Identify the disease in which the tree is suffering. Inspect the trunk, branches, stems, foliage, flowers and fruit of the tree. Look for disease symptoms such as yellowing, browning, blistering, oily, brown, or black spots on foliage, dieback, blight and other signs of poor health.
Check the tree's soil moisture and review the tree's current watering schedule. Look for signs of dehydration and overwatering such as wilting leaves, dropping branches or stems and dropped fruit. Increase the watering schedule for under-watered trees and decrease for overwatered trees.
Take a soil test. Check to see that the soil's acidic levels complement the needs of the selected orange tree variation. Take steps to correct the soil levels if the results indicate a need. Speak with a local nursery or horticultural specialist on the appropriate steps after identifying the disease, or for assistance in disease identification.
Trim away all wilted, dead and dying branches, stems, foliage, flowers and fruit. Discard the diseased clippings immediately to avoid infecting other trees and plant life. Cut away any developing cankers or rotted areas from the tree whenever possible.
Hose down the tree's infected areas to wash away any resting fungus. Use a combination of household detergent (1/3 cup), household bleach (1 qt.), trisodium phosphate (2/3 cup) and water (3 qt.) to wash the tree. Wear gloves and protective eye wear while washing.
Spray the orange tree with a fungicidal application during the early summer months. Use a fungicidal spray that is specifically designed for citrus trees. Apply the spray when weather conditions are moderately warm with little to no rain in the forecast. Follow the directions closely.
Inspect the orange tree regularly for signs of improvement. Reapply fungicidal sprays when necessary. Follow directions closely. Remove all developing fruit on more seriously diseased orange trees to promote rapidly healing.
Avoid fertilizing the orange tree while it is diseased to provide vigorous growth of the infecting fungus. Resume fertilization when the tree returns to good health.