citrus greening (Huanglongbing, HLB)

Symptoms

Leaves show a distinctive blotchy, asymmetric yellow mottle that does NOT match on the two sides of the midrib (unlike a nutrient deficiency, which is symmetric); shoots yellow sector by sector; fruit stay small, lopsided and fail to colour properly, staying green at the bottom ("greening") with aborted dark seeds and a bitter taste; the tree slowly declines and dies. Spread by the citrus psyllid Diaphorina citri.

Plants affected

• sweet orange
• mandarin
• lemon tree

Easily confused with

• iron-deficiency chlorosis

  How to tell them apart: On citrus, both yellow the leaves but only one is a quarantine emergency. Iron/zinc deficiency (abiotic): SYMMETRIC interveinal yellowing, mirrored on both sides of the midrib, green veins, no fruit effect, corrects with feeding/pH fix. Huanglongbing / citrus greening (Candidatus Liberibacter asiaticus, EU quarantine, NOTIFIABLE): ASYMMETRIC blotchy mottle that crosses the midrib unevenly, corky/yellow veins, and small lopsided fruit that stay green at the bottom and taste bitter - if the mottle is blotchy and asymmetric, treat as SUSPECTED HLB and report it.

• Mal secco

  How to tell them apart: Candidatus Liberibacter asiaticus (Huanglongbing/citrus greening) shows asymmetric, blotchy yellow leaf mottle that does NOT respect the midrib — the yellowing crosses from one side of a vein to the other unevenly, often with corky enlarged veins, and produces small lopsided fruit that stays green at the stylar end (bitter, with aborted dark seeds). Decline is slow and tree-wide. Plenodomus tracheiphilus (Mal secco, 'dry disease') instead causes shoots to wilt and dry from the TIP downward, and when you cut a wilting twig the wood underneath shows a diagnostic salmon-pink to orange-red discolouration in the vessels — a sign HLB never produces. Mal secco is favoured by cool, wet winters and wounds; HLB chlorosis is uniform-blotchy across the canopy with no wood staining.

• citrus tristeza virus

  How to tell them apart: Candidatus Liberibacter asiaticus (Huanglongbing/citrus greening) shows a diagnostic ASYMMETRIC blotchy mottle on the leaf blade: irregular yellow and green patches that do NOT mirror each other across the midrib, often with corky, swollen, yellowed veins and a leathery upright leaf. Fruit are small, lopsided, stay green at the bottom (colour inversion), have curved aborted seeds and a salty-bitter taste. Citrus tristeza virus (CTV) instead causes vein clearing/flecking (translucent veins held to the light), seedling yellows, and most tellingly STEM PITTING - pits and grooves in the wood under the bark of the trunk and twigs (peel the bark to see them). On sour-orange rootstock CTV gives sudden quick-decline collapse with honeycombed bark at the bud union, whereas HLB causes a slow sector-by-sector dieback. Use the asymmetric mottle for HLB versus the under-bark stem pitting for CTV.

• two-spotted spider mite

  How to tell them apart: Candidatus Liberibacter asiaticus (Huanglongbing / citrus greening) shows ASYMMETRIC blotchy mottle — patchy yellow-green islands that do NOT mirror across the leaf midrib, often with corky, swollen, raised veins and overall yellow shoots; fruit stay small, lopsided and green at the stylar (bottom) end with aborted, dark, abortive seeds and a bitter taste. Tetranychus urticae (two-spotted spider mite) instead causes fine, evenly distributed pale stippling and silvery-bronze speckling, mainly on the UPPER leaf surface, with fine silk webbing and tiny moving mites plus shed white skins on the LEAF UNDERSIDE; damage is symmetric and worst in hot, dry, dusty conditions. Look for webbing and live mites under a hand lens (mites) versus midrib-crossing asymmetric mottle and corky veins (HLB). Huanglongbing is an EU quarantine organism — if you see asymmetric mottle with corky veins and lopsided green-bottomed fruit, do NOT treat it as a mite problem; report it to your national plant protection organisation (NPPO).

• greasy spot

  How to tell them apart: Candidatus Liberibacter asiaticus (Huanglongbing/citrus greening) causes a distinctive ASYMMETRIC blotchy yellow mottle: the chlorosis crosses the midrib unevenly so the two halves of the leaf differ, and patches are diffuse with no sharp border. Look also for corky, raised, enlarged leaf veins, upright 'rabbit-ear' leaves, twig dieback and lopsided, small, hard fruit that stay green at the stylar end with aborted, dark, crooked seeds. Yellowing appears on scattered single shoots ('yellow shoot') across an otherwise green canopy. Mycosphaerella citri (greasy spot) starts as small chlorotic yellow specks on the UPPER leaf surface, but the diagnostic sign is on the LOWER surface: raised, blister-like, oily, greasy brown-to-black lesions that later become tan and crusty. Spots are discrete and roughly symmetric on both sides of the midrib, not the diffuse cross-vein mottle of HLB. Heavy infection causes premature leaf drop from the inside/bottom of the tree, and fruit may show a fine black 'greasy spot rind blotch' of tiny specks rather than the lopsided green fruit of HLB.

Common questions

How do I recognise citrus greening (Huanglongbing, HLB) on sweet orange?

Leaves show a distinctive blotchy, asymmetric yellow mottle that does NOT match on the two sides of the midrib (unlike a nutrient deficiency, which is symmetric); shoots yellow sector by sector; fruit stay small, lopsided and fail to colour properly, staying green at the bottom ("greening") with aborted dark seeds and a bitter taste; the tree slowly declines and dies. Spread by the citrus psyllid Diaphorina citri.

Do I have to report citrus greening (Huanglongbing, HLB)?

Yes. citrus greening (Huanglongbing, HLB) is a regulated quarantine organism in the EU. A suspected finding must be reported to your national plant protection organisation — they will confirm it and tell you what to do next.

How is citrus greening (Huanglongbing, HLB) treated?

Mainly with biological and cultural measures — removing affected material, changing the conditions that favour it, and treating with agents approved in your country. The exact steps and their timing depend on the plant, the season and where you live: Dr. Phyto works them out in a free diagnosis.