Growing your own lemon trees is a great way for warm-climate gardeners to have an abundant supply of citrus fruit. Lemons also grow well in greenhouse settings. These trees do require some special care and suffer from a number of ailments, including yellow leaves. Yellowing lemon tree foliage can be caused by bacteria, nutrient deficiencies or fungal infections.
Citrus Variegated Chlorosis
This bacterial disease spreads when sharpshooter leafhoppers feed on citrus leaves. These insects carry a bacterium, called Xylella fastidiosa, which attacks the xylem, or woody part of the tree. This disease damages lemon trees less than sweet orange cultivars, but can reduce productivity over time. Trees with CVC show chlorosis, or yellowing, between their veins. They also have gummy brown lesions on the lower side of the leaves. Lemon trees suffering from CVC grow more slowly and produce fewer fruits. The lemons that do ripen are usually small, hard and very acidic. There is no cure for this disease. Remove and destroy affected trees.
Yellow Vein Chlorosis
This yellowing condition starts at the leaf veins and spreads outward, while the leaf margins stay green. Yellow vein chlorosis indicates that the lemon tree isn't getting proper nutrition. It's usually caused by root rot, excessive water, insect damage or physical damage by machinery. Rotting or damaged wood prevents proper nutrient transmission through the lemon tree, turning the leaves yellow. According to the University of Florida, yellow vein chlorosis may affect the whole tree or just a damaged branch or twig. Correct any drainage problems around the tree and avoid injuring lemon trees to prevent this condition.
Lemon trees growing in poor conditions often suffer from nutritional deficiencies, which cause leaves to lighten or turn yellow. Too little zinc causes yellow banding along the leaf veins, while too little manganese turns leaves yellow between the veins. Plants with nitrogen deficiencies have light, yellow-green leaves which later drop from the tree. Too little iron causes lemon trees to produce light yellow, almost white, leaves with greener veins. Correct all these conditions by applying appropriate fertilisers.
Alternaria Brown Spot
This fungal condition causes brown spotting on leaves and fruits. These spots are surrounded by a large yellow areas. Leaves affected by Alternaria brown spot often fall off prematurely. Your lemon tree is most susceptible to this condition when the leaves and fruit are still young. Alternaria brown spot infects trees more readily when the canopy is damp or wet. Prevent it by planting disease-free trees in high areas and at a wide distance to encourage the leaves to dry quickly after rain. The University of Florida recommends controlling this condition with copper fungicides, applied when new leaves are ¼ to ½ expanded. Apply fungicide again when the leaves reach full expansion, and again shortly after petal fall.
Greasy spot is a fungal disease that causes yellow spots on the upper surface of leaves, with corresponding greasy brown blisters on the lower part. Eventually, the leaves may fall off of your lemon tree. This fungus infects primarily mature leaves. Like Alternaria brown spot, it favours damp conditions. Apply copper fungicides and fungicidal oils to the lower part of the leaves to control this disease.