• Orange trees usually have bright green leaves, but their leaves can become yellow due to disease or improper care.Anna Yu/Photodisc/Getty Images
  
  Mature orange trees reach up to 50 feet tall and produce attractive white flowers. They also grow one of the world's most popular fruits, which are high in vitamin C. Orange trees grow best in subtropical climates with temperatures between 55 and 100 degrees Fahrenheit in the warm season and 35 to 50 degrees during the cold season. Several treatable problems can cause the leaves of an orange tree to turn yellow.
  
  

Root Rot

• According to the University of Florida IFAS Extension, root rot can cause citrus tree leaves to yellow. Several different species of fungi can cause root rot. The fungi tend to infect trees when a gardener overwaters them or when they are planted in areas with poor soil drainage. Growers should examine orange tree roots for root rot by checking whether they are mushy and rotting. If a gardener finds rotting roots, he should reduce the amount of watering.
  
  

Alternaria Brown Spot

• The University of Florida IFAS Extension describes alternaria brown spot as a fungus that eventually causes brown spots on leaves followed by leaf yellowing. It can eventually cause leaves and fruit to fall off, severely damaging tree health and orange production. Fungicides can help kill the alternaria brown spot fungus after an infection. To prevent infections, orange tree growers should plant trees with enough space between them to allow for proper airflow, which helps the leaves dry out after rain or irrigation. Do not plant healthy trees near infected trees before treatment, because the spores can easily spread to nearby trees.
  
  

Salt Burn

• Salt burn can cause leaf yellowing and leaf dropping on citrus trees, including orange trees. According to Texas A&M University Horticulture, improper fertilizer use can cause salts to build up and harm the trees. To get rid of salt burn, growers should leech the soil by watering with pure water that contains no salts or fertilizers.
  
  

Nitrogen Deficiency

• Too little nitrogen in the soil can cause orange tree leaves to turn yellow, according to the University of Florida IFAS Extension. Specifically, new leaves start to grow in with a light green or yellowish color, and they grow somewhat smaller than normal. The leaf veins will have a slightly lighter color than the rest of the leaves. Growers can correct a nitrogen deficiency by applying a nitrogen fertilizer to the soil. Since other nutrient deficiencies and over-fertilization can also cause leaf yellowing, growers who suspect a nitrogen deficiency may want to send a soil sample to a lab for nutrient testing before changing fertilizer amounts.
  
  

Magnesium Deficiency

• A magnesium deficiency can cause splotchy yellowing on the leaves on an orange tree. According to the University of Florida IFAS Extension, yellowing from a magnesium deficiency usually starts on leaves closest to fruits and starts at the middle of the leaves. Gardeners can have the soil tested for a magnesium deficiency and then add Epsom salt (magnesium sulfate) if test results show low magnesium levels.