Citrus Leaf Diseases: 9 Problems Identified by Symptom

Citrus leaf diseases are easiest to tell apart by where the damage starts and what shape it takes. Blisters under the leaf point to greasy spot. Raised corky spots with a yellow halo point to canker. Lopsided yellow blotches can mean citrus greening, which has no cure, according to UC IPM. Use the table below, then confirm with your extension office or plant-health service before you spray.

This guide covers lemons, oranges, mandarins and grapefruit. It also shows the problems that look like disease but are not, such as zinc shortage and leafminer damage. You also get a calculator that adds up the copper in your spray plan.

Citrus leaf diseases by symptom

Check three things first: the side of the leaf, the age of the leaf, and whether the pattern is the same on both halves. Turn over several leaves from different parts of the tree. One marked leaf tells you little.

The symptom notes come from the University of Florida guide Citrus Problems in the Home Landscape and the 2026-2029 Florida Citrus Production Guide, plus UC IPM and EPPO for greening and mal secco. No single figure for yield loss applies across regions and varieties, so this table does not give one. Local disease pressure, cultivar and weather change the result.

Fungal citrus leaf diseases

Scab, Alternaria and melanose infect when the leaf is wet. How long the leaf must stay wet differs by disease, and that drives spray timing and pruning decisions.

1. Greasy spot

Greasy spot is caused by Zasmidium citri-griseum, formerly called Mycosphaerella citri. Spores come from rotting leaf litter on the ground and enter through the leaf underside. Yellow spots appear above, then brown blisters that look greasy form below. Leaves can fall before the full symptoms show.

Symptoms are slow. UF/IFAS says internal growth takes months, and leaves stay susceptible for life. In Florida, spore releases usually peak from April to early June, and infection conditions run from June to September. That means the spray that matters happens long before you see damage.

What growers do: Florida guidance for processing Valencias is one oil or copper plus oil spray from mid-May to June. Early and mid-season oranges and processing grapefruit may need a second spray after the summer flush expands. Spray the underside of the leaf. Rotate products, because strobilurin resistance has been recorded in this fungus. A lemon or orange grove outside Florida should use its own regional timing.

2. Alternaria brown spot

The useful clue is blackened veins running out from dark lesions, often with a yellow border. Young shoots may die back. Fruit can get sunken spots and drop.

Variety matters here. UF/IFAS lists Minneola tangelos, Dancy tangerines and Murcotts as hosts, and less often Orlando tangelos, Novas, Lees and Sunburst. Lemons are not on that list, so a brown spot on a lemon leaf calls for a closer look at other causes. Sugar Belle is reported as highly tolerant, while Marathon is at least moderately susceptible.

What growers do: Plant clean stock. Use higher sites with good air drainage. Space trees widely. Avoid excess water and fertilizer, which drive soft growth. In Florida, the first spray goes on when the spring flush is one quarter to one half expanded, and a later spray follows petal fall. UF/IFAS uses over 10 hours of daily leaf wetness at 68-83°F, along with rainfall, as a sign that conditions favor infection.

3. Citrus scab

Scab forms corky, wart-like pustules on leaves, shoots and fruit. It needs little water: one to two hours of wetting for spores, then three to four more hours for infection. Water splash spreads it. Scab on lemons and sour oranges tends to stand up higher than scab on grapefruit, which can look like wind scar.

What growers do: If last season’s leaves carry heavy scab, Florida guidance uses three sprays: at about one quarter of spring flush expansion, at petal fall, and about three weeks later. With little carryover, the first spray can be dropped. Copper is less effective where scab pressure is high. Fruit usually becomes resistant by about two months after petal fall. Processing fruit rarely needs scab control, with Temples as a possible exception.

4. Sooty mold

Sooty mold is a black film that grows on honeydew from whiteflies, scales, aphids and mealybugs. It does not infect the leaf, but it can reduce photosynthesis and delay fruit color. It is also a sign that insects have reached damaging levels. Treat the insect, not the film. Oil sprays loosen the mold and help against some insects.

If you see the film along with cupped leaves, look for aphids on new growth. Our guide to black spots on lemons covers black spots in more detail.

▶ Managing aphids and foliage spots on lemon trees, with satellite monitoring. · 10:52

How long leaves must stay wet

Scab can infect after a short wet spell of a few hours, while melanose needs a long wet spell. Melanose attacks young fruit, leaves and twigs and grows from recently dead twigs in the canopy, so pruning out dead wood reduces its spore source. It is mostly a fresh-fruit problem in mature groves, though dead wood from greening has made young groves worth watching too.

Chart

Leaf wetness needed for infection

Hours; scab counts time after spores form, in addition to 1-2 hours to produce them; selected diseases

Melanose data come from the UF/IFAS melanose chapter. If you are working with lemons in a cool, wet climate, use a leaf-wetness sensor or a nearby weather station instead of guessing. In Florida, growers can check daily leaf-wetness estimates through the state weather network.

Bacterial and viral citrus diseases

These groups cannot be cured with a spray program once a tree is infected. Regulators may also be involved. Report suspected cases and do not move plant material until you get advice.

5. Citrus canker

Canker starts as small raised blisters. Lesions turn tan to brown with a water-soaked margin and a yellow halo, and the center grows corky. They usually show on both sides of the leaf. Old lesions can fall out and leave round holes.

Wind-driven rain, mowers and people spread it. Leafminer tunnels give it a way in: UC IPM notes this link in Florida and says canker is not found in California, where annual surveys run. Check your own state or national status before assuming a diagnosis.

What growers do: UF/IFAS names copper as the control tool. Leafminer control reduces entry points where both are present. In canker-clear regions, report first and spray later.

6. Citrus greening (huanglongbing)

UC IPM describes the early sign as yellowing on one limb or one sector of the canopy, with a blotchy, uneven pattern across the midrib. Leaves can also stay small and upright. Asian citrus psyllids spread the bacterium. UC IPM says it can kill a tree in as little as five years and that no cure exists.

What growers do: Report suspected trees. In California, UC IPM says infected trees must be destroyed to protect nearby trees. Where greening is established, UF/IFAS advises good irrigation, weed control, soil and foliar nutrition, and psyllid control to keep trees productive. Do not treat a greening tree as a simple nutrient case; leaf nutrition supports it but does not clear the bacterium.

7. Citrus tristeza virus

Tristeza shows as tree decline, small pale leaves on affected seedlings and, for some strains, pits in the wood. EPPO reports that lemon, grapefruit and sour orange seedlings react strongly to seedling-yellows strains, and that sour orange is still a common rootstock in the Euro-Mediterranean area. The brown citrus aphid is the most efficient carrier. It was found in Portugal and parts of Spain in the early 2000s.

What growers do: Buy certified stock. Do not graft unknown budwood. Watch for aphids on new growth, since they carry the virus. Ask a lab to confirm the virus, since other problems also cause decline.

▶ Citrus greening symptoms, the role of the Asian citrus psyllid and control options. · 13:17

Trunk and root problems that show on leaves

Sometimes the leaf is just the messenger. If new spots are missing but the canopy is thinning, inspect wood, trunk and roots.

8. Mal secco

Mal secco is a wilt fungus, now named Plenodomus tracheiphilus, and lemon is its main host. EPPO calls it the most destructive fungal disease of lemons in the Mediterranean region. It enters through wounds, including those from frost, wind and hail, and blocks water movement. Leaves then show water stress and twigs die.

Tolerant lemon cultivars replaced Femminello in parts of Italy and Greece, but EPPO notes that they gave lower quality. Cut out infected wood, remove the prunings, and contact your plant-health service. Some countries regulate this disease. Spray advice depends on local product approvals.

9. Phytophthora root rot and foot rot

This soil-borne disease sloughs off feeder roots and peels bark at the soil line. Reddish-brown resin may ooze from the trunk. Above ground, leaves develop yellow veins and then drop, with dieback in bad cases. If you searched for citrus root rot treatment, begin with water, not a product.

What growers do: UC IPM lists resistant rootstocks, irrigation management, fungicides and fumigation as the tools. Drain wet sites before planting. Keep the budunion well above the soil. Water in the morning so the bark dries, and keep grass and weeds away from the trunk. For container lemons, the Royal Horticultural Society warns that overwatering in winter is a common problem and that it can yellow or shed leaves.

Not every leaf problem is a disease

Nutrient shortage is a frequent lookalike. UF/IFAS warns that greening symptoms resemble manganese and zinc deficiency, so use a leaf test and, for uneven blotches, a greening test. Infected trees can also run low on nutrients. The key difference from greening is symmetry: nutrient patterns tend to match on both halves of the leaf, while greening blotches do not.

Chart

Leaf levels below which UF/IFAS rates a tree deficient

mg/kg (ppm) in 4- to 6-month-old spring-flush leaves of healthy trees; UF/IFAS standards

• Zinc: irregular green bands along the midrib and main veins, light yellow between, and narrow pointed leaves in severe cases.
• Magnesium: yellow blotches on older leaves near fruit, which merge and leave an inverted green V at the base.
• Iron: pale new leaves with greener veins, often on calcareous or poorly drained soil.
• Manganese: yellowing between veins with a dark green network, evident in spring after a cold winter.
• Leafminer: shallow tunnels in young leaves. This is an insect, and heavy attacks on young trees rarely kill them.
• Cold or overwatering: yellowing that follows weather or irrigation, without spots.

For the broader picture, see lemon tree diseases and treatment, which also deals with fruit problems. Citrus black spot is mainly described from fruit lesions and is another reason to look at the rind as well as the leaf.

Spray timing and copper limits

Copper appears in many of these programs, so the total adds up. UF/IFAS gives these points for Florida groves:

• About 2 lb of metallic copper per acre is a usual per-spray rate for greasy spot and scab with average-quality products.
• For melanose on large trees, no more than 8-12 lb metallic copper a year is needed, even if copper is also used for scab, canker or greasy spot.
• From May to September, spray copper when temperatures are below 94°F, at no more than 2 lb metallic copper per acre, without petroleum oil, and in at least 125 gallons per acre.
• Do not use strobilurin (FRAC 11) products repeatedly. Resistance is recorded in scab and greasy spot.

These are Florida figures for commercial groves. Registered products and limits differ in California, Spain, Australia and elsewhere. Always check the label and your local rules, and get professional advice before you spray. Keep records for each block.

Try the numbers for your own farm below.

Where lemon leaf disease risk meets supply

Lemon and lime output swings widely by region. USDA FAS forecast global fresh production at about 10.1 million tons in 2025/26. It tied the 12% fall in the EU forecast to weather and pest pressure in Spain, and the 36% fall in Turkey to spring frost. Frost matters for disease too, because EPPO lists it as a wound that opens the way for mal secco.

Chart

Fresh lemon and lime production by region

1,000 metric tons; 2025/26 is a January 2026 forecast; selected producers

A scouting routine for your grove

1. Walk at each flush. Spring flush is when scab and Alternaria decisions start. Check later flushes for leafminer.
2. Sample by block. Check trees at the edges, in low spots and near windbreaks, plus a few in the middle.
3. Turn the leaves over. Greasy spot blisters and scale insects are easy to miss from above.
4. Photograph and date. Compare the same trees each visit, because slow diseases need a record.
5. Take a leaf test. Sample at the right time of year. Leaf nutrients change with age, so timing affects results.
6. Send suspects to a lab. Canker, greening and tristeza need confirmation.
7. Log sprays. Note product, rate, weather and block for your copper total.

Satellite images can show which blocks are losing vigor between walks. They show plant change, not the cause, so a person still has to visit the tree. Farmonaut provides this kind of field monitoring for orchards on the web and by mobile app. For planting basics, see how to grow a lemon tree orchard.

▶ An overview of common plant diseases and pests, and how to detect them early. · 17:29

FAQ

What are the most common citrus leaf diseases?

Sources do not rank them worldwide, since it depends on the region. UF/IFAS covers greasy spot, canker, Alternaria brown spot and scab in its Florida guides, and EPPO highlights mal secco on Mediterranean lemons.

What are the black spots on my citrus leaves?

If the black layer wipes off, it is probably sooty mold, so look for sap-feeding insects. If the spots are in the tissue, check for greasy spot blisters on the underside or Alternaria with blackened veins.

Can a lemon tree recover from root rot?

It may if damage is limited. UC IPM says corrective action may include longer irrigation intervals when feeder root loss is small. Trees with a girdled trunk often decline. Get the soil and roots checked before you spend on fungicide.

Why are the leaves yellow with green veins?

Green veins on a yellow leaf point towards zinc, iron or manganese shortage. Yellow veins on a green leaf suggest girdling, root trouble or cool weather. Uneven blotches call for a greening test.

Is copper enough for all citrus fungal diseases?

No. UF/IFAS says copper is less effective where scab pressure is high and can spot fruit in hot weather. It helps with canker but cannot cure greening or tristeza. Follow the product label and local rules.

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