Abstract
Of three adjacent blocks of orange trees (Citrus sinensis L. Osbeck) on rough lemon rootstock (C. limon S. Burm. f.), one was essentially citrus blight-free, and two were strongly affected. The three blocks were fertilized equally and irrigated from the same water source. The blight-free Grove 1 was planted on undisturbed soil, and Groves 2 and 3 were planted on filled-in land; Grove 2 was heavily limed in 1970 before planting. Annual analysis of leaves collected from healthy trees in July from 1986 to 1997 for 14 elements showed that the blight-free Grove 1 had lower leaf Ca and higher S, Zn, and Mn than the two blight-affected Groves 2 and 3. The leaves in Grove 2 had higher N, K, Na, Cl, and B concentrations than in Grove 3; P and Mg were lower. There were no significant differences in Ca, S, and the metal ions. The cause of citrus blight, a severe tree decline problem in humid citrus-growing areas, has never had an undisputed explanation (Childs, 1979; Wutscher, 1988; Timmer, 1990). Observations strongly indicate that soil influences the incidence of blight (Cohen, 1980; Wutscher, 1986; 1989b); pH has especially been implicated (Wutscher and Lee, 1988; Wutscher, 1989b; 1997). Heavy applications of basic slag and calcium hydroxide improved blight-affected trees but did not cure them (Wutscher, 1985; 1995). Abnormal leaf nutrient patterns, based on single-year determinations of low K (Anderson and Calvert, 1970), and low Zn (Smith 1974a), coupled with internal reallocations such as accumulation of Zn in the wood and the bark (Smith, 1974a; Wutscher and Hardesty, 1979) have been reported. Injection of water into the trunk (Cohen, 1974; Lee et al., 1984), zinc analysis of the outer trunk wood (Wutscher et al., 1977), and leaf analysis for characteristic proteins (Derrick et al., 1990) serve as diagnostic tests because visual symptoms are non-specific. Nutritional effects are best characterized by multi-year observations. Annual leaf analyses of three blocks of trees, located next to each other but varying sharply in blight incidence over a 12-year period, offered an opportunity to compare the leaf levels of 14 elements in blight-free and blight-prone blocks.
