Question: My grass is pale green in irregular spots. It started last year, but looked good this spring; it became worse in the last week or so. The pale blades are uniformly pale, the dark green blades are uniformly dark green. The pale blades are spreading through the yard. Any suggestions on what the problem is?

Yellowing grass can be caused by several factors.

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Answer: Pale, yellow leaf blades in St. Augustinegrass is often a result of low iron levels in the leaf tissue or nitrogen deficiency.

Notice the fertilized, solid green grass blades in the front (bottom one-third of the image) with the yellowing grass in the back two-thirds of the image.

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The low levels of iron in the plant is caused by alkaline (high pH) soils found in many areas of the state. If the soil pH is too high, called alkaline, essential elements such as phosphorus, iron, zinc and copper become unavailable for plant uptake even though these elements may be present in the soil. Unavailability of mineral elements is caused by physical encapsulation by soil calcium or chemical change of added nutrients. Iron is needed for chlorophyll development. Chlorophyll makes plants green so lack of iron and consequently lack of chlorophyll causes yellowing of plant foliage. Chlorosis is most common during early spring when grass is growing vigorously. This irregularity is characterized by streaks of green through a predominately yellow leaf. In severe cases, brown margins or spots will develop but this will occur after the leaf has been yellow for a period of time.

Notice the streaking of the grass blade in the bottom right corner. This is iron chlorosis.

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If the problem is iron chlorosis (yellowing), then an application of iron should correct the problem. Generally, a foliar application of a chelated iron or iron sulfate spray provides a faster greening than a soil application of iron -- spray an iron product such as Ferriplus or Iron Plus. However, soil applications of iron sulfate (Copperas) and Green Sand will also be effective. Ironite products have not proven to be effective even at extreme rates.

Yellowing grass can also be caused by lack of nitrogen fertilizer. This yellowing will differ from iron chlorosis in that there will be no parallel stripes of yellow and green on the grass blade. Instead, the grass blade will be solid yellow. The best way to solve this problem, as well as the iron chlorosis problem, is to apply a combination nitrogen-iron product such as Iron Plus. If you want to use Green Sand, just make an application of a slow-release lawn fertilizer such as 19-5-9 after the Green Sand is applied. Water thoroughly after applying.

Notice the grass blade in the center is solid yellow indicating nitrogen deficiency.

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Also, Take-All Root Rot (TARR), a major disease problem in St. Augustinegrass, will cause the leaf tissue to turn yellow. In the affected areas of the lawn, you will find both green and yellow leaf blades. Close examination of the stolons (runners) will reveal short, dark brown to black roots. If TARR is causing the problem, then an application of iron will not green up the yellow leaf blades. If TARR is the problem, then an application of sphagnum peat moss is one of the best treatments for this disease problem. Apply approximately 1 to 2 bales of the peat moss per 1,000 sq. ft. to the affected areas of the lawn. Then thoroughly soak the peat moss until it is wet. We have seen better results from a topdressing with peat moss than we have with the application of a fungicide in most cases. James McAfee, Extension Turfgrass Specialist, and Jerry Parsons, Extension Horticulture Specialist

For more information on TARR see the article at http://www.plantanswers.com/root_rot_fungus.htm