Brown spots on citrus leaves signal a problem requiring attention to maintain the tree’s health. These spots are areas of necrotic plant tissue, indicating the foliage has been damaged by external factors. Since a brown spot is a symptom, not a disease itself, identifying the exact cause is the first step toward effective treatment. Accurate diagnosis is necessary because numerous issues, from fungi to insect feeding and nutrient imbalances, can produce similar-looking lesions.
Pathogens Causing Leaf Spots
Fungal and bacterial diseases are common causes of brown spots on citrus foliage, often resulting in complex lesions.
Citrus Scab
One frequent fungal issue is Citrus Scab, caused by Elsinoë fawcettii, which produces raised, corky, and warty lesions that are light brown to gray. These irregular pustules typically appear on young leaves, often causing them to become distorted or puckered as they expand. The disease thrives in warm, moist conditions, spreading via fungal spores carried by wind-driven rain and water splash.
Melanose
Another fungal culprit is Melanose, caused by Diaporthe citri, which creates small, dark, gritty, and slightly raised pustules on the leaf surface. These lesions can initially be surrounded by a yellowish halo, and their texture is often described as feeling like sandpaper. The fungus survives primarily on dead wood within the tree canopy, and its spores are dispersed to new growth by water, such as rain or overhead irrigation.
Citrus Canker
The bacterial disease Citrus Canker, caused by Xanthomonas citri, is a serious concern that presents as distinct brown lesions. These spots are raised, spongy, and can range from tan to dark brown, often surrounded by an oily, water-soaked margin that transitions into a yellow halo. The bacteria enter the leaf through natural openings or wounds and spread rapidly through wind-driven rain or on contaminated tools. Damage caused by pests like the citrus leaf miner can create entry points, significantly increasing the severity of an infection.
Pests That Damage Citrus Foliage
Insects and mites cause brown or discolored spots either through direct feeding or by facilitating secondary issues.
Citrus Rust Mites
Citrus Rust Mites are microscopic pests that damage the epidermal cells of leaves and fruit as they feed. This activity causes the outer surface of the leaves to lose its glossy texture and develop a dull, bronze-like or silvery appearance, known as bronzing or russeting. This texture distinguishes mite damage from the raised lesions caused by fungi.
Scale Insects and Sooty Mold
Scale Insects contribute to browning indirectly by producing honeydew, a sticky, sugary liquid. This honeydew serves as a substrate for sooty mold, a black, non-parasitic fungus that grows as a thin layer over the leaf surface. Although the mold does not infect the leaf tissue, a heavy coating blocks sunlight, reducing photosynthesis and potentially leading to yellowing and premature leaf drop.
Citrus Leaf Miner
The larvae of the Citrus Leaf Miner create distinctive, silvery, serpentine tunnels just beneath the leaf surface. The tissue surrounding these mines often turns necrotic and brown, especially after the larvae have exited, leaving behind a distorted and curled leaf. While largely cosmetic on mature trees, this tunneling weakens new growth on young trees. The open wounds also provide an easy entry point for Citrus Canker bacteria.
Environmental and Nutritional Causes
Abiotic factors, such as nutrient deficiencies and environmental stress, can cause brown spots or browning patterns on citrus leaves.
Nutrient Deficiencies
Micronutrient deficiencies, including Manganese (Mn) and Zinc (Zn), display patterns of chlorosis that can eventually lead to necrotic spots. Manganese deficiency typically appears on young leaves as a mottled yellowing, where the main veins remain dark green while the interveinal areas turn a lighter green or yellow-bronze. Zinc deficiency, often called “mottle leaf,” presents as irregular yellow blotches between the green veins, leading to smaller leaves and stunted terminal growth. Both deficiencies are frequently observed in calcareous soils with a high pH, which makes these nutrients less available for the tree to absorb.
Environmental Stressors
Environmental stressors cause localized damage that appears as brown spots or scorched margins. Sunburn or heat stress causes browning on exposed leaf surfaces, while cold damage or frost leads to dark, water-soaked areas that later turn necrotic and brown. Incorrect application of chemicals, such as over-concentrated pesticides or certain fertilizers, can result in phytotoxicity, physically burning the foliage and leaving behind brown, scorched spots or margins.
Next Steps for Diagnosis and Control
Determining the underlying cause requires a close examination of the spot’s physical characteristics and location.
Diagnostic Clues
Pathogen lesions, such as those from Citrus Scab and Canker, are typically raised, corky, or scabby. In contrast, browning caused by rust mites is smooth, dull, or bronze-like. Insect damage is often identified by the presence of pests, sticky honeydew residue, or the distinctive serpentine tunnels of a leaf miner. The location of the damage also offers clues: nutrient deficiencies usually show a uniform pattern across new or old growth, while environmental burns are concentrated on the most exposed parts of the tree.
Management and Professional Consultation
General cultural practices help manage many issues. These include pruning to remove dead wood that harbors fungal spores and thinning the canopy to improve air circulation and reduce humidity. Ensuring proper irrigation and nutrition strengthens the tree’s natural resistance to pests and diseases. When the cause remains unclear, especially if spots are numerous, rapidly spreading, or exhibit the raised, haloed appearance of Citrus Canker, professional consultation is prudent. Arborists or local agricultural extension offices can provide laboratory testing for definitive identification of pathogens. Consulting experts is important for highly regulated diseases, as they advise on appropriate management protocols.