Is Your St Augustine Grass Turning Yellow? What are the Causes and Fixes?

Keith Hardy - Bio Photo
Keith Hardy
Senior Editor

Keith has been involved in the gardening and landscaping industry for the past 21 years. From a jobbing gardener to running his own landscaping services....

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It can be quite disturbing to see yellow and brown patches start to develop and spread across your lawn.  The truth is, is that St. Augustine grass is a relatively high-maintenance grass. It requires regular mowing (about once a week), watering, fertilizing, and pest control to keep it looking a deep beautiful green.

This is because without good lawn care it is readily susceptible to thatch buildup and consequently to fungal diseases that cause yellow/brown patches to form on the grass. In addition, poor soil nutrient balance, compaction and incorrect lawn maintenance procedures are usually the root cause of the yellowing. In this article, we will look at the most common causes of the problem and how to fix them.

The appearance of yellowing on your St Augustine grass can be due to a number of causes. These causes can be split into three groups, fungal diseases, soil nutrient issues, such as iron and nitrogen deficiency,  or poor maintenance issues such as poor watering practices. Provided that the cause is identified early most fixes are reasonably straightforward and good maintenance practices can prevent problems from reoccurring.

About St Augustine Grass

About St Augustine Grass

St. Augustine grass is a warm-season grass that is a popular choice for lawns due to its dense, carpetlike sod and its ability to crowd out most weeds and other grasses. It has a high tolerance for wear and tear, making it a good choice for areas that see a lot of foot traffic. It is also salt-tolerant and is therefore often found in coastal areas.

Fungal Diseases

Fungal diseases are one of the most common problems that can affect your St Augustine grass. There are many different types of fungi that can cause disease, and they can attack both the roots and the leaves of the grass. The two most common diseases that affect St Augustine grass are Take All Root Rot and Brown Patch. Another fungal infection called gray spot can also be common in areas with extremely high humidity.

Take All Root Rot

Take All is a fungal disease that is caused by the Gaeumannomyces tritici var. graminis pathogen. These fungi attack the roots of the grass, causing them to rot. The leaf blades of the grass will then turn yellow and die. It usually appears in spring, around April and May and is less likely to appear in the heat of summer. Although the fungus can generate spores, it spreads primarily through roots and stolons. This means that general foot traffic or mowing will not typically spread the disease. This disease is most common in wet, humid conditions.

In terms of fungicide, the best treatment is Azoxystrobin. You should be able to find it in most garden centers or nurseries. Apply as per the instructions and you should see improvement in around two to three weeks, if not sooner

Longer term, maintaining healthy grass is the best defense against take-all root rot. When the grass is weak or is stressed due to unfavorable climate conditions and or improper management, it is important to foster robust root growth to comate the issue.

In addition you need to relieve any compaction and ensure adequate drainage in the substrate as the disease spreads easily in damp turf by aerating once or twice a year. Monitor your watering practices carefully. It is better to water infrequently but deeply than to give the grass frequent, shallow waterings.

Brown Patch

Brown Patch is caused by the fungi Rhizoctonia solani and R. cerealis pathogens. These fungi attack the leaves of the grass, causing them to turn brown. The patches of brown grass that develop will then spread, killing more grass in that area. The disease usually affects grass that grows in warm, dry climatic conditions.

The main problem is that brown patch is hard to eradicate, so cultural control and prevention should be incorporated into your lawn care program, especially if your lawn is in climatic conditions conducive to the disease. Understanding the nature of the disease is essential to get rid of brown patch. It is critical to understand that the fungus can live in your soil despite the absence of visible signs of it in the grass.

To avoid these problems, you should apply fertilizer in autumn and don’t overfertilize in spring. You should also apply a fungicide containing benzimidazoles every month. The disease can also be easily spread by mowing, so be sure to clean your mower blades after each use.

Gray Leaf Spot

As the name suggests, gray leaf spot is a fungal disease that causes gray or brown lesions on the leaves of Augustine grass. The fungus Pyricularia grisea, also known as Magnaporthe grisea, is the most common cause of the disease, and it thrives in warm, wet conditions.

The fungus thrives in warm, wet conditions, with the disease is most common in the summer months, when extended periods of leaf wetness and high humidity create favorable conditions for the fungus to grow. This makes areas such as Florida a hot spot for the disease

Symptoms of gray leaf spot include small, round or rectangular lesions on the leaves of the grass. These lesions can be gray, brown, or black in color, and they are often surrounded by a yellow halo. The lesions can grow and merge together, eventually causing the leaves to turn yellow and die.

Gray leaf spot is most damaging to young, actively growing grass, and it can cause serious problems in new turf or sprigged areas. The disease can also slow the growth of mature turf and damage the leaves of the grass, leading to a loss of vigor and lead to a general thinning as a result.

There are a couple of things you can do to help prevent gray leaf spot from developing in your St Augustine grass. Firstly, water your turf early in the day, so that the leaves have time to dry before nightfall. Finally, avoid using excessive nitrogen fertilizer, as this can promote lush growth that is more susceptible to disease.

Chinch Bugs

Chinch Bugs

Chinch bugs are small insects that can cause big problems for your lawn. These common pests are tiny around 1/8 of an inch long, have white wings, and move lightning fast, making them hard to see.

An infestation of chinch bugs can severely damage St. Augustine grass with large yellow patches forming being an indication of their possible presence. The bugs feed on the grass, sucking out the juices and causing the grass to turn yellow and eventually die. They can also transmit viruses, including mosaic virus, which causes leaf distortion.

If you have these pests on your lawn, it’s important to take steps to get rid of them as soon as possible to avoid further damage. If you think you may have them in your lawn, there are a few things you can do to confirm their presence.

One way to check for chinch bugs is to mix a couple of tablespoons of dishwashing liquid in 1 gallon of water. Then, evenly pour the solution over a three feet square area of the affected turf. After a few minutes Look closely and should find them moving around. If you see more than just a few then chinch bugs, they are likely the cause of the turf problem.

Iron Deficiency

If lawn disease is not the cause of the yellowing of your St. Augustine grass, then nutrient issues are then the most probable cause. The most likely problem is that of iron deficiency. Iron is essential for grass like St. Augustine to undergo photosynthesis, form chlorophyll, and break down nitrogen, which creates that beautiful green color in your lawn. A healthy lawn should contain 30-100 parts per million (ppm) of iron.

However, things like high phosphorus content in the soil or high pH levels can prevent the grass from absorbing enough iron. Excessive phosphorus can bind with iron and make it unavailable for the plant to use. This is especially problematic for St. Augustine grass because it has shallow root systems and can’t reach down to get the nutrient from deep in the soil. When applying fertilizer to mature lawns, be sure to select products that have low phosphorus levels or avoid using phosphorus altogether.

In most cases, St. Augustine grass will do best in slightly acidic soil with a pH between 6.0 and 6.5. However, as the pH level increases, the amount of iron that the plant can absorb from the soil decreases. If you’re having problems with your St. Augustine grass yellowing, it’s worth checking the iron levels and pH of your soil to see if that’s the cause.

Detecting Iron Deficiency – Soil Testing

Detecting Iron Deficiency

One obvious sign of iron deficiency can be found in the nature of the yellowing. If a patch develops, look at any yellowing patterns on the affected leaf blades and their ages. The typical manifestation of chlorosis brought on by an iron deficiency is the appearance of speckled patterns, particularly on young blades.

While this is a tell-tale sign the only way to discover the extent of the iron deficiency or pH levels is to conduct a soil test. In practice,  you should be testing your soil at least twice a year and possibly more. The test will provide you will detail of key nutrient levels as well as the pH of the soil. If you discover problems or imbalances, as part of any good lawn maintenance program.

There are several ways to test your soil. You can purchase inexpensive soil test kits from garden centers or online, you can also buy digital testers, and finally, you can send off a soil sample to your local county extension office or one of the commercial external testing facilities.

The results of a soil test though will help you more accurately determine the right action to take and whether additional application of iron sulfate or chelated iron amendments are needed to improve iron levels.

Checking pH Levels and Managing Alkaline

The pH level of the soil is also critically important. St Augustine grass likes slightly acidic soil. If the soil pH levels rise too much above this it can affect the iron absorption into the grass. By reducing the alkalinity of the soil iron absorption will become easier and is another potential solution to the problem. 

One method of reducing the pH level is to apply sulfur in the form of sulfur pellets to the soil. The amount to add will depend on how much you need to bring down the pH. The effect of the sulfur application will be gradual as it takes a while for it the bring reduce the alkalinity. Don’t be tempted to overuse to speed up the process as this can damage the grass, you should follow the manufacturer’s guidelines

Nitrogen Imbalances

If you’ve noticed yellow patches on your St. Augustine grass, another common cause is nitrogen imbalances in the soil. This can be either through excessive nitrogen or through a nitrogen deficiency.

Excessive Nitrogen

Nitrogen is an essential element for healthy grass, but too much of it can damage the roots, and burn and dry out the leaves.

This form of chlorosis, or yellowing of the blades, is caused by high concentrations of nitrogen which could be the result of overfertilization, the use of the wrong type of fertilizer, or even the urine build-up over time from dogs as they generally like to urinate in the same spots on your lawn.

Excessive nitrogen can also cause various other problems such as an increased thatch buildup. Thatch is a layer of fresh or dried grass and roots that can block water and aeration. This can make your lawn more susceptible to diseases and stresses from extreme temperatures.

Again,, regular soil tests should flag up any issues and prevent you from overfertilizing or using the wrong type of fertilizer, one with unnecessarily high nitrogen content. A soil test will provide you with the extent of the problem and you can take steps to correct the issue.

This may involve changing your fertilizer or watering schedule. Because St. Augustine is a warm-season grass, it is recommended that slow-release fertilizers be applied only two or three times during the summer at four to six weeks apart. As your grass does dormant during the fall you should stop your applications.

Nitrogen Deficiency

Nitrogen Deficiency

Grass needs nitrogen to grow and strive. Too much causes problems but a lack of nitrogen can be just as damaging.

Nitrogen deficiency is a common problem that can lead to yellowing leaves, stunted growth, and poor plant health. Nitrogen deficiency is often caused by excessive leaching, which can happen when watering with too much force or frequency. It can also be caused by a lack of nitrogen in the soil, either because it was never added or because it has been used up by the plants.

The good news is that nitrogen deficiency is relatively easy to fix. The first step is to identify the problem. A soil test will provide you with the information you need to be able to add the right amount of fertilizer to correct the problem


Over-watering your lawn can lead to a number of problems, including root rot and your St Augustine grass dying. 

Root rot is caused by too much moisture around the roots of your grass. This can happen if you water too often or if you don’t allow the grass to dry out between waterings. Root rot can reduce the amount of iron in the soil, which can lead to chlorosis (yellowing of the leaves) and eventually the death of the grass.

So, how much water does your lawn really need? It depends on a number of factors, including the type of grass, the time of year, and the weather. In general, most grasses need about 1 inch of water per week. But during hot, dry weather, you may need to water more often to keep the grass from wilting.

If you’re not sure how much to water, it’s best to err on the side of too little rather than too much. Watering less often will encourage deeper root growth, which is more drought-resistant. And if you do happen to overwater, don’t worry – the grass will usually recover once the weather cools off and the rains return.

Summary: Is Your St Augustine Grass Turning Yellow?

Seeing your green lawn start to develop yellow patches can be quite concerning. As we have seen there are a considerable number of reasons why this might be happening but as we have also seen almost all the causes can be prevented by a good lawn care program. This is essential in that St Augustine grass is a relatively high-maintenance grass type and needs to be tended to carefully if you want to retain a beautiful-looking lawn.

Keith Hardy - Bio Photo
Keith HardySenior Editor

Keith has been involved in the gardening and landscaping industry for the past 21 years. From a jobbing gardener to running his own landscaping services. He is now dedicated to bringing you the latest in gardening news. Read more

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