Papaya ringspot disease

Papaya ringspot disease is caused by a plant virus called Papaya ringspot virus - type P (PRSV-P) which belongs to the genus Potyvirus in the family Potyviridae.

Two distinct types of Papaya ringspot virus are known:

• Papaya ringspot virus - type P (PRSV-P) which infects papaya and cucurbits
• Papaya ringspot virus - type W (PRSV-W) which only infects cucurbit plants such as cucumber, melon, pumpkin, squash and zucchini.

Symptoms on papaya (pawpaw or papaw)

Fruit

• Immature fruit develop small green rings or spots or c-shaped markings on the surface.
• As the fruit matures the rings and spots grow larger and become darker in colour, and as the fruit ripens they can change in colour to yellow and brown.
• The number of spots present on a single fruit can vary from a few to many.

Leaves

• Mottled and mosaic colour patterns of varying severity can develop, and the leaf surface can become ruffled or puckered.
• One or more leaf lobes may become severely distorted and reduced in size, giving a ‘shoestring’ appearance.

Stems

• Dark green, water-soaked streaks can develop on petioles and stems.

Plant stage and plant parts affected

• Papaya plants of all ages are susceptible to the disease, with symptoms generally appearing more severe during cooler weather.
• Papaya ringspot disease can cause symptoms on leaves, stems and fruit.

Plant damage

• Affected plants can become stunted and fruit set can be markedly reduced or absent.
• Resulting fruit can have poor flavour, a tougher texture and are more likely to develop secondary fungal rots or black spot (a common fungal disease in southern Queensland).
• The lifespan of infected plants is also reduced, and in severe cases whole plants can die within months of infection.

Cucurbits (cucumber, melon, pumpkin, squash and zucchini)

• The strain of PRSV-P present in Australia has almost no symptoms on cucurbits, with only very mild distortion and mottling of leaves observed.

A second (closely related, but separate) disease of cucurbits is caused by PRSV-W.PRSV-W does not infect papaya plants.

Some plant feeding mites can cause papaya leaves to yellow, but they do not cause distinctive ring spots or c-shaped markings on fruit.

Nutritional deficiencies and herbicide damage can cause discolouration and distortion of leaves, with papaya plants particularly sensitive to both. However, neither causes fruit spots.

Some common fungal diseases of papaya can also cause spots on fruit and leaves of plants. Although these spots can at times become quite large and numerous; they do not develop the distinctive ring and c-shaped patterned marks caused by papaya ringspot disease.Fungal diseases of papaya that produce leaf and fruit spotting include:

• anthracnose, which causes sunken, brown, rot spots on fruit
• black spot or brown spot diseases, which cause small furry looking black or brown spots on leaves and fruit.

Papaya ringspot disease is only present in South East Queensland in the northern and bayside suburbs of the greater Brisbane area and in isolated pockets in the greater Bundaberg to Gin Gin area.

The disease has not been found in Central or North Queensland or elsewhere in Australia. The significant distances between production areas in Queensland has greatly assisted in reducing further disease spread.

All varieties of papaya (pawpaw or papaw) currently grown in Queensland are highly susceptible to PRSV-P. Papaya and pawpaw/papaw are very similar. Papaya are often pear shaped with a green/yellow coloured skin and generally have red or pink flesh while yellow pawpaw/papaw are often rounder and larger with a pale orange skin and yellow flesh

Other hosts of PRSV-P are cucurbits such as cucumber, melons, pumpkin and squash.

PRSV-P cannot survive outside of a living host plant. Like all viruses, PRSV-P undertakes its whole life cycle within a plant cell. Viruses use the resources (energy, raw materials and 'machinery') of their plant hosts to reproduce themselves.

Upon entering a new host plant cell, the virus sheds its protein coat, and uses the host plants systems to replicate its genome (RNA) and to make coat protein, which the virus uses to create the new virus particles.

Once new virus particles have been produced, they remain in the infected plant cell and wait for a means to transfer to a new host plant. PRSV-P is most commonly spread by aphids (insect vector).

In 2017, the value of production of Australia’s papaya industry was $31.6M; 95% of this coming from North Queensland. It's been reported that Papaya ringspot disease has limited papaya production in parts of Southeast Asia, Africa, India, South America, the Caribbean Islands and North America.

When papaya ringspot disease was first detected in South East Queensland in 1991, it ended commercial production within a few years. If the disease were to spread to the North Queensland production area, it's anticipated to have a similar impact on the North Queensland papaya industry and nearby local communities. PRSV-P resistant varieties of papaya are available, but are not considered as flavoursome and are much less productive than currently grown varieties.

Home gardeners can be severely impacted by papaya ringspot disease, with plants usually dying with 1-2 years of infection. Once infected, backyard plants become stunted, fruit production declines and any resulting fruit have poor flavour and texture.

Human assisted movement of infected papaya (pawpaw or papaw) plants and cucurbit seedlings could easily spread the virus and is the most likely way this disease could spread over long distances.

Papaya ringspot disease is spread from plant-to-plant by aphids, which are small sap-sucking insects. There are many species of aphids that are capable of transmitting the virus. When an aphid feeds on an infected plant, its mouth parts become contaminated with the virus, which is then transmitted when the aphid moves to the next host plant to feed. Aphids only need 10–15 seconds of feeding to pick up the virus, and a similar amount of time to deposit it.

PRSV-P spreads most quickly when aphids feed on plants that have been sprayed with insecticide – the insecticide irritates the aphid, and so it moves from plant to plant, spreading PRSV-P to every host plant it probes ('tastes') along the way.

The rate of disease spread depends upon aphid activity, and the proximity of papaya host plants to one another. The disease spreads very quickly in commercial orchards. Spread in urban areas is usually much slower, as papaya host plants are less common and further apart.

No other insects are known to transmit the disease.

Seed transmission of PRSV-P is considered to be so extremely rare that it is not considered to be a pathway for spread of the disease.

PRSV-P does not survive in soil or dead plant material.

Monitor papaya (pawpaw and papaw) plants, particularly plants that are unthrifty or stunted looking, and examine them for:

• green, orange to brown rings or spots or c-shaped markings on fruit
• yellowish mosaic patterns on leaves and/or leaf distortion
• water soaked markings on stems and petioles.

If you find plants with these symptoms and the plant is located outside of the papaya ringspot biosecurity zone 2 (PDF, 768KB), then you must report the incident as soon as possible to Biosecurity Queensland on 13 25 23.

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

There is no cure for papaya ringspot disease and the disease is very difficult to control once it has become established.

In home gardens within the biosecurity zones, infected plants should be removed as soon as symptoms are noticed.

Movement restrictions are in place to prevent disease spread out of the papaya ringspot biosecurity zones to Queensland's papaya (pawpaw and papaw) production areas.

Two papaya ringspot biosecurity containment zones (PDF, 768KB) legally prevent the spread of papaya ringspot disease out of South East Queensland.

Restrictions apply to the movement of host plants. Refer to the Queensland biosecurity manual (PDF, 1.8MB) for the risk minimisation measures that must be undertaken before moving PRSV-P host plants out of the papaya ringspot biosecurity zone.

You will need a biosecurity certificate to move papaya (pawpaw and papaw) plants out of papaya ringspot biosecurity zone 1.

You must apply for a biosecurity instrument permit to move cucurbit plants out of papaya ringspot biosecurity zone 2 to a place in Queensland outside papaya ringspot biosecurity zone 1.

For more information about biosecurity certificates and biosecurity instrument permits, contact the Customer Service Centre on 13 25 23 or email qld.plantquarantine@daf.qld.gov.au.

If you suspect you have found papaya ringspot disease in Queensland outside of papaya ringspot biosecurity zone 2 (PDF, 768KB) call us on 13 25 23.