Why Are the Leaves of My Orange Tree Turning Yellow?

Yellowing of leaves is also known as chlorosis in some instances. It results from the lack of the green pigment, chlorophyll. Orange tree leaves are prone to yellowing when growing in unfavorable conditions.

So what causes the yellowing of orange tree leaves?

Orange leaves turn yellow due to overwatering, iron deficiency, and fungal diseases such as root rot. Apply nitrogen fertilizer or urea to fix the yellowing foliage. If the soil is soggy, repot the tree in a fresh soil mix then water it 1-2 times a week while allowing the soil to dry out between waterings.

Reasons for yellowing leaves

Yellowing of orange tree leaves is mainly caused by overwatering, nutrient deficiency, excessive use of organic manure, and diseases like root rot.

When the leaves are discolored, water and mineral absorption and transportation in the plant are reduced.

Here are the causes of yellowing in detail:

1. Overwatering

Orange trees prefer well-drained soils and moderate watering. Overwatering causes the leaching of vital nutrients beyond the root zone.

Leaching of nutrients below the root zone temporarily causes nutrient deficiency. Although it is a temporary effect, it causes yellowing and premature death of leaves.

The leached nutrients are unreachable by the shallow plant roots. However, when the roots grow deeper, they recycle the leached nutrients.

• Heavy rainfall is a major cause of nitrogen leaching especially if you’re growing citrus trees on porous soil.
• Overirrigation is also leads to waterlogging that occasions denitrification and temporary nitrogen deficiency in the soil. Leaves of your citrus tree will turn yellow as a result of N deficiency. (Source – Mongi Zekri and Tom Obreza, IFAS Extension).

2. Leaching soil

Studies have shown that some regions have vulnerable soils that leach nitrogen and other nutrients easily, causing yellowing in citrus trees such as oranges.

For example, some regions of Florida have a “thin surface soil layer, high water table, and porous limestone in many areas, make the soil susceptible to downward movement of nutrients.” (Source).

Such soils are low in organic matter and drain very easily. They leach nutrients required by plants and soon, your orange trees will develop yellowing foliage from malnutrition.

3. Root rot disease

Root rot disease damages healthy roots. Root decay limits the absorption and transportation of nutrients to the few healthy roots.

With only a few roots functional, the leaves do not get enough minerals for chlorophyll formation.

3. Nutrient deficiency

Plants, including orange trees, need iron, nitrogen, zinc, and manganese for chlorophyll formation. Chlorosis will develop when plants lack these nutrients in the soil.

The two major types of chlorosis due to poor nutrition include the following:

• Iron chlorosis
• Nitrogen chlorosis 

In soils with high pH levels (above 7.0), high phosphorus, and the presence of lime, iron solidifies and becomes deficient in the plant since solid iron is unavailable for absorption by the orange trees.

Solid iron forms a plastic-like layer that limits soil aeration and causes waterlogging. Iron is an immobile nutrient and its deficiency affects only the new growth of leaves.

Chlorosis causes stunted growth, reduces fruit size, and decreases yield. You want to apply a fix as soon as you notice the signs of chlorosis to prevent serious issues such as fruit and leaf loss in citrus trees.

Nitrogen deficiency can also make your orange trees turn yellow. It causes a type of yellowing called nitrogen chlorosis and is common when you feed your orange tree with a lot of organic matter with high carbon content.

Microbes in the soil use nitrogen to break down carbon contents and end up depleting the available nitrogen nutrients in the soil. Nitrogen is a mobile nutrient that moves from the older foliage to the young leaves. So, nitrogen chlorosis begins from the older leaves towards the new growth.

Is chlorosis treatable? Yes, you can treat chlorosis through foliar spraying, soil fertilization treatment with a good citrus fertilizer, and trunk injections.

Inadequate manganese and zinc in the soil also cause leaf discoloration. Manganese and zinc are immobile nutrients and their deficiency will start showing in the young and middle growth of leaves.

4. Excessive use of manure

Manure contains high contents of salts and ammonia. The high concentration of salts and ammonia damages the roots and beneficial microorganisms. It is for this reason that ammonia can be used to get rid of weeds in the yard.

Reduced microbial activity limits the proper decomposition of organic matter. And, injury to the roots prevents the absorption and transportation of nutrients. This leads to a lack of enough nutrients to form chlorophyll, thus causing chlorosis.

5. Diseases

The primary citrus diseases causing yellow-orange leaves are root rot and Citrus Variegated Chlorosis (CVC). Root rot (Phytophthora gummosis) results from fungus or overwatering.

Root rot leads to the decay of healthy roots, thus hindering the absorption of nutrients. CVC survives in the xylem, thus affecting its functions. So, CVC limits the transportation of water and minerals to other plant parts.

How to stop orange tree leaves from turning yellow

Yellowing of orange tree leaves is treatable. However, with the longer lifespan of orange trees, there’s no permanent treatment for chlorosis.

Therefore, treatment approaches require reapplication after a certain duration to achieve better results.

Here’s how to fix yellowing orange tree leaves:

1. Apply a citrus fertilizer treatment

This is a slow treatment option for greening up yellow foliage on orange trees. So, to treat mild chlorosis, use nitrogen-rich or sulfur-rich fertilizers. This method is not effective for severely chlorotic trees. Below are two ways of soil-fertilization treatment:

• Application of nitrogen-rich and sulfur-rich fertilizers.
• Adding iron chelates into the soil.

Soil fertilization treatment is easy to administer. Apply the fertilizer or iron chelates into the topsoil. Ensure it is not beyond two inches of the topsoil. The fertilizer will dissolve into the soil upon watering or when it rains.

2. Treat with a foliar spray

This treatment method involves the application of water-soluble nutrients. It is a short-term method of treatment but gives effective results. For continuous desirable results, apply the foliar treatment every 60 to 90 days.

Here is the procedure for using foliar spray: 

• Add iron sulfate or iron chelates to water in a ratio of 1 pound to 20 gallons of water.
• Add soybean flour to the solution to help the solution stick to the leaves.
• Add a tablespoon of detergent to help wet the leaves.
• Use a sprayer pump and spray the solution on the leaves. Avoid spraying surfaces such as building as the solution can cause rust stains.
• Spray the solution in the evening or during cold weather. This is to give it ample time to absorb into the leaves. 

3. Trunk injection treatment

This is the application of iron or manganese-rich compounds to the trunk of chlorotic trees. This is an expensive treatment method that gives fast results and remains useful for up to 3 years. Here is the procedure for trunk injection treatment:

• Drill holes in the lower part of the trunk or the root flares near the soil.
• Introduce iron citrate or manganese-rich compound into the trunk through the holes.
• The best time to administer trunk injections is in the spring. But, if the leaves show any signs of chlorosis, you can apply them in any other season.

Use an arborist to administer trunk injections. This is to ensure safety and reduce the chances of trunk injuries.

4. Lower the soil pH to 6.0

This approach involves lowering the soil pH to prevent solidifying of iron nutrients. Lowering soil pH is a permanent way of treating chlorosis.

However, it is quite impossible to lower the pH levels of deeper soils. Thus, a more workable approach is the application of iron sulfates or chelates, or urea into the soil.

How to Prevent Leaves Yellowing and Falling

There is no permanent treatment for the yellowing of orange tree leaves. Thus, prevention is a suitable method to protect leaves from yellowing and falling. Below are some of the prevention methods:

• Avoid over-watering: Keep the soil of your orange trees well-drained. Water enough to avoid water-logging and causing root rot which leads to chlorosis.
• Fertilize the soil: Frequently fertilize the soil with nitrogen and iron sulfate-rich fertilizers. Fertilizing enriches the soil with vital nutrients, thus avoiding chances of nutrient deficiency.
• Avoid excessive use of manure: Use well-decomposed manure moderately. This is to avoid injuries resulting from high contents of salt and ammonia. Also, well-composted manure will not need nitrogen to break down. Thus preserving nitrogen available in the soil for the plants only.
• Soil pH: Do not plant your orange trees in soils with high pH levels. High pH levels solidify iron nutrients.

References

1. Utah State University, Forestry extension: Preventing-iron-chlorosis
2. University of Illinois, Extension Services: Yellow-leaves-can-indicate-plant-problems