7 Lemon Tree Leaf Diseases and How to Manage Them

Everglades Farm is dedicated to providing premium citrus trees cultivated under optimal conditions, which ensures you will receive healthy specimens that exhibit greater resistance to diseases. Our Fast-Growing Plants collection features vibrant, high-yield specimens that not only enhance your garden but also increase your chances of a fruitful harvest.

1. Citrus canker

Citrus canker is a bacterial disease, and lemons are highly susceptible. The first symptoms are tiny, slightly raised, blister-like spots on young leaves. As they age, they turn tan to brown and develop a raised corky center with a yellow halo. Lesions show on both sides of the leaf, and some centers drop out and leave round holes. Heavy infection causes defoliation and early fruit drop.

Wind-driven rain spreads the bacteria, and so do mowers, tools, clothing, and people moving between trees. There is no cure. Plant in full sun where air moves freely and keep water off the foliage. Copper fungicide on newly emerging leaves shields tissue for only a short time and does not cure an infection.

If you catch it early on a few leaves or one branch, prune it out and double bag the material for disposal. Do not compost it, and disinfect everything that touched the tree. Heavily infected trees decline, and at that stage removal is an option.

2. Citrus greening

Citrus greening, also called HLB, is a bacterial disease carried by the Asian citrus psyllid. Early signs show on one branch or section of the tree: blotchy yellow mottling with yellow or corky veins, and small, upright leaves, followed by heavy leaf drop and dieback. Fruit comes out small and lopsided with a bitter taste.

Yellow veins alone do not prove greening, because yellow vein chlorosis also comes from root rot or girdling at the trunk, and cool fall weather can cause it too. Greening yellowing can also resemble zinc or iron deficiency.

No cure exists. Infected trees decline over several years, and removal is recommended once a tree is confirmed, because psyllids carry the bacteria to nearby citrus. Horticultural oil or neem oil against psyllids, plus good watering and feeding, may keep a tree productive longer. Where greening is widespread, even well-kept trees can become infected. If yellowing persists after fertilizing, ask your Extension office about testing.

3. Greasy spot

Greasy spot is a fungal disease that thrives in the rainy season, May through September in Florida. Yellow spots show first on the upper leaf surface. Irregular brown blisters follow on the underside, turning dark and slightly raised with a greasy look, and they eventually appear on top as well. Heavy infection drops leaves early, which weakens the tree and leaves it more open to cold damage and other pests. Lemons are among the susceptible types.

Rake and discard fallen leaves, because they feed the next round of infection. Copper with horticultural oil, applied thoroughly to the leaf undersides, controls it, and one or two sprays during the rainy season is typical.

4. Citrus scab

Scab is a fungal disease that looks like a rash of corky, wart-like outgrowths on leaves and fruit. Leaves and twigs distort and stunt. Water from rain or overhead irrigation moves the spores, and new spores form within days, so it spreads fast.

Remove infected leaves and twigs early, and switch to drip or micro-sprinkler irrigation. Time copper to the young spring flush and petal fall. Inspect nursery trees for lesions before buying.

5. Melanose

Melanose is a fungal disease. Early symptoms are small, dark brown, raised spots that feel like sandpaper. It rarely causes leaf drop and is usually minor on lemons. The fungus lives on recently killed twigs, and splashing rain or overhead water carries spores to young leaves, so older trees with plenty of dead wood suffer most.

Prune out small dead twigs and avoid overhead watering. Copper helps if the spots bother you, but a mature tree can live with melanose.

6. Algal spot

Algal spot is caused by a green alga, and it hits lemon and lime harder than other citrus. Raised lesions form on either side of the leaf, sometimes with mild yellowing around them, and later dry and flake off. Small depressions remain. Leaves rarely suffer much. The bigger concern is the limbs. Most of the year the alga is a gray-green film on the bark. Between June and September it blooms into rusty red rings that can crack bark and girdle branches.

Older advice called for copper, but newer UF trials found copper products do not appear effective. Keep the canopy open and the tree healthy, and ask your Extension office what is currently labeled for home use.

7. Sooty mold

Sooty mold is a fungal film, a thin black mat on leaves and twigs. The fungus grows on the sugary honeydew that whiteflies, scales, aphids, and mealybugs leave behind. A heavy coat reduces photosynthesis and delays fruit color.

The mold washes off easily. Oil sprays loosen it and help control the insects, and controlling the insects is the best fix.

Other lemon tree leaf problems

Bacterial blast kills individual leaves, with black lesions that spread from the leaf stem into the leaf axil, and it follows cold, wet weather. Frost or Dothiorella blight, now called bot gummosis, can cause similar damage. Botrytis forms gray, velvety mats on leaves and twigs. This can happen with prolonged wet, cool weather.

Yellow leaf veins can also point below ground. Yellow vein chlorosis, where the midrib and main veins yellow while the rest of the leaf stays green, comes from girdling by Phytophthora gummosis or root rot, and from cool fall weather. Armillaria root rot brings small yellow leaves and early leaf drop, and root nematodes shrink the leaves as slow decline sets in.

Lemon tree pests

Aphids cluster on new growth, curling and twisting young leaves. They are under 2 millimeters long, and the ladybird beetle is among their natural enemies.

Scale insects, mealybugs, and whiteflies all suck sap and leave honeydew that grows sooty mold. Snow scale also splits bark and can kill limbs. Horticultural oil at a 1/2 to 1% solution usually controls red scale, and snow scale needs several applications.

The citrus leafminer leaves curled leaves and silvery, serpentine mines. Young trees suffer most, and oil on a new flush, when the leaves are 1/2 to 1 inch long, usually protects them. The Asian citrus psyllid distorts new flush and carries greening, but frequent oil sprays and neem oil can take care of that.

The citrus rust mite russets leaves and browns fruit, peaking in June and July. Oil suppresses it, but never spray oil within three weeks of a sulfur spray.

The citrus nematode feeds on fine roots and causes a decline with slowed growth, a thinning canopy, small leaves, and twig dieback. Start with certified nematode-free plants and resistant rootstocks.

The lemon tree gall wasp, an Australian native, lays eggs in citrus stems, and the tree grows woody galls around the larvae. 

Nutrient deficiencies that mimic disease

Nitrogen and magnesium deficiencies show first on older leaves, while zinc and manganese deficiencies show first on the youngest. Leaf discoloration from nutrient deficiencies usually looks the same on both sides of the midrib, which helps separate it from an actual disease.

Nitrogen deficiency turns older leaves light yellowish-green, thins the canopy, and stunts the tree. Magnesium deficiency starts as yellow blotches along the midrib of mature leaves and spreads until only an inverted green V remains at the leaf base.

Iron deficiency bleaches new leaves nearly white while the veins stay green, a classic pattern of interveinal chlorosis. It is most common on calcareous soils, and waterlogged or cold soil makes it worse. Iron chelate applied to the soil is the most reliable fix.

Zinc deficiency leaves green bands along the midrib and veins on a pale background, and the leaves come out small, narrow, and pointed. Manganese deficiency looks similar, but the leaves keep their normal size, and it shows up in spring after a cold winter. Both usually call for a foliar spray on the young spring flush.

Midrib chlorosis, where the midrib and main veins turn yellow, can also come from foot rot or root rot girdling the trunk or roots, or from cool fall weather. Greening produces chlorotic patterns that imitate zinc and iron deficiency. If roots are the problem, nematode-resistant rootstocks and certified clean plants prevent the slow decline that thins the canopy and shrinks the leaves.

Environmental and cultural disorders

Not every problem is a disease with a lemon tree. Fruit drop is partly natural, as the tree thins its crop in the weeks after bloom. Fruit splitting follows heavy rain or watering after a long dry spell, and low potassium makes it worse. Even watering and feeding can prevent this.

Premature fruit drying, also called granulation, follows delayed harvest, water shortage, and very vigorous growth. Sunburn dries the peel on fruit exposed beyond the leaf canopy. Leaf drop and twig dieback occur after freezes, drought, and heavy pest pressure or root damage.

Lemons are not freeze tolerant. Freezing weather can drop the leaves and kill wood, flowers, and young fruit, so move potted trees to shelter and plan protection for in-ground trees before frost arrives. Cold soil slows nutrient uptake and can cause yellow veins in winter. Poor drainage and overwatering cause iron deficiency and root rot.

Preventing and treating lemon tree disease

Start with a certified tree from a reputable nursery. Inspect the leaves and trunk before buying, and plant with the bud union well above the soil line.

Sanitation is crucial. Rake and discard fallen leaves, prune dead wood, and remove infected twigs and fruit early. Keep foliage dry with drip or micro-sprinklers, and water in the morning so the bark dries.

Copper fungicides handle most fungal leaf diseases when timed to young leaves, and horticultural oil suppresses insects, mites, and scales while loosening sooty mold. Follow the label exactly, and skip copper while the tree is in bloom because it can aggravate postbloom fruit drop. Also avoid spraying in hot, dry, windy weather. A vigorous tree with a few spots often needs no spray at all. A windbreak can cut fruit scarring and slows wind-driven spread.

A seasonal checkup routine

Give your trees a good look over once a week through the spring and summer flushes. Check the underside of new leaves, and photograph anything odd on both sides. In late winter, after the last freeze, prune lightly before the spring flush. Before summer rains begin, rake up fallen leaves and clear the area under the canopy. Hold off on copper until petals have fallen.

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