Pierce's disease

Xylella fastidiosa subsp. fastidiosa (the bacterium which causes Pierce's disease) is one of a number of recognised Xylella fastidiosa subspecies.

Xylella fastidiosa is currently being research, which may result in further changes to various subspecies in the future.

Recent changes in our understanding of the species has clouded the usefulness of many older host studies, which do not recognise the various subspecies.

Currently recognised subspecies of Xyllela fastidiosa:

• Xylella fastidiosa subsp. fastidiosa – Vitis spp. (commercial, ornamental and wild grapevines), Coffea spp. (coffee), some Citrus spp., forest and ornamental trees. This is the subspecies that causes Pierce's disease.
• Xylella fastidiosa subsp. multiplex – Prunus spp. (almond, peach, plum), pecan (not grapevine).
• Xylella fastidiosa subsp. sandyi – oleander, Jacaranda spp., day lily and magnolia (not grapevine or almond).
• Xylella fastidiosa subsp. pauca – Citrus spp. (coffee, olives).
• Xylella fastidiosasubsp. tashke* – leaf scorch of Chitalpa tashkentensis.

Other proposed subspecies of Xyllela fastidiosa:

• Xylella fastidiosa subsp. morus* – red mulberry (Morus rubra).

*Subspecies are rare.

Symptoms

Pierce's disease causes symptoms on grapevines that are very similar to those caused by water stress, as the bacteria produces gums that block the xylem of the host plant. Symptoms can also appear similar to those caused by salt toxicity.

Fruit production is severely reduced, with leaf scorching and wilting leading to shrivelled unsaleable fruit. The disease can be so severe that it can kill commercial grapevines within 1–4 years.

Plant stage and plant parts affected

All host plant growth stages can be affected.

In order to distinguish Pierce's disease symptoms from other grapevine problems, look for a combination of any of the following:

• dead leaf edges, especially tips
• 'scorched' looking spots in the middle of leaves
• shrivelled fruit clusters
• petioles (leaf stems) attached to the cane (shoots at least 1 year old)
• wood on new canes (shoots) maturing irregularly, producing patches of green surrounded by mature brown bark.

Pierce's disease of grapevines has been reported from parts of North, South and Central America, Taiwan and Italy.

The host range for Pierce's disease of grapevine caused by Xylella fastidiosa subsp. fastidiosa includes table and wine grapes (Vitis vinifera) as well as a broad range of ornamental and wild Vitis species.

As well as infecting grapevines, Xylella fastidiosa subsp. fastidiosa has been reported to cause other diseases of varying severity in a range of trees and shrubs, including:

• Prunus spp. (almond, peach, plum, nectarine)
• Coffea spp. (coffee)
• some Citrus spp., but not Citrus sinensis (navel, blood and sour oranges)
• forest and ornamental trees.

Xylella fastidiosa subsp. fastidiosa lives in the water-conducting cells of host plants. It is transferred from plant to plant by insects that are capable of carrying the disease in their mouthparts (vectors), of which the glassy-winged sharpshooter (Homolodisca vitripennis) is the most efficient.

Several other (more minor) plant sucking insects are known to be able to carry and transmit Pierce's disease. The ability of Australian native leafhoppers (and other xylem-feeders) to transmit the bacterium is unknown.

Australian grape production contributes significantly to Australia's economy, jobs and local communities. About 1,000 growers produce table grapes, much of which is exported. In 2015–16 Australian table grape production was valued at $431 million. During the same financial year, over 5,000 wine grape growers grew grapes which, once made into wine at 2,300 wineries, generated export sales valued at $2.56 billion in 2017. It is estimated that wine grapes contribute $40 billion to the Australian economy annually. (Source: Plant Health Australia – table grapes, wine grapes).

Pierce's disease has the potential to devastate Australian grape production, especially if it arrived in combination with its key vector, the glassy-winged sharpshooter. A 2017 Australian Bureau of Agricultural and Resource Economics and Sciences study estimated that the economic impacts of X. fastidiosa on Australian grapevine industries could be in the order of $2.8–7.9 billion.

Internationally, the immediate impacts of Pierce's disease include significant reductions in fruit and juice quality and quantity, with affected plants usually dying within several years of infection, and the death of entire vineyards quite common.

The presence of Pierce's disease may also have interstate and international trade implications for fresh fruit and other plant product exporters and propagation nurseries, within Australia and internationally.

Backyard growers would also be affected.

Local spread

Spread from plant to plant is by xylem feeding insects, such as the glassy-winged sharpshooter. Although the glassy-winged sharpshooter is not known to be present in Australia, there are many other xylem feeding insects here that may be able to spread the disease. These insects include members from the sub-order Auchenorrhyncha, such as leafhoppers (Cicadellidae), spittlebugs and their relatives (Cercopoidea) and cicadas (Cicadoidea).

When introduced by insects, disease symptoms generally appear first on the windward (side facing the prevailing wind) edges of vineyards. The disease then moves along the rows where plants touch each other.

Depending on flight capacity of the specific species, insect vectors can spread Pierce's disease from vineyard to vineyard within production regions.

Using infected rootstock or scion material can also transmit Pierce's disease. When introduced this way, the symptoms often appear randomly across the paddock depending upon where the infected material was used.

Pierce's disease is not spread by cutting tools or on other surfaces.

Long-distance spread

The main method of long distance spread is by the illegal importation and movement of infected planting material or through the movement of plants or fruit infested with insect vectors like the glassy-winged sharpshooter that are carrying the disease.

The Australian Government closely regulates and approves importation of host plant material that may contain Xylella fastidiosa or its insect vector, and monitors for illegal plant movement. This helps to prevent the introduction of diseases such as Pierce's disease into Australia.

The potential for long distance spread of Pierce's disease by infected insects being blown in extreme weather events is unclear. Although theoretically possible, it has not been previously reported.

Pierce's disease is not seed transmitted.

The biggest risk period for spread is when plants are actively growing and insect vector populations are likely to be high.

Refer to the Vineyard biosecurity manual (PDF, 2.9MB) for specific advice on how to monitor for exotic grapevine pests and diseases.

Refer to the Wine Australia website for advice on how to recognise and manage endemic grapevine pests and diseases.

Report suspected Pierce's disease to Biosecurity Queensland immediately on 13 25 23 or contact the Exotic Plant Pest Hotline on 1800 084 881.

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

There is no cure for Pierce's disease. In the United States of America, the movement of items capable of distributing Pierce's disease and its vectors (such as glassy-winged sharpshooters) are strictly regulated.

In Queensland, Pierce's disease is prohibited matter under the Biosecurity Act 2014. Report suspected Pierce's disease to Biosecurity Queensland on 13 25 23 without delay.

By law, everyone has a general biosecurity obligation (GBO) to take all reasonable and practical steps to minimise the risk of spreading this disease.