The lowdown on downy mildew in grapes

The 2024 summer weather in Ontario has been ideal for downy mildew development.  Typically, we see a flush of downy mildew after bloom for about a month and then it slows down until we get into August.  But this year the pressure from downy mildew seems to be non-stop.

The downy mildew fungus survives as oospores in the soil.  The oospores produce sporangia that rely on rain splash to be moved to grapevines where they release motile zoospores that initiate infections.  These primary infections produce lesions that produce the sporangia that can cause spread in the vineyard.  Leaves are most susceptible until they are fully expanded. Berries are infected and support profuse sporulation until 2 weeks post-bloom. Pedicel tissue remains susceptible until 4 weeks post-bloom and downy mildew can follow this pipeline into the fruit, causing the “leather berry” symptom — a hard and dry berry with no DM spores produced upon it.

Historical perspective

Historically, we thought that early downy mildew infections were produced by sporangia produced from the oospores in the soil only until bloom.  The 10-10-10 rule (10°C, 1/10 inch of rain, and 10 cm of shoot growth) was applied for the first infections in the spring.  After infection, new sporangia emerge on the underside of lesions.  Subsequent spread of downy mildew was by sporangia produced on the undersides of lesions and the flush of downy mildew we see in late summer was the result of the reactivation of some of the earlier infections. The requirements for sporulation and secondary infection are humidity of at least 98% and the temperature is 13°C or above (optimum 20–25°C) in at least 4 hours of darkness and leaves are wet for 2-3 hours predawn.

New”ish” information

• Oospores can survive for up to 2 years in the soil and can produce sporangia throughout the growing season whenever conditions are suitable. Late summer infections are new ones coming from oospores on the soil or in leaf litter rather than earlier lesions on grape tissues. So not only can grapevines be infected by the repeating cycle of sporangia on lesions, they can also be infected by new sporangia from oospores.
• It’s not just the amount of rainfall that has an effect on sporangium release.  The intensity of rain (millimeters per hour) has a key role in splashing water drops from the ground to grape leaves; increasing intensity resulted in an increased coverage by splashed drops, as well as an increased number and size of the splashed drops.  As rain intensity increases, mean raindrop size increases and more splash drops are formed. Raindrop size influences both the numbers of spores dispersed and the distance dispersed.  Large raindrops are more effective than small ones because they carry more spores and can splash them farther. The bottom line: we’ve had ideal conditions for downy mildew dispersal throughout this summer.
• Sporangia can be released without rainfall.  Leaf litter moisture due to water from the atmosphere makes oospore development possible during non-rainy periods.
• Lesions do not continue to produce sporangia for the entire growing season. They have a defined period for producing sporangia: after 3 sporulation events, the number of sporangia produced on a lesion significantly decreases and the lesions die (turn brown), as shown in the photo below.

Fungicide Resistance

Ontario-based surveys of downy mildew conducted in 2021 and 2022 revealed a high proportion of isolates had the G143A mutation that confers resistance to Group 11 fungicides.  Therefore, it is not recommended that any Group 11 fungicides (Flint, Pristine, Sovran) be used for downy mildew control.  A group of isolates from a specific region in Ontario had the G1105S mutation which confers resistant to Group 40 fungicides (Revus, Forum and half of Zampro).  While this does not eliminate these products for downy mildew control, it emphasizes the importance of practicing good resistance management.

Resistance to phosphorous acid products?

We’ve been relying on phosphorous acid (“phos acid”) products because of their systemic activity and post-infection mode of action.  However, it’s important to understand how they work.  They do not “burn out” downy mildew infections.  Rather, if applied before lesions start to sporulate, they will reduce or even prevent sporulation.  They do not kill off the sporangia that have already been produced.  Phos acids are systemic and will move from sprayed leaves to newly developing leaves to provide protection from infection after approximately 24 hours.  According to the label, they should be applied every 7 days under optimum conditions for downy mildew development.  Stretching the interval beyond that will result in reduced efficacy of the treatment.  Concentration is also important: using a rate lower than that on the label will result in less control and going over the label rate can cause damage to leaves.

Several growers have mentioned that phosphorous acid products, Phostrol specifically, do not seem to be working as well and wondered about resistance.  These products are generally considered to be at low risk for resistance because they activate plant defences and also have fungistatic activity directly on the pathogen.  This means that, unlike Group 11 or Group 40, multiple gene mutations are necessary in order for an individual to become resistant to these products.  While resistance to phos acid has been found in a related disease and in a research trial, applying 5 consecutive applications of phos acid resulted in poor control of downy mildew. No one has identified this resistance in the downy mildew fungus in a field situation.  Since anyone who is interested in resistance management (I hope all Ontario growers!) would never do this, it is more a lesson in caution.  Do not use these products repeatedly without a break to or tank mix with another fungicide group.  Field experience (as opposed to replicated trials) has shown that a tank mix with powdered copper or folpan dries up the sporangia that have already been produced while the phos acid prevents more sporulation and protects from infection.

So what are the options?

The severe restrictions on captan and mancozeb products have definitely cramped our style as far as downy mildew control is concerned.  Ridomil Gold MZ is no longer for sale and if you have any, it can’t be used after bloom.  We do have some pretty effective new chemistries to rely on, including some biologicals that are more effective than the old ones.  For information on relative efficacy of fungicides see: Activity of Fungicides on Grape Diseases and Impact on Honeybees (gov.on.ca). 

Keep it covered!

If downy mildew is severe in the vineyard in the fall, it will show up earlier and be more severe by bloom the following year. Typically I recommend not spraying for downy or powdery after labour year. But this isn’t a typical year so if there’s still active downy in your vineyard into September, keep that foliage protected. Just make sure you keep pre-harvest intervals in mind. For information on restricted entry intervals, pre-harvest intervals and maximum applications per season see:  Products used for Disease Control for Suppression on Grapes (gov.on.ca)