Arboriculture & Urban Forestry 35(5): September 2009 Chlorophyll Measurements A Minolta chlorophyll meter SPAD-502 was used. Chlorophyll was measured at the midpoint of the leaf next to the main leaf vein. Calibration was obtained by measurement of absorbance at 663 and 645 nm in a spectrophotometer (PU8800 Pye Uni- cam) after extraction with 80% v/v aqueous acetone (regres- sion equation = 5.92 + 0.060x; r2 adj = 0.92, P = <0.01) (Lich- tenthaler and Wellburn 1983). Ten leaves per tree randomly selected throughout the crown were used for measurements, and the mean per tree was calculated for statistical purposes. Relationship Between Leaf Calcium Content and Scab Resistance Collection of Leaf Material for Calcium Analysis Mature, fully expanded leaves near the top of the canopy (gen- erally about the fourth leaf from the apex) of a range of Malus species—eight trees per species—classified as resistant (M. ‘Red Jade’, M. huphensis, M. floribunda, M. ‘Profusion’), intermediate (M. sargentii, M. ‘Golden Delicious’, M. ‘Royalty’, M. ‘Indian Magic’), and sensitive (M. × purpurea ‘Eleyi.’, M. ‘Cox’s Orange Pippin’, M. ‘Hopa’, M. ‘Red Delicious’) to apple scab (Durham et al. 1999) was collected in early May ca. six weeks after leaf flush—a time when leaves show maximum photosynthetic per- formance (Kitao et al. 1998) with no visible signs of apple scab. Leaf material was collected from a range of local commercial sup- pliers of ornamental apple located within the Reading area. Collected leaves were transported in insulated boxes sheltered from light, and all material prepared within two hours of collection. Leaf calcium content of calcium and penconazole treated trees was analyzed in the same manner only in this instance leaves were collected at Week 2 following the final calcium and penconazole spray treatment. Calcium Content Assessment Eight trees per calcium treatment, with twelve leaf samples per tree, were selected at random from the canopy and pooled to form sufficient material for calcium analysis. Samples were 265 then thoroughly washed, dried in a convection oven at 85°C (185°F) for 48 hours before being ground through a 0.5 mm (0.02 in) cyclone mill (Retsch, Middlesborough, UK). Each separate sample [0.5 g (0.02 oz)] was placed into 150 ml (5 fl oz) volumetric flasks and digested in 20 ml (0.7 fl oz) of 7:1 ni- tric/perchloric acid. After cooling, the solutions were brought to volume with deionized water and analyzed by Inductively Coupled Plasma-Emission Spectroscopy (ICP) elemental analy- sis. Calcium values were expressed as % total leaf dry weight. Data Analysis Data were analyzed using Genstat 8. Levene’s (1960) test was used to determine the homogeneity of variances, and data were trans- formed [log (y+0.5)] when necessary. Visual severity index data were transformed using arcsine [sqrt (y/100)]. Treatment signifi- cance from controls was separated using Fisher’s least significant difference (LSD) at the P < 0.05 level. Correlation equations and coefficients of multiple determi nation (r2 ) of foliar calcium con- tent against leaf scab severity at the cessation of the growing sea- son (September) were calculated using the curve fitting feature of Genstat V. A linear model was used for correlation analysis as this model provided the highest percentage goodness of fit. RESULTS Field Trials Irrespective of year, no significant influence of the plant pen- etrant Silwett L77 used to facilitate entry of calcium ions into the leaf was recorded (Table 1; Table 2). Measurements of leaf and fruit scab severity, chlorophyll fluorescence Fv/Fm values, leaf chlorophyll content SPAD values, and foliar calcium con- tent were, in all cases, not significantly (P < 0.05) different from noncalcium treated control trees (Table 1; Table 2). Applications of calcium sprays in virtually all instances had a significant influ- ence on foliar and fruit scab severity, chlorophyll fluorescence Fv/Fm values, leaf chlorophyll content SPAD values and fo- liar calcium content (Table 1; Table 2). Application of calcium Table 1. Selected calcium foliar sprays compared for the control of apple scab (Venturia inaequalis) on apple cv. Crown Gold as measured by leaf and fruit scab severity, chlorophyll fluorescence (Fv/Fm values), leaf chlorophyll content (SPAD values), and leaf Ca content. Data from 2006. Treatment Control Silwett Calcium chloride Calcium sulfate Calcium nitrate Calcium nitrate borate Calcium hydroxide Calcium metalosate Calcium amino acid complex Penconazole DF LSD Significance of Ca (P value) z y Values mean of 8 trees. Values mean of 8 trees, 10 leaves per tree. Asterisk (*) indicates value significantly different from controls according to Least Significant Difference (LSD) at P < 0.05. ns = not significant different from control value. ©2009 International Society of Arboriculture Leaf scab severityz 3.50 3.62ns 2.13* 2.50* 3.00ns 2.50* 2.13* 2.75ns 2.38* 79 1.70* 0.826 0.002 Fruit scab severityz 2.85 2.50ns 1.38* 2.75ns 1.75* 2.13* 1.50* 2.13* 2.25ns 79 1.00* 0.678 <0.001 Fv/Fmy 0.723 0.713ns 0.813* 0.803* 0.797ns 0.800ns 0.799ns 0.723ns 0.793ns 79 0.811* 0.082 0.059 SPADy 12.26 15.69ns 19.12* 18.34* 14.3 ns 13.05ns 18.70* 12.80ns 18.75* 20.51* 79 3.86 <0.001 Leaf Ca contentz 1.22 1.24ns 1.47* 1.39* 1.51* 1.46* 1.58* 1.37* 1.59* 79 1.24ns 0.134 <0.001
September 2009
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