Widespread testing for downy mildew resistance to fungicides in grapevines has revealed varying levels of sensitivity in different regions, dialling up the pressure on existing disease management programs and the need for new fungicides.
Downy mildew remains one of the top three diseases affecting grapevines, costing growers millions of dollars annually.
Bayer Crop Science recently carried out resistance testing of the fungicides commonly used for downy mildew control, as well as a new mode of action foliar fungicide it plans to introduce this year.
Darren Alexander, Bayer horticulture territory business manager in South Australia and the Sydney Basin, said downy mildew infection samples were collected from various field locations across Australia and analysed for sensitivity to fungicides, mainly at the South Australian Research and Development Institute (SARDI) in Adelaide.
“The results showed the downy mildew fungi had varying sensitivities to the products tested depending on the region. Throughout SA, there was not a lot of resistance to existing products, however in other regions such as Griffith and the Hunter Valley in NSW, down to the Yarra Valley in Victoria, there was different sensitivity to products like metalaxyl.
“This is one of the important active ingredients with curative effects that growers have long reached out to under high pressure conditions,” Mr Alexander said.
“If there is a loss of sensitivity to the performance of metalaxyl to the point it starts failing in the field, that’s potentially one less tool growers will have.
“A couple of products showed mid to high sensitivity loss and could have reduced effectiveness for disease control, indicating the current effective chemistry available to growers is likely to diminish in the near future.”
He said growers not rotating or mixing up their fungicide programs risked resistance to those products in their vineyards.
“By mixing up chemicals within programs and from season to season, growers can extend the effective life of different chemistries against downy mildew.
“Combining multiple groups of fungicides to ensure there is less pressure on any single fungicide mode of action is an approach that is encouraged by the Fungicide Resistance Action Committee (FRAC) – and is becoming well adopted.
“Growers should also make sure they use products at the right rate and spray volume appropriate to the canopy.
“With climate change increasingly changing regional weather patterns, it’s becoming harder to predict a normal season, so running a protective program is increasingly becoming a beneficial approach – being ahead of the game, rather than chasing your tail.
“In regions of high pressure, where downy mildew persists year-in year-out, there is a need to look at new chemistry, adopting different modes of action to make sure all available tools are used to combat the disease and ensure crops are protected through to harvest.”
Xivana Prime, a unique mode of action (Group 49) fungicide registered in grapes by Bayer Crop Science, containing the active ingredient fluoxapiprolin, showed no resistance to downy mildew isolates in any of the samples tested from across all regions.
Registered for use in both wine and table grapes, Xivana Prime acts on all stages of the downy mildew lifecycle, providing a strong tool in a protective program.
Mr Alexander said the new mode of action will add extra protection to the programs growers are using and help take the pressure off and prolong the life of existing fungicides. “Some growers have a limited number of products for the applications they want to apply,” he said.
“Xivana Prime is best applied in a program for protective activity. If it is applied following conditions that are conducive to downy mildew, it can provide effective protection, so long as it is applied before oil spots are seen in vineyards.
“It has a wide application window for downy mildew within the growing season, from 10cm shoots (E-L 12) through until 7mm berries (E-L 31).
“Another major benefit with Xivana Prime is the residue profile, allowing it to be used quite late in the growing season, with no measurable transfer of fluoxapiprolin to wine when used as directed.
“Additionally, Xivana Prime offers a longer lasting spray interval than some leading products, being 10–21 days depending on growth dilution and disease pressure, and it has a great safety profile for pollinators and beneficial insects when used as directed.”
To ensure the longevity of the new mode of action for years to come, and to support the continued effectiveness of existing fungicides, Mr Alexander said Bayer Crop Science recommended Xivana Prime always be used in a mixture with another effective downy mildew product, as per resistance management guidelines.
Trials conducted with Xivana Prime have shown that for effective downy mildew control, it is essential to use an adjuvant, with Maxx, Pulse Penetrant, Hasten or Agridex all recommended.
Trials have also been conducted to demonstrate that Xivana Prime has excellent compatibility with other products that may be added for powdery mildew, botrytis bunch rot and light brown apple moth control. Excellent compatibility has also been shown with other products used in viticulture.
“Some of the tank mixes for powdery mildew included the use of wettable sulphur, Prosper and Talendo, and we also looked at botryticide tank mixes including with Teldor, Switch, Scala, chlorothalonil and the biological product, Serenade Opti, and all were found to be compatible with Xivana Prime, even when used with other tank mix partners such as copper and mancozeb,” Mr Alexander said.
