For more than a century, producers in the United States of America have battled to manage the effects of the Pierce’s Disease-causing bacteria Xylella fastidiosa (Xylella) and their progress is drawing increased attention as countries around the world struggle to contain the spread of the bacteria.
The National Xylella Preparedness Program – led by Wine Australia and Hort Innovation under the Plant Biosecurity Research Initiative – is closely following international progress as it works to improve our preparedness for the detection of Xylella, and in planning how we can minimise the impact on the diverse range of wine and horticultural sectors that are at risk from a Xylella incursion.
Manager of the National Xylella Preparedness Program, Craig Elliott, recently returned from a visit through northern California where he met with growers, viticulturists, researchers and government staff working to manage the impact of Xylella.
‘This trip provided a great insight into the impact that Pierce’s Disease has had on the wine sector in California as well as the progress that is being made to protect the sector into the future’, Mr Elliott said.
Shrivelled berries, an early sign of Pierce's Disease
Managing the spread of sap-sucking insects
Xylella can be transmitted by the movement of infected plant stock and through xylem-feeding sapsucking insects (vectors), although the effectiveness of transmission can vary between different insects.
While Xylella was first identified in California in the 1880s, it was the arrival of the Glassy-winged sharpshooter (GWSS) in 1994 that accelerated Xylella’s spread across California and among different plant species.
The GWSS proved to be a much more effective vector than endemic species and with its introduction the sector saw widespread devastation from Pierce’s Disease across southern Californian vineyards.
In the northern growing areas, the focus is on exclusion of the GWSS and trying to minimise the impact from native vectors that can still spread the bacteria but are considered to be less effective than the GWSS.
Given there is no cure currently for Xylella infection, the focus in California has been on preventing the spread of the more damaging GWSS vector to northern areas while working on solutions to sustain the sector into the future.
‘That strategy been quite successful with the GWSS only making marginal range expansions and 18 incursions eradicated’, Mr Elliott said.
‘What I saw though is the importance of understanding the bacteria and the vectors. UC Berkeley researchers have identified more than 360 Xylella genotype isolates and there is still more to learn about the evolution of Xylella and how it interacts with vectors and host plants.’
Managing the impact within vineyards
Mr Elliott said California’s experience shows that there is extensive movement of the vectors, and that area-wide management and managing hotspots were an important part of their program.
Vineyards that bordered riparian (the boundary between land and rivers or creeks) and other insect attracting areas were most commonly heavily affected, with up to 10 to 12 rows from the border being infected by Xylella.
Most growers remove the infected vines upon detection to reduce the pressure on the rest of the block, but these then need to be replanted to reduce the risk of the vectors moving deeper into the block searching for feeding sites.
‘This of course generates significant costs from planting and caring for new vines, but we looked at the importance of maintaining what you might call a “sacrificial set” of vine rows on that exposed edge to protect the remainder of the block’, said Mr Elliott.
There are some very positive developments in breeding genetically resistant and tolerant grape varieties at UC Davis and feedback suggests that the later generation varieties are constantly improving in quality.
‘The experience in Napa and Sonoma Counties showed the value of having a buffer protecting the vineyard and, in future, more tolerant vines in the buffer could be used to reduce production losses’, he said.
Replanting vines in Napa County.
Replanting vines in Sonoma County.
Preparing for Xylella in Australia
‘We still have a lot of uncertainty about how Xylella would behave in the Australian environment, but we are planning research projects to look at the potential of our native insects to act as vectors and to develop our diagnostic capability.
‘This work – coupled with contingency planning, identifying the Australian native plants that may be impacted plus those that may also act as a wild Xylella reservoir, and trying to improve the effectiveness of property and supply chain biosecurity and traceability – means we can build a higher degree of resilience into our sectors and community’, said Mr Elliott.
‘While we still don’t have a cure, there is work continuing overseas in this area and we’re all aiming to ensure that, if Xylella is detected in Australia, we can minimise the impacts as much as possible.’
For more information on Xylella, including the symptoms of Pierce’s Disease on grapevines, visit www.planthealthaustralia.com.au/pests/xylella-fastidiosa/.