Citrus greening

What you are looking at

Leaves with blotchy, asymmetric yellow patches that do not mirror across the midrib the way a nutrient deficiency usually does. Fruit from an infected tree is small, misshapen and lopsided, often staying partly green at the base even once the rest has coloured up, which is where the common name comes from; cut one open and the flesh tastes bitter and salty, with small, dark, underdeveloped seeds. Over several years an infected tree declines steadily: twigs die back, the canopy thins, and yellow shoots appear at random through it rather than in one identifiable area.

The insect responsible for spreading it is easy to overlook: a mottled brown-and-white psyllid only 3 to 4 mm long, found feeding on new shoot growth with its wings held tent-like over its body. It picks up the bacterium from an infected tree while feeding and carries it to the next healthy one it visits.

The world’s most destructive citrus disease

This bacterium is native to Asia and has no cure once a tree is infected, which is the fact that shapes every control strategy against it. It reached Florida in the early 2000s and devastated the state’s citrus industry within a decade, an outcome now treated as the cautionary case study everywhere else growing citrus commercially. It has since established across the southern United States, much of Central America, the Caribbean and parts of South America, and a related pathogen spread by a different vector, the African citrus psyllid, causes a similar disease across much of sub-Saharan Africa.

Europe has so far avoided the disease itself, though its insect vector was recorded for the first time in the European Union in Cyprus in 2023, and a biological control programme against the psyllid there began the following year aimed specifically at preventing the disease from following it in. Australia and New Zealand remain free of both the vector and the disease and treat keeping it that way as one of their highest-priority citrus biosecurity goals, given what happened to Florida’s industry once it arrived there.

What to do about it

Buy citrus trees only from a certified, disease-free nursery, since infected budwood and young trees are a primary route the disease travels by.

Where the vector is present, monitor new shoot growth regularly for the psyllid, since keeping the insect off healthy trees is the only real defence available once the bacterium is in a region. Support parasitic wasp release where a regional programme runs one, and reserve insecticide for a confirmed local psyllid population rather than a preventive schedule, given how much a single infected tree threatens the trees around it.

Remove and destroy a tree with a confirmed infection rather than attempting to treat it; there is no cure, and a standing infected tree is a continuing source of infection for every other citrus tree nearby.

Screen young or high-value trees with fine insect mesh in an affected region.

Outside the established range, report a tiny mottled psyllid on new growth, or fruit matching the lopsided, partly green pattern described here, to the national plant health authority rather than treating it as routine.

What it gets mistaken for

Nutrient deficiencies, particularly zinc and manganese shortage, cause similar-looking yellow mottling, but the pattern is symmetrical either side of the leaf’s midrib and the fruit is normal. This disease produces asymmetric blotching that ignores the midrib entirely and comes paired with fruit that stays lopsided and partly green no matter how the tree is fed.