Huanglongbing

Citrus leaves

• Blotchy mottling, that is not uniform across the leaf.
• Yellowing of leaves and growing shoots, these leaves and shoots stand out from the normally green canopy.
• Small, upright, yellowish leaves with thickened leaf midribs and veins, sometimes resembling deficiencies (e.g. zinc or boron nutrient deficiency symptoms).
• Unseasonal leaf flushing that is out of phase with healthy trees.
• Leaf drop and dieback of branches.

Citrus flowers and fruit

• Out-of-season heavy flowering and fruiting on diseased branches.
• Small, lopsided fruit with small, dark, aborted seeds.
• Unevenly coloured mature fruit (particularly sweet oranges and mandarins in temperate and subtropical regions).
• Premature and excessive fruit drop.
• Bitter tasting fruit.

Whole citrus trees

• Become progressively unthrifty.
• Have leaf and fruit drop.
• Develop branch dieback and root rot, leading to eventual tree death.

In Murraya

• Yellowing and mineral deficiency-like patterns on leaves with potential for shoot dieback.

Plant stage and plant parts affected

All stages of citrus tree plants from young to mature are affected, however young plants may be more susceptible because their young flushing growth attracts the psyllids that carry the disease.

Some non-citrus hosts of huanglongbing may not develop symptoms.

Citrus conditions that can be confused with huanglongbing

Insect pests, some other diseases, physical damage and herbicide damage can appear similar to huanglongbing.

Australian citrus dieback

Australian citrus dieback can cause very similar symptoms to huanglongbing, including yellowing and mottling of leaves, reduced fruit size on the affected limb and dieback of foliage. This condition is most often seen in grapefruit varieties and Seville oranges, particularly trees adjacent to native vegetation.

Laboratory tests are required to distinguish Australian citrus dieback from huanglongbing, so any suspect symptoms need to be reported.

Winter yellowing

The sudden onset of cold temperatures in autumn can cause yellowing of the most recent leaf flush (usually the late summer or autumn flush). In young trees the whole tree may become yellow. This condition is commonly called 'winter yellows' because the foliage remains yellow throughout winter. Generally, leaves will re-green with the return of warmer weather in spring.

Nutrient deficiencies

Nutrient deficiencies can often cause yellowing or mottling of leaves and shoots; however, these symptoms are typically uniform (symmetrical) on each side of the leaf midrib. Huanglongbing in comparison causes asymmetrical yellowing or mottling of leaves. Nutrient deficiencies also tend to appear uniformly across the canopy on shoots of the same age, whereas huanglongbing symptoms tend to first appear on single shoots or branches.

Root rots

Citrus trees can often suffer root or collar rot (caused by the Phytophthora fungus), especially in poorly drained soils. Trees can also suffer from 'wet feet' during periods of prolonged wet weather or overwatering, characterised by blackened, rotted roots. These conditions can cause yellowing of leaves, leaf fall and tree dieback. Check the roots and trunk for signs of tissue browning and root death.

Citrus tristeza virus (stem pitting strains)

On grapefruit and oranges, stem-pitting strains of citrus tristeza virus (CTV) can cause:

• yellowish mottling and nutrient deficiency-like symptoms on leaves
• shoot yellowing
• production of small, lopsided fruit
• stunting and twig dieback.

If in doubt, always report suspect symptoms.

Huanglongbing occurs in many parts of the world.

The Asian strain is known to occur in Asia, the Middle East, North, South and Central America, the Caribbean and in Papua New Guinea.

The African strain is known to occur in Africa and the Middle East.

The American strain is known to occur only in Brazil.

Species and cultivars of citrus that are affected include:

• sweet and sour orange
• grapefruit
• mandarin
• calamandarin
• tangelo
• kumquat
• lemon
• lime
• pomelo
• tangelo
• citron
• trifoliate orange
• citrange.

Some Australian native citrus species may also be affected.

Other host plants of the Asian strain include:

• mock orange/orange jasmine (Murraya spp.)
• wampee (Clausena lansium)
• Chinese box orange (Atalantia buxifolia)
• Limonia acidissima
• possibly dodder (Cuscuta australis).

As well as citrus, the African strain has been detected in Cape chestnut (Calodendrum capense) and white ironwood (Vepris lanceolata).

Some non-citrus hosts of huanglongbing may not develop symptoms.

The bacterium lives in the host plant's food conducting tissue (phloem), where they impede the movement of nutrients.

There can be a time lag between infection and development of the first symptoms, and the type of symptoms that develop, due to a number of factors including:

• the strain of bacteria
• the species/cultivar
• the age of the plant
• the growth stage of the plant
• environmental conditions such as temperature.

Huanglongbing (Asian strain) can be transmitted by both the Asian and African citrus psyllids. The Asian strain is acclimatised to hot climates, so is able to survive in subtropical and tropical climates, as well as in cooler areas.

The African strain appears to be sensitive to hot climates and is only known to occur in cooler climates. It can be transmitted by both the African and Asian citrus psyllids.

The American strain is transmitted by the Asian citrus psyllid. The African citrus psyllid has not been found where the American strain occurs. The American and Asian strains occur in the same areas in Brazil, so it is likely that the American strain can tolerate the same climatic conditions as the Asian strain of the disease

Citrus is a significant crop in Australia, there is over 28,000 hectares of citrus planted and around 1,900 growers. Citrus production for the year ending June 2017 was valued at $724.4 million, with Queensland the largest producer of mandarins.

Significant economic damage would occur if huanglongbing were to arrive in Australia. The disease causes serious decline in affected trees and loss of productivity. Young trees may never produce marketable fruit while more mature trees may gradually become unproductive. If fruit are produced, the flavour can be affected and can taint extracted juice products. The lifespan of an affected orchard is markedly reduced as tree deaths take their toll.

Production costs would increase as growers try to manage the disease and the insects that can spread it, and as nurseries take steps to produce disease free replacement plants. Market access would be disrupted.

Home gardeners would also be affected as citrus is a common backyard plant.

Although bitter tasting, the fruit from an infected tree is safe to eat.

Movement of infected plant material (budwood, grafted trees, rootstock, seedlings) and plant material infested with infected psyllids can introduce the disease to new areas and spread the disease. Infested ornamental plants such as mock orange, orange jasmine and curry leaf have been known to spread the psyllids.

Infected adult psyllids can fly short distances, so are capable of spreading the disease locally. Tropical storms and cyclones could carry infected psyllids much further.

The bacteria can also be spread by grafting and marcotting with infected plant material.

The Australian Government closely regulates approved imports of plant material and monitors for illegal plant movement.

Regularly monitor citrus plants for signs of huanglongbing:

• Examine unthrifty trees, or trees with unexplained leaf and fruit drop, or dieback.
• Look for Asian and African citrus psyllids.
• Look for leaves with asymmetrical blotchy mottling, particularly when the plants nutritional status is known; i.e. when regular applications of citrus specific fertiliser with trace elements have been applied. This is a distinctive sign of huanglongbing.
• Look for out-of-season heavy flowering and fruiting on sick looking branches and/or small, lopsided fruit with small, dark, aborted seeds.

Report it if you suspect huanglongbing or the citrus psyllids that can spread the disease.

Protect your farm from plant pests and diseases:

• Protect your farm from emergency plant pests
• Understand good farm biosecurity
• Visit the farm biosecurity website

There is currently no cure for the disease.

Huanglongbing and the Asian and African citrus psyllids that can carry it are prohibited plant pests under the Biosecurity Act 2014.

Report them to Biosecurity Queensland immediately on 13 25 23 or contact the Exotic Plant Pest Hotline on 1800 084 881.

If you think you have found them, you must take all reasonable and practical steps under your control to minimise any associated risks. This is called a general biosecurity obligation (GBO).