Biohazards

Biohazards are the endemic and exotic pests, diseases and weeds that affect Australia’s plant industries, community, and environment. Biosecurity protects our plant resources for future generations.

Rose Aphids – the sap-feeding enemy of the Rose

Species: Macrosiphum rosae

Aphids are small, soft-bodied insects that are members of the insect order Hemiptera which also includes psyllids, cicadas, leafhoppers and bed bugs.

Macrosiphum rosae is one of the thousands of aphid species found globally. It’s host-specific, which means it feeds only on or in one species. This Aphid has a penchant for feeding on the sap of rose shoots, tips, and buds. Rose Aphids suck up nutrients from rose plants which damage their foliage and flowers.  

Aphid secretions cause Sooty Mould!

The damage to the Rose doesn’t stop there; the insect’s secretions, also known as Honeydew, cause a secondary disease, Sooty Mould. Sooty Mould disrupts the plant’s photosynthesis and respiration. Aphids also spread other diseases as they move from plant to plant. They are most active during spring and summer when tender rose shoots and buds grow.

Etymology

Macrosiphum is derived from the Greek ‘macro’ meaning large and the Latin ‘Sipho’ meaning siphon, both of which reference the Aphid’s siphoning mouthparts that suck up their host’s sap. Rosae has its origin in the Latin for Rose, rosy or pink.

Garden Sign Story – Rose Aphids

These tiny, soft bodied bugs can migrate hundreds of kilometres, and have offspring at a rate of 12 per day! 

Image caption: In a hurry to reproduce, aphids can lay eggs or give birth to live young. 
Photo: C. Quintin/FLICKR CC BY-NC 2.0 

War of the roses 

Aphids are a gardener’s nightmare. They reproduce in the millions, all busy slurping sap from buds and new shoots with their straw-shaped mouthparts.  

But we’ve got good insects on our side! Lacewings, Ladybug and Hoverfly larvae are released into the Garden to munch on aphids. Another helpful predator, parasitic wasps (Aphidiius rosae) lay their eggs inside aphids. The wasp larvae hatch and eat the aphid from the inside out.   

Image caption:  Look closely to find the bodies of parasitised aphids — sometimes you can see where the wasp emerged from the host. 
Photo: M.Yokoyama/FLICKR CC BY-NC-ND 2.0 

Having babies 

Aphids never stop multiplying. Two remarkable methods are parthenogenesis — where females bypass the need to mate and give birth to clones. And paedogenesis — where juvenile aphids are born pregnant! 

Armillaria – Armillaria luteobubalina

Common name: Honey Fungus

Armillaria is a genus of plant pathogens that are found worldwide and includes the parasitic fungi Armillaria luteobubalina. 

There's nothing sweet about Honey Fungus

Also known as Australian Honey Fungus, Armillaria luteobubalina, is a native fungus that causes the disease Armillaria root rot. Root rot is a significant cause of tree death and forest dieback; it infects many plants, including Australian natives.

Armillaria luteobubalina feeds on live tissue, and on dead and decaying wood. While most of the Root Rot is located underground, and in the water transport systems of the trees it has infected, it can spread some way up the trunk of a tree.

Etymology

Armillaria is derived from the Latin armilla, ‘bracelet,’ which refers to the frill on the fungi’s fruiting body, which resembles a bracelet. Luteobubalina refers to the olive, yellow colour of the mushroom (luteo) and the shape of the mushroom.

Garden Sign Story – Armillaria

A parasite lurks just out of sight. Growing ever larger, it feeds… draining this tree’s life away. 

Hidden infection 

It can be hard to spot, but if you look at this tree’s lower trunk, you can see lesions — a tell-tale sign of Root Rot (Armillaria luteobubalina.). These have been spreading slowly since around 2017.  

Armillaria fungi live as large underground webs of hyphae — root-like structures. They feed on roots and invade the host trees, feeding on living and dead plant material. Once infected, a tree cannot be saved.   

Image caption: In May and June you can see the fungal fruiting bodies growing up the trunk. 
Photo: B. Summerell  

Canna Rust – Puccinia thaliae

Common Names: Roya de la canna de Indias (Spanish), Rouille du balisier (French)

Canna Indica’s enemy

Puccinia thaliae is a fungal pathogen that causes brown spots known as Canna Rust on Canna indica (Canna lilies), a popular floral display plant with colourful blooms. 

Canna Rust occurs all over Canna leaves but is most prominent on the underside, eventually cloaking the entire leaf. The spots on the upper leaves eventually turn brown/black, and holes develop in older leaves. Finally, they become dry, wilt and fall.

Etymology

Puccinia thaliae was named to honour Tommaso Puccini, an Italian anatomist thaliae, is derived from the Greek word ‘thaleia’, which means ‘blooming’ or ‘bounteous’ and refers to Canna Rust’s abundance of spores.

Garden Sign Story – Canna Rust

Rust is a super-parasite. Billions of tiny fungal spores can travel for thousands of kilometres to find their next victims.   

Control, not cure 

There’s no cure for Canna Rust (Puccinia thaliae). Instead, the Garden’s Horticulture Team use control measures:  

1. We choose canna varieties that are rust resistant.  

2. Cannas are planted in full sun and spaced to create air flow around the plants.  

3. We water at soil-level, to keep the leaves dry. 

4. Healthy plants are stronger, so we provide extra nutrients and mulch.  

5. Infected leaves are removed, and we cut the plants to the ground in autumn.  

Image caption: Lines of rust pustules erupt parallel to the leaf veins on Canna indica. 
Photo: S. Nelson/FLICKR 

Rust fungi infect plants through the stomata — leaf pores. Once inside, they steal nutrients, growing ever larger while starving the plant.    

Image caption:   
Photo: Cesar Calderon, Pathology Collection, USDA APHIS PPQ, Bugwood.org CC 30.0 

Fig Psyllids – Mycopsylla fici

Common name: Fig Psyllid, Lerp

Psyllids are a group of sap-feeding insects related to aphids and scale insects that can spread bacterial infections and damage many plant species. 

Scale insects love to dine on Morton Bay Figs 

Psyllids are fussy eaters and only feed on specific species of host plants. The fig psyllid Mycopsylla fici favours the Moreton Bay Fig, Ficus macrophylla. There are many mature, magnificent specimens throughout the gardens, and arborists regularly check them to assess the damage and treat it.

Etymology

Fici is derived from 'ficus', Latin for figs or fig trees and the Italian word fico, which means “a fig”.

Garden Sign Story – Fig Psyllids

Feeling itchy? This tree has lice! Look closely and you might see the infestation of tiny sap-sucking insects.  

Unwelcome guests 

Fig Psyllids (Mycopsylla fici) live out their full life cycle on the host tree. Hatching from tiny eggs on the leaf-edges, the nymphs suck sap from the leaves as they develop into winged adults.  

Healthy figs can cope with some leaf-loss, but psyllid numbers increase in dry, hot weather.  Over time, trees can suffer die-back and death. We carefully maintain the fig’s water and nutrient levels. Keeping fallen leaves protects tree-roots and encourages natural psyllid predators.  

Sugar-house snacks 

Each soft-bodied psyllid nymph excretes sugar which crystalises, forming a lerp — a protective cover. These are a tasty treat for many birds and small mammals. 

Fusarium Wilt of bananas

Fusarium oxysporum f.sp. cubense

Common names: Panama disease of banana, Fusarium wilt

Fusarium Wilt is a vascular wilt disease; it attacks a plant’s vascular tissues, which contain water, nutrients and sap. The common fungal soil-borne pathogen that causes Fusarium Wilt in plants is Fusarium oxysporum; different plant species are susceptible to different strains of Fusarium. 

One of the many strains is Fusarium oxysporum f.sp. cubense which causes Panama Disease, is host-specific, growing only on bananas and is one of the most destructive diseases in banana crops globally.

Panama Disease is incurable

Fusarium Wilt was the first banana-specific disease to spread worldwide in the first half of the 20th century. The epidemic began in Central America on ‘Gros Michel’, a variety that dominated the global banana export trade. It had no cure; however, a resistant cultivar ”Cavendish”  was discovered and replaced Gros Michel all over the world wherever bananas are grown.

Growers can no longer save their skins with Cavendish Bananas 

In the past, Australian growers relied on Fusarium Wilt resistant cultivars such as the Cavendish Banana to avoid financial and crop losses due to Panama disease. Unfortunately a new strain of the fungus “Tropical Race 4” has evolved to attack Cavendish bananas and is now spreading around the world threatening banana production. The race is on now to discover disease resistant cultivars to replace Cavendish bananas.

Etymology

Fusarium is derived from the Latin 'fusus', meaning a spindle, which references the shape of the microscopic spore of the fungus. 

'Oxus' is from the Ancient Greek word for sharp and 'sporos' for seed. 'Cubense' tells us that it comes from Cuba.

f. sp. (forma specialis) indicates that this is a special form of the pathogen. Botanists use this fungi naming protocol to distinguish between different, usually pathogenic, fungal strains.

Garden Sign Story – Fusarium Wilt

Australians eat five million bananas daily! One of the world’s biggest crops, bananas have been fighting disease since the 1950s.  

Image caption: Biosecurity measures at banana plantation in Queensland. 
Photo: Karegg/Shutterstock 

Banana battles 

Since the 1950s, Panama Disease has caused devastating banana crop losses around the world. The disease is caused by Fusarium fungi — super-parasites that survive fungicides, fumigation and soil sterilisation.  

Cavendish bananas were introduced to Australia because they were resistant to the disease, but the fungus has evolved. Now, outbreaks are managed by strict quarantine, and destruction of infected plants. Our best hope is to develop new, resistant banana varieties. 

Image caption:  A cross-section of an infected banana stem shows how the fungus liquifies the plant from the inside.  
Photo: S. Nelson/FLICKR 

Clone consequences 

Banana crops are clones, grown from root-divisions rather than seed. They are genetically identical — making them particularly vulnerable to disease.  

Myrtle Rust threatens endangered Myrtle Lenwebbia sp. Main Range

Lenwebbia sp. Main Range is a critically endangered endemic member of the Myrtaceae family of plants – the family also includes eucalypts, paperbarks and lillypillies. Small in stature, it grows to 4 metres in height, has brownish fibrous bark, branches that are densely covered in short hairs, and has glossy green oval-shaped leaves. Like many of the Myrtle family, It is threatened by the exotic rust fungus Austropuccinia psidii.

First Nations uses of the Myrtaceae family

First Nations peoples have used members of the Myrtaceae family as plant medicines, bush foods and building materials for thousands of years.

Etymology

Lenwebbia, was named for the Australian plant ecologist Dr Leonard Webb who worked as a research scientist at the CSIRO Rainforest Ecology Section.

Main Range – is a reference to the Main Range National Park in South East Queensland, where this species is found.

Garden Sign Story – Myrtle Rust

Like families, friends and workmates: Communities of plants, animals and fungi interact, compete, protect and support each other.  

A sheltered life 

Australia is an island. Separated from other land masses for millions of years, unique life-forms evolved together — creating one of the most biologically diverse places on the planet. Unfortunately, this isolation has also made Australian plants vulnerable to exotic diseases like Myrtle Rust.

Three-quarters of Australia’s forests are dominated by gum trees and other members of the Eucalypt family, and they are under attack. Losing our Eucalypts would change Australia forever.  

Caption: Myrtle Rust has pushed some species to the brink of extinction. (Lenwebbia sp. Main Range).  
Photo: G. Phillips.

Brown Marmorated Stink Bug – Halyomorpha halys

A relative of the aphid, the Brown Marmorated Stink Bug is a member of the order Hemiptera and the Family Pentatomidae.

Etymology

Halyomorpha – is derived from the Latin ‘halare’, which means ‘to be fragrant’ or ‘emit vapour’, which references the stench the bug emits when touched.

The colouring of the bug is referenced by ‘marmorated’ from the Latin ‘marmoreus’, which means ‘of marble’.

Garden Sign Story – Brown Marmorated Stink Bug

These bugs don’t just smell; they’ve got nasty table-manners! They inject saliva into fruit as they feed, causing it to rot. 

Image caption: Adult and nymph BMSBs use their needle-like mouthparts to pierce an apple. 
Photo: US Dept of Agriculture/FLICKR CC BY 2.0 

Big bug attack 

Brown Marmorated Stink Bugs (BMSB) are on the march. From Asia, they’ve spread to Europe and the Americas, and invaded New Zealand. They’re a top priority for Australian biosecurity — they’ve already been stopped at the border several times.  

BMSBs are a serious threat to food-production. They feed on crop-plants including apples, corn and tomatoes. They’re resistant to insecticides and have no natural predators in Australia. Be on the look-out!  

Image caption:  You can help: If you spot a BMSB, take photos, capture it if possible, and call the Exotic Plant Pest Hotline: 1800 084 881. 
Photo: David R. Lance, USDA APHIS PPQ, Bugwood Org. 

Stench defence  

No-one wants to be eaten. Predators and people be warned — whenever Stink Bugs feel threatened, they spray putrid chemicals from holes in their abdomen.  

Visit the Invasive Species Compendium

Peronosporaceae, a family of Water Moulds and Downy Mildew

Peronosporaceae is a family of water moulds with more than 600 species, most of which are downy mildews. The Phytophthora species threaten native ecosystems as well as agricultural and horticultural crops. 

Phytophthora spread via spores through waterways and soil, which can then be mechanically spread via shoes, tent pegs, gardening tools, and bike and car tyres. It has a wide host range (over 1000 species), and once introduced into an area it can exist on the roots of many different plants, sometimes without causing visible symptoms on the foliage.

Kauri Dieback – Phytophthora agathidicida

Common name: Kauri dieback, Kauri collar rot

Agathis australis, the Araucariaceae that towers above all others

Phytophthora agathidicida is a plant pathogen named for the tree species it attacks, the Kauri Pine (Agathis australis).

The Kauri is a magnificent coniferous tree, towering over its neighbours in the forests of the northern North Island of Aotearoa (New Zealand), often reaching heights of 40-50 metres tall. It is a member of an ancient Gondwanan plant family that goes back 175 million years in the fossil record.

First Nations' uses of Kauri

Māori Peoples consider Kauri to be ‘taonga' – treasures of great significance. 

Māori had many uses for kāpia (Kauri gum) they made gum torches and Kauri ash was mixed with oil and fat then used to colour moko (facial tattoos). It was also an ingredient in chewing gum. In the early years of European colonisation, Kauri gum was exported in large quantities as a varnish.

The Etymology of the pathogen:

Phytophthora is derived from the Greek word ‘Phyto’, meaning plant, and ‘phthora’, which means to decay, ruin or perish.

The species name, agathidicida references Agathis, the tree genus affected by the water-borne pathogen.

Agathis is the Greek word for a ‘ball of twine, which refers to the shape of the tree’s male pinecones.

‘Cida’ is derived from the Latin for strike or kill, hence – ‘kills Agathis’.

Garden Sign Story – Kauri Dieback

Giants of the forest, New Zealand’s Kauri Pines can stand for two thousand years. Now they’re under attack… and falling.   

Giants are falling 

In 2006, a virulent new strain of the deadly water-mould (Phytophthora agathidicida) was found in New Zealand. Spread by the movement of people and animals through the forests, it only attacks Kauri Pines, causing dieback and death.  

If the disease reaches Australia, Kauris across the country will be vulnerable. There’s no cure, but you can help prevent the spread of the water mould: After visiting bushland, disinfect your shoes, tent-pegs, bike and car tyres.   

Phytophthora dieback – Phytophthora cinnamomi and other Phytophthora species

Common Names: Root rot, water mould, Phytophthora dieback

Considered a threat to Australian plant communities and the animals that depend on them, P. cinnamomi and a number of other Phytophthora species are major soil-borne pathogens of crops, forestry and natural vegetation, especially in southern Australia. 

It has been distributed worldwide by man and globalisation; its initial spread is thought to have been via infected nursery  and crop plants, which are still a means of transmission. It is also dispersed via seepage, drainage and irrigation water and via spread of infested soil on footwear, digging implements and tyres

Root rot is an unrelenting threat

When environmental conditions are not suitable for its growth, P. cinnamomi lies dormant, germinating and spreading when conditions improve. 

Etymology:

Phytophthora is derived from the Greek word ‘Phyto’, meaning plant, and ‘phthora’, which means to decay, ruin or perish.

Garden Sign Story – Phytophthora dieback

Last survivor of an ancient line, the Wollemi Pine is an enduring link to Gondwana. Now it’s under threat. 

Plant Pandemic 

Death stalks Australia’s forests: Phytophthora means ‘plant destroyer’ and this introduced water mould lives up to its name. It travels through soil and water, infecting plants through their roots.  

It is deadly, but there’s still hope. Our expert plant pathology team is working to protect Australia’s plants from disease. Years of research and testing has identified the best way to apply Phosphonate; a cure that triggers the tree’s immune system to fight off the infection. 

Conservation partners: National Parks NSW; Wollemi Pine Recovery Team; Australian Institute of Botanical Science.  

Image caption: Plant pathology team member Maureen Phelan painting Phosphonate onto Wollemi Pine bark, testing to find the most effective dosage.  
Photo: E. Liew 

You’re my hero 

You can stop the pandemic in its tracks! Before and after a bushwalk, the simple act of cleaning your shoes and spraying them with disinfectant will work wonders.  

Image caption: Phytophthora cinnamomi is microscopic, spreading through waterways and soil transfer on shoes, tent pegs, gardening tools, bike and car tyres.  
Photo: S. Bullock 

A White Myrtle naturally resistant to Myrtle Rust – Rhodamnia argentea

Common names: Silver Myrtle, Brown Malletwood, Silver Malletwood, White Turpentine

The White Myrtle (Rhodamnia argentea) is a member of the Myrtaceae family with some natural resistance to Myrtle Rust, a devastating fungal disease. This resistance may prove significant in informing treatments for other species in the fight against Myrtle Rust. 

Learn more about Myrtle Rust in Episode 4 of What the Flora: Myrtle Rust: Pushing the Myrtle Family to Extinction. 

Etymology

Rhodamnia argentea is derived from the Greek word “rhodamnos” for a young branch, which references the tree’s slender branchlets, and the Latin word “argenteus” means silver.

Garden Sign Story – Myrtle Rust

Australia’s iconic forests are in danger. An exotic fungus is spreading fast, and we’re working against the clock to fight it.  

Fight the fungus 

Yellow pustules of Myrtle Rust (Austropuccinia psidii) smother soft new growth, killing fully-grown trees. Rainforest myrtles are particularly vulnerable: Some species are close to extinction, and so far there’s no cure.  

This graceful rainforest tree, Silver Myrtle (Rhodamnia argentea), has a natural tolerance to Myrtle Rust. How does it fight the disease? Dr Jason Bragg is studying its DNA to unravel the mystery. He hopes to find a way to boost resistance in other related trees.   

Image caption: Jason with another iconic tree affected by Myrtle Rust: Broad-leafed Paperbark (Melaleuca quinquinerva). 
Photo: L. Schwanz 

Depending on Eucs 

The ‘mewing’ call of Green Catbirds echo through Australia’s east-coast forests. Like Regent Honeyeaters and many other birds, they depend on the flowers and fruits of the Eucalyptus family.  

Image caption: Green Catbirds (Ailuroedus crassirostris) are a type of bowerbird. 
Photo: S. Mantle/ FLICKR 

Xylella fastidiosa – a bacterial pathogen that causes yellow leaves

Common Names: Bacterial Leaf Scorch, Golden Death, Phony Peach Disease, Pierce’s disease when found on grapevines – the disease’s name differs depending on the host plant it is affecting.

Xylella fastidiosa damages ornamental and fruit trees, and although it’s not present on Australian shores if it arrives, it poses a severe threat to our almond, macadamia, and pecan industries. 

It is a bacterial pathogen transmitted by sap (xylem)-feeding insects, mainly from the Cicadellidae family. Xylem transfers water and nutrients from a plant’s roots to its stems and leaves. Species in the Cicadellidae family include leafhoppers, sharpshooters, froghoppers, and spittlebugs.

Etymology

The pathogen’s genus name Xylella is a reference to its transmission of bacteria via the plant’s xylem. Xylem is derived from the Greek word ‘xylon’, which means “wood”, and is the most well-known xylem tissue.

Garden Sign Story – Xylella fastidiosa

Spring without blossoms, or autumn without colour — unthinkable!  But now a global disease threatens plant species, including cherries and maples. 

Image caption: Not just beautiful, Taiwan Cherry flowers are the main food-source for Taiwan Yuhinas during their breeding season.  
Photo: AWAI/Shuttrstock Id 783352681

Super-spreaders 

Many of the world’s most loved ornamental trees and fruit trees are under attack. First detected in the Americas, the bacteria Xylella fastidiosa has now spread through Europe, the Middle East and parts of Asia.  

Just like mosquitoes transfer diseases, sap-sucking insects spread bacteria between plants. They are big eaters — flying between plants and slurping hundreds of times their own body weight in sap every day. 

Image caption: In North America, Glassy-winged Sharpshooters drink sap so fast they also pee constantly, tiny droplets falling like rain on unsuspecting passers-by. 
Photo: Russ Ottens/University of Georgia/Bugwood.org

Plant quarantine 

Keeping the Botanic Garden disease-free is a challenge. All new plants are isolated for several weeks in our Nursery, where we monitor for symptoms and hitchhiking pests.

Pierce’s Disease – Xylella fastidiosa

Pierce’s Disease is a grape-specific bacteria spread by sap-feeding insects as they move from plant to plant, sucking and piercing foliage. After the insects have dined out on the leaves, the bacteria they’ve spread takes residence in the plant, where it lives in the sap. 

The bacteria blocks water and nutrient-carrying vessels, and the plant attempts to defend itself by producing more sap, adding to the blocked vessels. This series of events causes the leaves to turn brown or ‘scorch’, stunts the roots, causes dieback and then death. 

Pierce's Disease has no cure, and if it arrives on our shores, it is a potential threat to grapes and viticulture. We have border controls, including plant quarantine and biosecurity measures to prevent its entry into the country.

Etymology

Pierce's Disease was named for Newton Pierce, one of the United States’ first formally trained plant pathologists. Pierce’s descriptions from the late 1800s became the key symptoms that are used to identify the disease today.

Garden Sign Story – Pierce’s Disease

Attacking grape vines from California to Europe… Pierce’s Disease is a serious threat to wine production, and it’s heading our way.   

Image caption: Grape leaves showing symptoms of Pierce’s Disease, caused by the bacteria Xylella fastidiosa. 
Photo: E. Mazzoni /Alamy

A new threat

A new threat for Australia, these bacteria (Xylella fastidiosa) aren’t fussy.  They attack a wide range of important agricultural crops including grapes, olives and stone-fruit.   

Easily spread by sap-sucking insects, the bacteria reproduce quickly inside the host plant. They invade stems, roots and leaves, blocking water and nutrient-flow. Infected plants show symptoms like browning and withering leaves, stunted and misshapen fruit. Death occurs within two-three years, and there is no cure.

Image caption: Xylella fastidiosa has killed thousands of olive groves in Southern Italy.  
Photo: C. Palma/Shutterstock

Border control

Quarantine is one of the most important defences against biosecurity threats. You can help protect Australian agriculture and our natural ecosystems by declaring all plant material at the border.

Yellow Dragon Disease – Candidatus Liberibacter asiaticus, a bacterial infection in Citrus trees

Common Names: Yellow Dragon Disease, Asian Greening, Huanglongbing (Mandarin)

Candidatus Liberibacter asiaticus, or Yellow Dragon Disease, is one of the most dangerous citrus diseases globally. 

Caused by a bacterial infection, Yellow Dragon Disease spreads via small insects known as psyllids that carry the disease from plant to plant. It affects citrus in the Rutaceae family, and different species of citrus have different levels of susceptibility. 

Yellow Dragon Disease Symptoms

The symptoms include small, yellow, mottled leaves with blotching, yellow shoots, enlarged and corky leaf veins, and small and distorted fruits. 

There is no cure for Yellow Dragon Disease. At the time of writing, it has not been found in Australia. 

Etymology

When scientists classify bacteria, Candidatus is put before the name of a bacterium that cannot be grown in culture. Liber refers to the interior bark of plants, which is found next to the wood. Bacter is derived from the Greek word bacterion which means “small staff” and is linked to ‘baktron’ which means “stick, rod, staff or cudgel”, describing the first bacteria that were observed, which were rod-shaped. Asiaticus is from the Latin word meaning a native of Asia or occurring in Asia.

Garden Sign Story – Yellow Dragon Disease

Huánglóngbìng (Chinese), Yellow Dragon Disease, Candidatus Liberibacter asiaticus 

Citrus fruit are one of summer’s sweetest pleasures. But Yellow Dragon Disease is on the rise, killing and stunting citrus across the globe. 

Sour story 

Yellowing leaves and stunted fruit are early warning signs, and many plants infected by the bacteria (Candidatus Liberibacter asiaticus) will die. Luckily some citrus, including lemons, grapefruit and sour oranges, are able to tolerate the disease. 

A nasty trick helps the disease spread: It’s carried by psyllids — tiny sap-sucking insects — and the bacteria can hide inside them for three months. So far, Australia’s quarantine controls have stopped both infected plant material and psyllids.  

Image caption: Sweet orange infected with Yellow Dragon Disease. 
Photo: H.D. Catling/Bugwood.Org 

Eaten alive 

Parasitic wasps are a grisly but effective biological control! Female wasps lay eggs on psyllids, which are then devoured by the wasp larvae.  

Image caption:  1mm Tamarixia radiata wasps laying their eggs on psyllid larvae. 
Photo: US Dept. Of Agriculture