Why Is My Lemon Tree Dying? Diagnose Citrus Greening, Canker, and 6 Other Diseases

In March 2026, California’s Department of Food and Agriculture declared a new quarantine around Ramona, affecting more than 100 local businesses, after two backyard citrus trees tested positive for huanglongbing, better known as citrus greening. It’s the disease every citrus grower fears, and it’s spreading into home gardens, not just commercial groves. But greening is only one of at least eight problems that get lumped together as “lemon tree disease” — and most of them won’t kill your tree.

I’ve had readers send me photos of blotchy yellow leaves, convinced their lemon tree was dying of greening, when the real cause was nothing more than a cold windowsill. The mistake goes both ways: treating a fatal infection like a minor annoyance is just as costly as panicking over cosmetic leaf spotting. This guide sorts the two apart by symptom, tells you which diseases are fatal and which aren’t, and walks through what to actually do for each.

Quick Diagnosis: Symptom, Cause, and Whether It’s Fatal

Start here. Match what you’re seeing to the closest row, then jump to the matching section below for the full explanation.

Citrus Greening (HLB) — The One With No Cure

Huanglongbing is the disease that matters most, because it’s the only one on this list that always ends the same way. It’s caused by a bacterium spread mainly by the Asian citrus psyllid, a tiny insect that feeds head-down at a 45-degree angle on new leaf flush. The psyllid picks up the bacterium from an infected tree and injects it into the next one it feeds on — grafting can transmit it too, but that’s rare in home gardens.

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The signature symptom is blotchy, mottled yellowing that looks different on each side of the leaf’s midvein — a genuinely asymmetric pattern, not the mirrored yellowing you’d get from a nutrient deficiency. As the disease progresses, fruit turns lopsided and small, stays green at the blossom end even when ripe, and tastes bitter with aborted, dark seeds. Shoots die back and the whole canopy thins out.

Here’s the part that catches people off guard: once a tree is infected, there is no treatment that clears the bacterium. Management options — nutritional programs, gibberellic acid sprays — can extend a tree’s productive life by a few years, but they don’t cure it. That’s why the response to suspected greening is different from every other disease on this list: don’t spray anything. Photograph the leaves and fruit and report them to your county Extension office or, in California, CDFA at 1-800-491-1899. If you’re mistaking ordinary nutrient deficiency for greening, our guide to citrus leaf yellowing patterns walks through the nitrogen, iron, and magnesium symptoms that get confused with it most often.

The Ramona quarantine is a useful reminder of how this actually spreads in residential areas: psyllids move on wind and on infected nursery stock, which is why moving citrus cuttings or potted trees across county lines — even as a gift to a neighbor — is the single riskiest thing a home grower can do.

Citrus Canker — Quarantine-Regulated, Rarely Fatal to Lemons

Canker is a bacterial disease, caused by Xanthomonas axonopodis pv. citri, and it’s easy to confuse with greening on first glance because both cause leaf damage and both trigger a quarantine response. The lesions look different once you know what to check: canker produces raised pustules on both sides of the leaf, on fruit, and on stems, which turn corky and crater-like with a distinctive yellow halo and a water-soaked ring around the margin.

The bacterium travels on wind-driven rain and contaminated pruning tools, and it gets a major assist from citrus leafminers, whose larval tunnels create wounds the bacteria can enter through. Grapefruit and trifoliate orange are highly susceptible; lemons are moderately susceptible, which means infection is possible but severe defoliation and dieback are uncommon on lemon trees specifically. Fruit becomes cosmetically unmarketable well before the tree itself is at real risk.

As with greening, there’s no home treatment — canker is federally quarantine-regulated, and the correct move is reporting a suspected case to your Extension agent rather than reaching for a bactericide yourself.

Phytophthora Root Rot and Gummosis — When Wet Feet Kill the Tree

If greening is the disease you can’t stop, gummosis is the one you almost always can. It’s caused by Phytophthora species — water molds, not true fungi — that produce swimming zoospores whenever the soil around a tree’s trunk stays wet. Those spores splash up onto bark during rain or overhead irrigation and infect through the base of the trunk, which is why the classic symptom is a gummy, dark ooze — gummosis — right at the soil line rather than higher up the tree.

Left alone, the lesions expand and can girdle the trunk completely, cutting off the flow of water and nutrients and killing the tree within one to several years depending on how wet the site stays. This is the mechanism that connects almost every drainage mistake back to a dead citrus tree: a heavy, poorly draining potting mix or a container without real drainage keeps the root crown wet exactly where Phytophthora needs it to be.

Prevention beats treatment here. Switch sprinklers to drip irrigation so water never hits the trunk directly, plant on a slightly raised mound so the graft union sits above the soil and mulch line, and choose a tolerant rootstock if you’re planting new. If gummosis has already started, scrape away the infected bark back to a border of healthy, greenish tissue, let the wound air-dry, and treat it with a copper fungicide or Bordeaux mixture.

Melanose and Greasy Spot — Cosmetic Fungal Spotting

These two get mistaken for something more serious because the spotting looks alarming, but neither one threatens the tree’s survival the way greening or gummosis can.

Melanose, caused by the fungus Diaporthe citri, produces small tan-to-chocolate-brown raised bumps on young leaves and fruit rind. Its spores come almost entirely from dead twigs already in the canopy — not from soil or neighboring trees — which is why the single most effective control is pruning out dead wood before spring rains wash spores onto new growth. Grapefruit is the most susceptible citrus, but any citrus, lemons included, can pick up rind blemishes. If pruning isn’t enough, a copper fungicide applied from mid-to-late April, while fruit is still small, is the standard follow-up.

Greasy spot, caused by Mycosphaerella citri, shows up as dark, greasy-looking lesions on the underside of mature leaves with a matching yellow spot on top — on lemons specifically, the lesions tend to look more diffuse and yellowish than the sharply raised spots you’d see on tangerines. Its real cost isn’t the blemish; it’s that heavily infected leaves drop early in fall and winter, and a tree that loses leaves prematurely year after year does lose vigor and fruit yield over time. Raking and disposing of fallen leaves each autumn — rather than leaving them to compost under the tree — removes the fungus’s main source of new spores for the following season.

Sooty Mold — Not a Disease, a Symptom of a Pest Problem

Sooty mold looks like the scariest thing on this list — a tree coated in black soot — and it’s actually the least worrying. It isn’t a plant pathogen at all. It’s a non-parasitic fungus that grows on honeydew, the sugary waste that aphids, whiteflies, and soft scale excrete while feeding on sap. The mold itself doesn’t infect the leaf; it just sits on the surface, which is why you can wipe a light coating off with a damp cloth.

That said, a heavy coating does block light from reaching the leaf and can measurably slow photosynthesis, so it’s not purely cosmetic if it’s left unchecked for a full season. The fix is to treat the insect causing the honeydew, not the mold — horticultural oil applied when temperatures are between 45°F and 85°F controls whiteflies, aphids, and mealybugs, and once the pest population drops, the mold weathers away on its own within a few weeks. Our ranked comparison of citrus pest treatments covers which product fits which infestation.

Indoor and Potted Lemon Trees — When It’s Not Disease at All

If you’re growing a lemon tree in a container, especially indoors or in a UK conservatory, the odds that any of the above pathogens are the culprit drop sharply. The RHS is blunt about it: citrus grown under glass or indoors is usually disease-free, and the leaf drop, yellowing, and failure-to-fruit problems container growers report are almost always cultural, not infectious.

In my own zone 6 sunroom, a lemon tree dropped half its leaves every November for two seasons before I worked out the cause was an 8°C draft from a nearby window, not disease. Lemons need a minimum night temperature of around 10°C (50°F); anything colder, especially combined with overwatering — which is the single most common mistake with indoor citrus in winter — triggers leaf drop and, if the soil stays soggy long enough, root rot. The mechanism is the same low-oxygen root suffocation that drives Phytophthora infections outdoors, just without a soil-borne pathogen required to finish the job.

Before you treat a potted lemon tree for disease, rule out the cultural causes first: check the temperature at night, ease off watering through winter, and increase humidity with a tray of damp gravel rather than misting the leaves. If the tree still looks unwell once those are corrected, our full Meyer lemon care guide covers the watering and light baseline most indoor problems trace back to.

When Not to Treat, and How to Prevent Reinfection

Over-treating is as common a mistake as ignoring a real problem. Don’t spray copper fungicide as a blanket preventative for melanose or greasy spot on a tree that’s never shown symptoms — it’s an unnecessary cost and, sprayed during bloom, can harm pollinators. Don’t treat sooty mold directly with a fungicide; it does nothing if the sap-sucking pest underneath is still feeding. And never apply a bactericide at home for suspected canker or greening — both are quarantine-regulated, and unauthorized treatment can delay the official response that actually protects your neighborhood’s trees.

Where prevention genuinely pays off is sanitation and water management, because those two factors govern most of the diseases here that aren’t insect-vectored: rake and discard fallen leaves each autumn to starve out greasy spot spores, prune dead interior wood every year to remove melanose’s main source of inoculum, and keep irrigation off the trunk to deny Phytophthora the standing moisture it needs. None of that requires a spray program — it’s maintenance you’d be doing anyway.

Frequently Asked Questions

Can a lemon tree recover from citrus greening?

Not fully. There’s no cure once a tree is infected, though nutritional programs and gibberellic acid treatments can extend a tree’s productive life by a few years in commercial settings. For a backyard tree, the practical choice is usually reporting it and, once confirmed, removing it to protect neighboring citrus.

Is it safe to eat fruit from a tree with melanose or greasy spot?

Yes. Both are surface-level fungal infections that affect the rind’s appearance, not the fruit’s edibility. Peel or wash affected fruit as you normally would.

Why does my potted lemon tree keep dropping leaves even though I can’t find any pests or spots?

This is almost always cultural rather than disease-related — most commonly a cold draft, a night temperature under 10°C (50°F), or overwatering. Rule these out before assuming disease, particularly on indoor or container trees.

How do I tell citrus greening apart from a nutrient deficiency?

Greening produces yellow blotches that are asymmetric across the leaf’s midvein — different on each side. Nutrient deficiencies like nitrogen or magnesium shortage typically produce symmetric, mirrored patterns. If you’re unsure, photograph the leaf and send it to your county Extension office; they can confirm it far more reliably than a visual guess.

Sources

• UF/IFAS Gardening Solutions. Citrus Greening FAQ. University of Florida. gardeningsolutions.ifas.ufl.edu
• UC Statewide IPM Program. Asian Citrus Psyllid and Huanglongbing Disease. UC ANR Pest Notes. ipm.ucanr.edu
• San Diego County News Center. State Declares Citrus Quarantine in Ramona. countynewscenter.com
• University of Georgia Cooperative Extension. Citrus Canker. UGA Citrus Blog. site.caes.uga.edu
• UC Statewide IPM Program. Phytophthora Gummosis of Citrus. Home and Landscape. ipm.ucanr.edu
• UF/IFAS Ask IFAS. 2025-2026 Florida Citrus Production Guide: Melanose (CG019). University of Florida. ask.ifas.ufl.edu
• UF/IFAS Ask IFAS. Greasy Spot. University of Florida. ask.ifas.ufl.edu
• Clemson Cooperative Extension. Citrus Insects & Related Pests. Home & Garden Information Center. hgic.clemson.edu
• Royal Horticultural Society. Citrus Problems. RHS Advice. rhs.org.uk