Boron Deficiency in Citrus
Foliar symptoms of boron deficiency on citrus are not very characteristic, and a deficiency suspected on the basis of leaf symptoms should be confirmed by fruit symptoms. The first signs appear on the younger leaves as water-soaked spots which become translucent. The veins tend to be thick, cracked, and somewhat corky. The young leaves wilt and curl, and have a dull brownish-green color without any luster. Dieback of leaf tips is common. A gummy exudate may appear on the twigs and fruit pedicels.
The fruits, which are small, shrivel and go hard on the tree. They characteristically show internal gum formation, usually in the albedo but also in the pith. Normally, the gummed spots cannot be seen unless the fruit is cut. This feature helps to distinguish the deficiency from citrus impietratura disease. The rind is thick and the fruit has a low juice content. Excessive fall of young fruit occurs, resulting in very poor yields. This may be the first sign of a boron-deficiency problem. The seeds are likely underdeveloped and the seed coat dark and shriveled.