Management options
Cultural
Oospores may spread from property to property and region to region by the movement of infected leaves and litter in the soil or on vines in late summer and autumn. Avoid distribution of infected soil and plant matter by equipment and machinery (for example, mechanical harvesters, leaf pluckers, trimmers and utilities), by soil still adhered to rootlings, or by potted vines from nurseries.
Sporangia may spread across property boundaries by wind. It is very difficult to prevent the spread of these spores. It is believed that sporangia rarely spread more than 200m by wind.
Canopy management practices that encourage air movement will help to dry out leaves and improve sunlight and spray penetration. This will help to prevent infection. Such practices include:
- lower planting density;
- trellising and pruning to open the canopy;
- shoot training to open the canopy;
- vine trimming and hedging;
- lateral shoot thinning; and
- leaf plucking.
Vegetative growth may also be managed by the selection of appropriate rootstocks prior to planting and by careful application of fertilisers (for example, nitrogen). Excessive growth leads to dense shaded canopies that may encourage the development of downy mildew.
Chemical
All chemicals registered for use against downy mildew of grapes in WA are listed in the department's Viticulture Spray Guide. Consultation with the Australian Wine Research Institute (AWRI) 'Dog Book' should occur prior to use of chemicals for any further application requirements.
Pre-infection fungicides (applied as close as possible but prior to an infection event)
Pre-infection (protectant) fungicides help to prevent downy mildew zoospores from entering the green vine tissue. Spray coverage needs to be excellent to adequately protect all of this green tissue. In particular they need to be applied to the underside of leaves and the back of bunches. It is important to time their application as close as possible but prior to the possible infection event (for example, when possible primary or secondary weather events are forecast).
A pre-infection spray program tends to be used where downy mildew is well established in a region or vineyard and occurs frequently. Growers who are unable to conduct careful monitoring tend to use a pre-infection spray program. Large vineyards and table grape growers tend also to use pre-infection spray programs or those with soils which make access limited after a rain event, delaying post-infection spray application.
Pre-infection fungicides have limited movement from the areas where they are deposited and any new growth after the spray has been applied will not be protected. Rain and overhead irrigation will dilute or may wash the protectant sprays off the vine. Hence, further applications will be required before the next possible infection event.
A pre-infection spray program often requires application on a 7-14 day schedule. This may be expanded to a 21 day program later in the season as shoot growth slows and possible infection events are less. As flowering is the critical period to prevent crop loss, the spray program may need to be tightened to every 5-7 days to coincide with possible infection events.
(Note: In table grapes pre-infection fungicides should only be used provided it is prior to 10mm berry size as spray residues on berries may occur).
Post-infection fungicides (applied as soon as possible after an infection event)
Post-infection (eradicant) fungicides are systemic and penetrate the vine tissue killing the downy mildew fungus from within the vine tissue. Use of these fungicides involves withholding sprays until an infection event has occurred. Relying on post-infection fungicides requires careful monitoring and has a greater risk of downy mildew becoming established. However, if downy mildew is not established in the region or vineyard and few possible infection events occur it has the advantage of using fewer sprays that have greater effectiveness.
Post-infection fungicides work best when applied as soon as possible after an infection event - within five days of infection and before oilspots appear. No additional spraying should be required until weather conditions favour another possible infection event. In this situation, pre-infection fungicides may be used once again.
Once the fungus is visible it is difficult to kill. A single post-infection spray is usually not effective, although it may reduce the number of spores and limit spread of the disease. Hence, follow up sprays of post-infection fungicides may be required after the initial post-infection fungicide spray.
(Note: In table grapes not all post-infection fungicides can be used after 10mm berry size for the control of downy mildew. Contact your nearest Department of Primary Industries and Regional Development office or consultant for further information).
Did the post-infection fungicide work? Use The bag test for downy mildew of grapes to check whether the post-infection fungicide was effective. Conduct the tests 24 hours and then, if required, another three days after spraying with samples of healthy bunches, limp and browning bunches and leaves with oilspots. Keep each sample in a separate moist bag.
Consider immediate repeat spraying with a post-infection fungicide if fresh white down is evident on the underside of the leaf or on bunches of these samples when inspected the next morning.
Post-infection products should be used after an infection event if there is any concern that the pre-infection fungicides applied were not adequate. Browning bunches may be too badly infected for the post-infection fungicide to prevent crop loss and only normal looking bunches will benefit from the spray.
Fungicide resistance
Due to the ability of Plasmopara viticola to sexually reproduce within Australian vineyards and the use of eradicant fungicides for control it is considered high risk for the development of resistance. Fungicide resistance has been detected in WA for this disease. The continual use of one fungicide or one group of fungicides increases the risk of resistance developing to that fungicide or that group of fungicides. To reduce the risk of resistance developing within your vineyard always read the chemical label and regularly consult the resistance management guidelines for grapevine downy mildew produced by Croplife Australia. Always read the chemical label thoroughly prior to use.