Abstract: This study confirmed that leaf vein splitting of citrus was multi-pathogeny caused by both Mg-deficiency and B-deficiency. Spraying 1.0% Mg(NO₃)₂ • 6H₂O had significant effect on remedying leaf vein splitting of‘Newhall’[Citrus sinensis（L.）Osbeck] caused by Mg-deficiency in April. According to the investigation in Fujian citrus orchards，86.2% of leaf vein splitting was suffered from Mg-deficiency，11.5% from Mg-deficiency and B-deficiency，only 2.3% from B-deficiency. Infection rates of leaf vein splitting caused by Mg-deficiency ranked in descending order was as follow：Newhall navel orange，‘Guanxi-miyou’[C. grandis（L.）Osbeck]. Other infected varieties included‘Kumquat’（Fortunella crassifolia Swingle），Satsuma mandarin（C. unshiu Marc.‘Early variety’）and‘Ougan’（C. suavissima Hort. ex Tanaka）. Leaf vein splitting susceptible variety Newhall [C. sinensis（L.）Osbeck] and resistant varieties Ponkan（C. reticulate Blanco）were used as intermediate stocks in graft of Ponkan or Newhall. Comparing with Ponkan，Newhall absorbed less Magnesium and Born，but more potassium，which may explain Newhall sensitive to leaf vein splitting. There was significant difference symptom between Mg-deficiency and B-deficiency. Infected leaf caused by Mg-deficiency had ∧-shaped etiolate in leaf top and vein splitting normally appeared in this part. Leaf caused by B-deficiency kept green. Infected leaf caused by Mg-deficiency and B-deficiency had symptom of full leaf etiolate，main and lateral veins splitting obviously or veins splitting to the leaf base with ∧-shaped green.

Key words: citrus, leaf vein splitting, magnesium, boron