The lowdown on downy mildew this summer, 2026

The rainy, cloudy weather we’ve been having is ideal for downy mildew infection. We’ve found sporulating lesions on leaves and developing fruit. Young leaves are susceptible to infection for about 40 days and berries are susceptible as soon as the caps come off and continue to be susceptible for about 5 weeks.

Frequent heavy rains will wash off contact fungicides. See a previous ONFruit post Rainfastness of fungicides. Locally systemic products are more resistant to wash-off.  The table below shows the Fungicide Resistance Action Committee (FRAC) group, activity in the vine (contact, locally systemic or systemic) and the relative efficacy vs downy mildew.

Products	FRAC group	Activity in Vine	Downy mildew efficacy
Ridomil Gold 480 SL	4	Systemic	4
Flint, Flint Extra	11	Locally systemic	1*
Diplomat 5 SC	19	Contact	3
Torrent 400SC	21	Locally systemic	4
Allegro 500 F	29	Contact	3*
Forum	40	Systemic	4
Revus	40	Locally systemic	4
Timorex Gold	46	Contact	2*
Xivana Prime	49	Locally systemic	4
Zampro	40+45	Systemic	4
Pristine WG/Empire WG	7+11	Locally systemic	1
Stargus	BM2	Contact	2
Guardsman Copper Oxychloride	M1	Contact	3
Copper 53 W	M1	Contact	3
Cueva Commercial	M1	Contact	3
Cobranza	M1	Contact	3
Copper Spray	M1	Contact	3
Kocide 2000-O	M1	Contact	3
Follow WDG	M4	Contact	4
Maestro 80 WSP	M4	Contact	4
Supra Captan 80 WSP	M4	Contact	4
Folpan 80 WDG	M4	Contact	4
Sharda Captan 48 SC	M4	Contact	4*
Oxidate	NC	Contact	2
Aliette WDG	P07	Systemic	4
Confine Extra, Rampart, Phostrol	P07	Systemic	4
LifeGard WG	P6	Systemic/SAR	3-4

LifeGard, which works soley by activating vine defence mechanisms, will not eradicate existing sporulating lesions and is best used before disease develops or once disease is controlled by other products.

Xivana is a new locally systemic downy mildew product in FRAC group 49. This gives us a new mode of action with which to rotate.

Surveys conducted in Ontario vineyards suggest that the gene for resistance to Group 11 fungicides (Sovran, Flint, Pristine) is very prevalent in downy mildew populations here.  Therefore, these products are not recommended for downy mildew management in Ontario.

We also surveyed for resistance to Group 40 fungicides (Revus, Forum and ½ of Zampro) in Ontario. Although the gene for resistance was detected in most of the vineyards tested, resistance to Group 40’s is more complicated and warrants a separate blog post in the future. If these products are used BEFORE sporulating lesions are present, they should still provide good protection from downy mildew. Issues reported last year are likely due to trying to use these products as eradicants instead of protectants. Research in Europe suggests that solo Group 40 fungicides are best used early season and in a tank mix with another mode of action.

Researchers in the U.S. recently determined that downy mildew was not as susceptible to phosphorous acid products as previously found (See Resistance Management for Phosphorous Acid Fungicides in Grapevines). There were reports in Ontario that phosphorous acid products weren’t performing as well as they used to. However, phosphorous acids work in 2 ways: directly on the pathogen and by activating defence compounds in the vine. The U.S. research did not investigate SAR. This year, we are collecting downy mildew isolates in Ontario and testing them using both the published method and also looking at whether systemic acquired resistance activity is affected. Early applications of phosphorous acid this spring have resulted in oil spots without sporulation, even when incubated in the lab, which means it is working. Regardless fungicide resistance management strategies should be followed:

• The “burn-out” approach with sprays applied to sporulating lesions leads to resistance a lot faster. Site-specific fungicides should NOT be applied alone to sporulating lesions (and there are a LOT of those out there!) unless they are tank mixed with a fungicide from a different group, preferably a product with multi-site activity like copper, folpet, captan, Stargus or Timorex Gold.
• Alternate among fungicide groups for every application. Except for Groups M and BM.
• Do not use any product containing only 1 fungicide group more than twice in a season.
• Do not cut rates. Use full label rate and sufficient water volume to provide excellent coverage of both sides of the leaves and clusters.

Of course, the other fungal pathogens also flourish in this soggy weather and this is the peak susceptibility period so make sure to choose your products wisely. Check out the Grape Fungicide Efficacy page on the Crop Protection Hub.