The yellowing of orange tree leaves is a common symptom known as chlorosis. Chlorosis is not a disease itself, but a visual signal that the plant cannot produce sufficient chlorophyll, the green pigment necessary for photosynthesis. This lack of chlorophyll points to an underlying problem preventing the tree from functioning normally. Diagnosing the specific cause requires observing the pattern of yellowing and identifying which part of the tree is affected.
Yellowing Caused by Nutrient Shortages
Nutrient deficiencies are a frequent cause of chlorosis, and the pattern of yellowing often helps pinpoint the missing element. Nitrogen deficiency is characterized by a general yellowing or pale green color across the entire tree, starting with the older leaves. Since nitrogen is a mobile nutrient, the tree pulls it from older foliage to support new growth, causing mature leaves to become uniformly yellow. Correcting this requires a high-nitrogen fertilizer application for immediate use.
Micronutrient shortages, such as iron, zinc, and manganese, appear primarily on new growth. Iron deficiency causes interveinal chlorosis, where the tissue between the veins turns light green or yellow while the veins remain distinctly green. This pattern occurs because iron is an immobile nutrient and cannot be moved from older leaves to new shoots. A solution involves applying chelated iron to the soil or a foliar spray directly onto the leaves for rapid absorption.
Zinc and manganese deficiencies also cause interveinal chlorosis on new leaves, often appearing as a mottled or blotchy pattern. Severe zinc deficiency can lead to “little leaf,” where new leaves are noticeably smaller and narrower than normal. For these immobile micronutrients, a foliar application of zinc and manganese sulfate is the most effective way to provide relief.
Magnesium deficiency, conversely, affects older leaves first, creating a distinct inverted V or chevron pattern of yellowing at the leaf tip while the base remains green. This can be treated with soil application of dolomite or a foliar spray of Epsom salts to restore chlorophyll production.
Yellowing Due to Water and Soil Problems
The physical condition of the soil and the amount of water it holds are linked to a tree’s ability to absorb nutrients, often leading to chlorosis that mimics a nutrient deficiency. Overwatering is a common culprit, as continuously soggy soil displaces oxygen, leading to anaerobic conditions that suffocate the roots. Damaged roots cannot breathe or may succumb to fungal root rot, preventing the uptake of water and nutrients, causing leaves to yellow and wilt. Addressing this requires immediate cessation of watering and improving soil drainage, perhaps by repotting container trees or incorporating organic matter to increase aeration.
Conversely, underwatering causes leaves to dry out, turn dull yellow, and eventually drop as the tree conserves moisture. Consistent, deep watering is required to ensure the entire root zone is hydrated, but allowing the top layer of soil to dry slightly between waterings is important. Soil pH imbalance is another issue that limits nutrient availability, known as nutrient lockout. Citrus trees thrive in a slightly acidic soil range, ideally between 5.8 and 6.5.
If the soil becomes too alkaline (a pH above 7.0), micronutrients like iron, zinc, and manganese become chemically “tied up” and insoluble, making them inaccessible to the tree. This high pH-induced deficiency is corrected by applying acidifying fertilizers, such as ammonium sulfate or elemental sulfur, to bring the soil pH back into the optimal range. The symptoms of this nutrient lockout appear identical to a true nutrient shortage.
Yellowing Caused by Insects or Disease
Biological stressors from pests and pathogens also induce leaf yellowing, which can be distinguished from nutrient issues by closer inspection. Sap-sucking pests like aphids, scale, or spider mites feed by piercing the leaf tissue and extracting plant fluids, causing localized stress. This feeding results in a mottled or speckled yellowing, often accompanied by leaf distortion or the presence of honeydew. Inspection, particularly on the underside of leaves and stems, confirms the presence of these insects, which can be treated with horticultural oil or insecticidal soap.
A more serious cause of chlorosis is the bacterial disease Huanglongbing (HLB), or citrus greening, spread by the Asian citrus psyllid. The defining symptom of HLB is an asymmetrical blotchy mottle pattern of yellowing on the leaf, meaning the pattern differs on one side of the central vein compared to the other. This asymmetrical pattern is a visual difference from nutrient deficiencies, which typically display symmetrical yellowing. Unfortunately, HLB has no cure, and infected trees show a general decline, producing small, lopsided fruit that remain partially green.