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The most common type of iron (Fe) deficiency in California is called iron chlorosis or lime-induced chlorosis. It is not caused by low iron but by high levels of bicarbonate ions (HCO3-) which frequently occur in high pH soils. Bicarbonate interferes with the uptake and use of iron by the plant. The most characteristic symptom is a loss of chlorophyll between veins, leading to yellow leaves with a fine network of green veins. Eventually leaves can lose all their chlorophyll and become completely white. In this state they are more susceptible to sunburn, so necrotic areas and defoliation can occur, starting from the tip of the shoot. With severe iron chlorosis, shoot growth and production are greatly reduced.
Management
Analyzing leaves for iron is generally not effective because iron may be present in the leaf but somehow inactivated by bicarbonate. Therefore, even though leaf threshold values have been published (survey), diagnosis of the disorder should rely more on leaf symptoms.
Over-irrigation or water logged soils can increase soil bicarbonate levels and thus lead to iron chlorosis symptoms. These symptoms can be alleviated easily by careful irrigation management.
If the problem is due to high pH soils with high levels of calcium carbonate or limestone (calcareous soils), it is more difficult to correct. The long term solution is to use a tolerant rootstock. Iron chelates applied to soil or foliage have sometimes been effective, but they are expensive and results are often short lived. Applications of sulfur to lower soil pH can also be effective. The amount needed to neutralize all the bicarbonate in the root zone is prohibitive, but even acidifying a portion of the root zone can be effective. Finally, some success has been shown with injecting iron containing materials into the trunk and with planting certain cover crops.
Further Reading
Managing iron chlorosis in Colorado
Managing trees with iron chlorosis in Utah
Intercropping with perennial grass species