Issued: 18 November 2021, updated 19 November 2021
Recent weather conditions have been conducive to downy mildew infection and symptoms of the disease have been reported in some regions. This eBulletin provides a reminder about downy mildew control.
CHEMICALS REGISTERED FOR USE
The recommendations section of Agrochemicals registered for use in Australian Viticulture 2021/2022 (‘Dog book) lists the active constituents registered for downy mildew control in Australian viticulture that can be used on grapes destined for export wine. The restrictions on use are imposed to ensure that the chance of residues is minimal. A list of all the active constituents registered for downy mildew control can be found on the AWRI online search portal here.
Phosphorous acid and captan are active constituents that are not currently (2021/22) accepted for use on grapes for export wines. The use of phosphorous acid is discussed on the AWRI’s phosphorous acid webpage.
MANAGING THE DISEASE
There are two main types of downy mildew fungicide, classified according to their timing in relation to infection events (pre-infection and post-infection), which also describes when they are most effective.
Pre-infection fungicides (protectants)
These protect the vine by preventing infection. Some protectant fungicides are taken into the grapevine tissue, but most provide a protective barrier on the surface of the foliage where they stop spores germinating. Since downy mildew invades through the stomates, contact fungicides should be applied to cover the undersides of leaves – the most difficult place to spray. They must be re-applied prior to an infection event if there is sufficient growth of leaves or of developing berries. Sprays should be applied as close as possible before infection events while allowing time for the spray to dry. Access to forecasts of downy mildew events will help time these sprays best.
Post-infection fungicides (eradicants)
These kill the pathogen inside infected tissue if they are applied at the right time. They do not eradicate the disease from the vineyard but, being systemic, are quickly absorbed into the sprayed foliage and are ‘rain fast’ within two to three hours of spraying.
The post-infection fungicides are more expensive but are able to stop the downy pathogen from within infected tissue, provided they are applied before oil spots appear. Consequently, these fungicides should only be applied when needed, as soon as possible after an infection and before oil spots appear (i.e. in warm conditions, within five days post-infection). In cool conditions oil spots may take three weeks to appear, so the monitoring period needs to be extended. Like all fungicides, good coverage is important.
The post-infection fungicides include metalaxyl and metalaxyl-M (Group 4) and phosphorous acid (Group 33). Because metalaxyl and metalaxyl-M are mixed with mancozeb or copper (protectants) these mixtures simultaneously provide a protective shield against new infection while treating infected tissue. However, as post-infection fungicides are single-site ‘actives’ and sprayed while the disease is present, they are at a high risk for resistance development. Resistance to metalaxyl and metalaxyl-M (Group 4) has been detected in Australian vineyards. Follow resistance management guidelines in the ‘Dog book’.
In wet seasons, weather conditions may prevent sprays from being applied when desired. Protectant fungicides should be applied to restrict the disease from getting established. Growers may need to shorten the window between protectant applications if weather conditions promote disease development and active growth is occurring. Relying on curative agrochemicals will not work if sprays cannot be applied. In low disease pressure years, timing sprays in relation to primary and secondary infection events may be a viable strategy. For this to work, growers need to monitor weather forecasts and at predicted times of high disease risk spray before the disease develops. Monitoring and interpreting the weather that follows is important to determine if infection occurred and adjust spray type and timing accordingly. If available, seek assistance from regional disease alert services.
Checking spray efficacy – the ‘bag test’
The efficacy of sprays can be checked by ‘bag testing’ oil spots to see if the downy mildew is still active. The bag test aims to stimulate sporulation conditions and involves putting the oil spot leaf in a ziplock plastic bag with a moist tissue and storing it at room temperature (ideally above 13°C) in a drawer overnight. If fresh sporulation (down) is present on the underside of the oil spot the following morning then the downy mildew is still active. Sometimes, the centre of the oil spot is dead but the margins can still remain active. Conducting bag tests on multiple oil spots from across the vineyard can provide a good picture as to whether the downy mildew is active or not.
Important considerations when spraying
- Spray coverage is critical for good control. It’s important to calibrate sprayers and check coverage. Make sure the water volume applied is sufficient to wet the entire canopy to the point of run-off. Sunscreen products can assist in assessing spray coverage on all tissue including hard to reach targets such as inflorescences.
- Confirmed resistance to metalaxyl has been identified in some regions. This is the result of ongoing use in high pressure sites after an infection (i.e. as an eradicant). The key lesson is that if you can’t apply metalaxyl immediately post-infection (i.e. prior to oil spots), you shouldn’t use it.
- To be effective, a post-infection spray must go on a dry canopy. A canopy carrying moisture will not take up the chemical and will not provide adequate control. Cultural practices such as trimming to allow more air into the canopy to assist with drying can help reduce disease pressure and improve coverage on remaining and new foliage.
- Flowers, caps and anthers hold water for longer than leaves. This means they can meet the conditions for infection even if the leaves appear dry and are no longer susceptible. To reduce primary infection conditions, cultural practices such as trimming allow more air into the canopy to assist with drying.
- Reduce spray intervals in high pressure seasons to maintain protection. Copper sprays at 10-12 day intervals are recommended while the vines are growing strongly and rain persists. Vines can ‘grow out of cover’ at longer intervals of 14 days, allowing downy mildew a chance to infect unprotected new growth.
Phosphorous acid is a registered fungicide with post-infection capabilities, but residues in wine are to be expected if it is used at any time during the growing season. For this reason, some exporting wineries have no tolerance for fruit that has been treated with this active ingredient. The ‘Dog book’ recommendation states that phosphorous acid should not be applied to fruit destined for export wines. Growers should contact their winery or grape purchaser prior to any phosphorous acid application.
Information about the tolerance for phosphorous acid residues in different markets can be found on the AWRI’s phosphorous acid webpage.
ADDITIONAL RESOURCES ON DOWNY MILDEW:
- WA Department of Primary Industries and Regional Development’s information on downy mildew
- The AWRI’s resources on downy mildew
- Wine Australia’s resources on downy mildew
- Control strategies for downy mildew and Botrytis (AWRI webinar 9 November 2018)
Liz Riley, Vitibit and Andrew Taylor, WA Department of Primary Industries and Regional Development are thanked for their contributions to this eBulletin.
For more information or assistance, please contact the AWRI helpdesk on 08 8313 6600 or email@example.com.