Technical Abstract: It is increasingly apparent that economic loss due to nickel (Ni) deficiency likely occurs in horticultural and agronomic crops. While most soils contain sufficient Ni to meet crop requirements, situations of Ni deficiency can arise due to antagonistic interactions with other metals. This study assesses the impact of excessive availability of iron (Fe) on Ni deficiency in pecan [Carya illinoinensis (Wangenh.) K. Koch]. Several field and greenhouse experiments indicate Ni deficiency can occur due to excessive Fe. Morphologically symptoms of Ni deficiency are either triggered or enhanced in young ‘Wichita’ cv. pecan shoots by foliar sprays of Fe [as Fe-EPTA 1.1995 g/L (l lb/100 gal); sodium diethylenetriamine pentaacetate (EPTA)] during early shoot growth immediately after budbreak. Ni deficiency is similarly inducible by soil application of Fe-EPTA to greenhouse grown ‘Desirable’ cv. seedlings prior to budbreak. Analysis of the Fe:Ni ratio in symptomatic Ni deficient foliage found that excessive endogenous Fe triggers Ni deficiency associated distortions in growth and morphology when the Fe:Ni = ˜ 150, with severity increasing in proportion to an increasing Fe:Ni ratio. Extreme dwarfing of seedling growth occurs at Fe:Ni = 1,200. Ni deficiency is also weakly associated with elevated foliar concentration of other transition metals [i.e., V, Cr, Co, Cu, Zn, and Mo (but not with Ti and Mn)]. Timely foliar sprays of symptomatic seedlings with Ni-sulfate quickly restored normal growth; whereas, sprays with V, Cr, Co, Cu, Zn, or Mo salts failed to correct or prevent symptoms. These results indicate that excessive endogenous Fe in organs and tissues during early post-budbreak growth potentially causes Ni deficiency, and are suggestive that high Fe substrates, or excessive Fe fertilization, can trigger Ni deficiency. This antagonistic effect of Fe on Ni nutritional physiology is of potential importance to horticultural, agronomic, and silvicultural crops.