Relationship of Xylem Plugging to Reduced Water Uptake and Symptom Development in Citrus Trees with Blight and Blightlike Declines

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Abstract
Citrus trees with declines of unknown etiology (which are called blight in Florida [USA], "declinio" in Brazil, and "declinamiento" in Argentina) all had significantly higher Zn levels in the trunk wood, lower water uptakes, and higher frequency of amorphous occlusions in the xylem than did comparable healthy trees. Filamentous plugs were not consistently associated with declining trees, and the number of filamentous plugs was not highly correlated with water uptake (r = -0.43). The logarithm of the number of amorphous plugs was significantly correlated with water uptake (r = -0.61). The canopy rating was significantly correlated with the logarithm of the number of amorphous occlusions (r = 0.71). When 5-10 amorphous plugs per 200 xylem vessels were present, water uptake was reduced to 0.30 ml/s as compared to 0.82 ml/s in healthy trees, and decline symptoms developed.