Fruit size and quality is quickly affected by drought stress and nutrient deficiency.
The symptoms of iron deficiency are yellowing or chlorosis of the interveinal areas of the emerging leaf (Plate 1 and 2). Later, the entire leaf turns yellow, and finally white. If the deficiency is severe, the entire tree may become chlorotic and die. Iron deficiency can easily be mistaken for nitrogen deficiency, but if affects emerging leaves first. Nitrogen deficiency, on the other hand, affects the older leaves first. Iron deficiency is a common micronutrient disorder on neutral, calcareous and alkaline aerobic soils. The severity of the disorder increases with the pH.
It may also be observed on upland acid soils, and on upland iron calcareous soils. A high concentration of calcium carbonate in the soil or irrigation water is likely to aggravate iron deficiency. Iron deficiency can be induced by high levels of nitrate, which raise the pH of the rhizosphere. Heavy phosphate applications may induce or aggravate iron deficiency, by precipitating iron, and by the inactivation of iron uptake of translocation to the shoot, and of iron metabolism.
Iron deficiency can be amended through a foliar spray of 2-3% ferrous sulfate solution, or the soil application of about 30 kg/ha of iron in the form of ferrous sulfate. Because of the low mobility of iron in the tree, split applications may be necessary. Ammonium sulfate and ammonium chloride are recommended nitrogen sources for soils with a high pH.
Plate 1 Iron deficiency of young leaves of star fruit. The lower left leaf is normal; the upper six leaves show serious iron deficiency. Photo by Mr. Ching-Hsee Lin, Taiwan ROC
Plate 2 Serious iron deficiency in young leaves of star fruit, which are white in colour. |
