Last week I submitted and article about leaf scorch. I would like to follow that up with this article about iron chlorosis, because both of these conditions appear when the weather is very hot and windy, and both will make the leaves on shrubs and trees turning yellow. 

 Iron chlorosis is the most common micronutrient problem of ornamentals, shrubs, vines, small fruits and trees in western States.  

New leaf growth on affected plants are yellow, light green, or white with distinct green veins resulting in interveinal chlorosis.  In severe cases, the leaves may be entirely white. 

The margins of severely chlorotic leaves often scorch and die during hot periods.  Iron deficiency chlorosis may be persistent or it may vary during the season or year to year depending on environmental conditions. If iron chlorosis is persistent for several years, individual limbs or the entire plant may die.

Iron chlorosis is the result of the inability of the plant to extract sufficient iron from the soil. Nevada soils are typical of arid and semiarid soils around the world with lime or calcium carbonate in most of the soil profile. These soils are alkaline with pH ranging between 7.2 and 8.3. Iron chlorosis is common in these soils and is exaggerated by poorly drained soils and excessive soil moisture, soil salinity, high concentrations of phosphorous, low or high soil temperatures, large additions of organic matter, or inefficient root function caused by nematodes or fungal pathogens.

 Plants vary in their ability to obtain and utilize iron. This is particularly evident when adjacent plants may show marked differences in chlorosis. Some plant species are capable of obtaining iron from alkaline soils whereas others cannot be grown successfully in native high Ph soils.

 Preventing and controlling iron chlorosis is difficult and often gives poor results. The following are recommended procedures for controlling iron chlorosis:

Soil moisture 

management

Water management is probably the most important consideration when growing plants in alkaline soils. In excessively wet or poorly drained soils, the chemistry of the soil changes and iron becomes unavailable. Irrigation applications should wet the plant root zone and should not be repeated until the soil moisture has been reduced by plant use and evaporation. Frequent irrigation in heavy clay soils or cold temperatures often results in a persistent deficiency of iron.  Less frequent irrigations with at least 1-2 inches of salt-free water per application are recommended.



Correction with iron fertilizers or soil amendments

Numerous iron compounds are available for treating iron chlorosis; however, responses to soil and foliar applied materials vary considerably, and no single product has proven to be consistently successful.

Soil Application: Many compound labels claim to correct iron chlorosis; however, most do not work in our highly buffered, alkaline soils. If your soil pH is below 7.2, it is possible that some products will work, but most of the soils in Nevada have a pH greater than 7.2. Under these conditions, it is very difficult to correct iron deficiency. Soil pH should be lowered with elemental Sulfur or fertilizers containing sulfur. 

Inorganic iron sulfate will give good results when applied to turf but their utility in other situations is generally unsuccessful. Iron sulfate will cause unsightly rust-colored spots on pavement when misapplied.

Chelated iron compounds consist of an organic molecule that binds iron and makes it more available to plants. One of the best compounds is EDDHA, a synthetic chelate that is available as Sequestrene 138 and Millers Ferriplus.

Chelated compounds must be placed into the root zone to be most effective. Incorporate lightly into the soil or irrigate in. Applications should be made in the spring to coincide with the first flush of growth. 

In most cases, it is necessary to treat every year. Soil incorporation of inorganic compounds such as iron sulfate is not effective in most Nevada soils.

Foliar Application: Iron compounds sprayed on leaves give the most rapid but temporary response. Usually green spots can be seen on the leaves a few days after spraying. Repeated applications are necessary as new foliage appears.

Chelated iron compounds or 0.1% ferrous sulfate can be applied as foliar sprays. Use a spreader-sticker surfactant to obtain better results. Avoid applications when fruit are present because staining may occur.

Source: Utah Plant Disease Control No. 45, Revised February 1994, Sherman V. Thomson/Extension Plant Pathologist, Scott C. Ockey/Plant Disease Diagnostician