Use of Minolta SPAD‐502 chlorophyll meter to quantify the effectiveness of mid‐summer trunk injection of iron on chlorotic pear trees

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Abstract
The severity of leaf chlorosis in iron (Fe)‐deficient fruit trees is often characterized using a semi‐quantitative visual rating index that is subject to evaluator bias. Analytical instruments are now available that provide a quantitative measure of leaf green color that could substitute for visual ratings. We injected limbs of mature chlorotic pear trees (Pyrus communis L. cv. Bartlett) with distilled water or a solution of 0.1% Fe (w/v) as FeSO4‐7H2O on 17 July 1995. Treatments were replicated eight‐fold. On 18 August 1995, a Minolta SPAD‐502 chlorophyll meter was used to measure the green color of 30 randomly sampled leaves located above the point of injection on each injected limb. Average leaf green color was higher in the Fe‐injected tree than in the water‐injected tree of each experimental block. Leaf green color (mean±SD) averaged 34.7±3.8 SPAD units for the Fe‐injected trees and 27.3±3.8 SPAD units for the water‐injected trees. The absolute increase in mean leaf color of 7.4 SPAD units was equivalent to a relative increase of 27%. Iron injection also induced more negative skewness and increased kurtosis in the frequency distribution curve for leaf SPAD meter readings. These results suggest that the SPAD meter can provide an unbiased quantitative measure of the severity of leaf chlorosis associated with Fe deficiency, and confirm that mid‐summer trunk injection of Fe can partially ameliorate Fe‐chlorosis symptoms.