (Conway et al. 1991). For example, direct inhibition of polygalacturonase—a cell wall degrading enzyme produced by M. fructicola as a con- sequence of calcium application—slowed the infection process and allowed constitutive and induced host responses to protect the plant from infection (Forbes-Smith 1999). The synthesis of plant protectant phytoalexin and phenolics substances has been reported to increase as a result of calcium application (Kohle et al. 1985; Miceli et al. 1999). Other calcium based resistance mechanisms include calcium ions interfering with chemotaxic re- sponses required for fungal infection, calcium induced abrasion of hyphae leading to ineffective infection and dilution of other nutrients at the plant surface that reduces pathogen vigor and growth rate (Glenn et al. 1999; Glenn et al. 2001). Pre-harvest micro-cracks in the cuticle of stone fruits have been identified as an important site of infection for Monilinia spp. in prunes (Michailides and Morgan 1997) and B. cinerea in sweet cher- ries (Børve et al. 1998) Because of this, it is a common prac- tice amongst sweet cherry growers worldwide to apply calcium to reduce the risk of micro-cracking and pre-harvest fruit infec- tion (Cline and Tehrani 1973; Børve et al. 2000). The effect of calcium on micro-cracks of apple fruit was not recorded in this investigation but may account for the significantly lower extent of scab severity recorded on fruit in both the 2006 and 2007 trials. Additionally, application of calcium fertilizers to trees has been shown to induce a suite of morphological and physiologi- cal adaptations associated with enhanced tolerance to environ- mental stresses such as salinity, freezing, and elevated tempera- tures. In these instances, enhanced tolerance of calcium treated plants are achieved via alterations to the structural stability of cell walls and plasma membranes resulting from calcium links between phosphate and plasma lipids and a concomitant increase in physical cell wall strength (Legge et al. 1982). Calcium has also shown to have been important in controlling enzyme ac- tivity involved with freezing resistance, inducing formation of shock proteins in response to elevated temperatures, and regu- lating stomatal closure in response to excess salinity, in turn preventing an excess build-up of reactive oxygen species within the leaf photosynthetic system (Price 1990). Furthermore, low temperature stress increased levels of a calcium-dependent NAD kinase responsible for activating enzymes, causing the synthesis of proteins necessary to influence cold acclimation. The prog- ress of freezing injury can also be halted by bathing or washing freeze-thaw injured tissue in calcium-based solutions (Monroy et al. 1993; Berbezy et al. 1996). Such improvements in tree vitality as a direct result of calcium fertilization may also have contributed to the reduced scab severity recorded in this study. Of all seven calcium products evaluated, only trees treated with calcium chloride and calcium hydroxide had significantly less leaves and fruit with scab infection compared to untreated control trees in both the 2006 and 2007 trials, indicating calci- um chloride and calcium hydroxide provided greatest protection against scab on both leaves and fruit. Differences in the extent of scab protection between commercially available calcium prod- ucts are consistent with other research (Wojcik 2001), indicating that calcium form affects susceptibility of a plant to scab infec- tion and the anion attachment (chloride, sulfate, nitrate etc.) may ©2009 International Society of Arboriculture
September 2009
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