Walt Mahaffee, Research Plant Pathologist and Research Unit Leader, USDA‑ARS Horticultural Crops Disease and Pest Management Research Unit
2025 and 2026
The 2025 battle against powdery mildew was a bit rough for many. The season had less rain, and the temperatures were in the optimal range for powdery mildew development a lot longer the normal before and after bloom. In addition, market conditions resulted in a lot of “less than optimally managed” vineyards. These less than optimally managed vineyards are not just those receiving fewer fungicide applications but also those being farmed with management philosophies that often result in levels of mildew that neighboring vineyard managers consider unacceptable.
These factors made for very high disease pressure especially in regions with a lot of adjacent vineyards. If you happened to be near one of those less than optimally managed vineyards, you may have experienced an especially challenging year. If you weren’t shortening fungicide application intervals early and ensuring excellent coverage, your vineyard probably had more disease than you wanted. You likely spent more money trying to clean it up than you would have tightening intervals early in the season.
The 2026 season is likely to be more of the same challenges as 2025 presented. Lots of “less than optimally managed” vineyards and weather conditions that will likely make mildew develop a bit faster than we would like. We could have early budbreak, low rainfall, and rapid shoot growth meaning disease will be significantly greater than normal. It would be prudent to start preparing for a Spring season that requires tight fungicide application intervals and applications beginning soon after budbreak without much of break until well after bloom. To get ready now for 2026, read the FRAME Networks checklist.
Start now: sprayer maintenance and calibration
Start the season now by doing maintenance on your sprayer and getting it calibrated. If it breaks down during the season and you miss an interval or even a spray, it will be very difficult to recover. Regardless of the marketing literature, there still aren’t products that are effective as eradicants fungicides. At best, they might kill some 1-3 days old colonies. In addition, there isn’t a product in the world effective against mildew if it does not land on the tissue you want protected.
To get good coverage the speed travel is a important as the product used regardless of the volume applied. Applications using most sprayers need to be done at 2 to 2.5 mph. Going faster than this will likely mean poor coverage since it takes time to get leaves moving around and the spray to penetrate the canopy and only the outside leaves get good coverage while the flowers/clusters and inner leaves are left unprotected. This can be offset by early and aggressive cluster zone leaf pulling.
Adjust spray intervals by growth & weather, not calendar
Adjust fungicide application intervals based on shoot growth and weather instead of the calendar. Most of the available fungicides do not move far enough in the plant to protect 6 inches of shoot growth. Every new leaf since your last spray is unprotected, and the powdery mildew that was sporulating when you sprayed is likely still sporulating. This means spores are landing on the unprotected tissue and very likely infecting it. Look at the weather forecast, and if you see a period of warm, dry weather before bloom, shoot growth will likely be very rapid and will need shorter spray intervals to keep the vines protected.
Leaf pull early and aggressively
Leaf pull early and aggressively, particularly in high vigor or high-density vineyards. Remove leaves within 2 nodes of the inflorescence or cluster to improve spray coverage and increase light penetration. Immediately following shoot thinning and leaf removal with a fungicide spray is a good idea, since opening the canopy also exposes the cluster to a higher chance of a spore landing on it and you just moved a lot of spores around, even if you can’t see them. Patty Skinkis and I did some research years ago that showed leaf removal in the cluster zone from inflorescence elongation (pre-bloom) through fruit set did not reduce end-of-season yield or cause sunburn but it resulted significantly less powdery mildew. We didn’t see a difference in fruit ripeness or tannins either.
Fungicide chemistry, resistance, and risk
“What chemistry and when” is a question often asked but there isn’t a single answer. Like many things there are numerous semi-optimal solutions that will get the job done. Considerations depend on your management philosophy, disease pressure, and your risk tolerance. Fungicide resistance is also still a consideration.
We are still seeing about half the samples we process with genetic markers for QoI (FRAC 11) and DMI (FRAC 3) tolerance. QoI tolerance seems less frequent early in the season and increases as the season develops. DMI tolerance based on the Y136F marker appears stable in the population. The presence of this marker appears to mean that myclobutanil and tebuconazole might not be the best choice for management, but some of the newer (to grapes) DMI chemistries appear to be effective (e.g. mefertrifluconazole, difenoconazole).
SDHI (FRAC 7) tolerance is a rising concern. Four years ago, our research indicated that genetic markers for SDHI tolerance were present but at low levels (~10%). While this is still true in Oregon (<10%), this past year we found rather a high proportion (<50%) of samples from California with these tolerance markers. An important caveat is that all these samples were sent to us because of management concerns, thus the results are not representative of the whole population. They do, however, indicate that SDHI (FRAC 7) tolerance is likely a rising concern. We also need to be careful with quinoxyfen (FRAC 13) since we continue to find isolates with tolerance, though they don’t seem to survive as well as other isolates.
Basically, all this means is the job of managing powdery mildew has gotten much harder and will remain rather difficult. Rotating fungicide mode of action and tank mixing multiple modes of action, if using conventional fungicides, is paramount to success for you and the rest of the industry. Mildew does stop at fence lines and will move from your vineyard to others and vice versa. Your problems are theirs and their problems are yours. The closer together vineyards are the more this is true.
Below are helpful resources to prepare yourself for the 2026 season:
Resources
- 2026 Pest Management Guide for Wine Grapes in Oregon
- FRAME Network (Fungicide Resistance Assessment, Mitigation, and Extension Network)
- FRAME Networks Grower Information
- Sprayers 101
Source: Walt Mahaffee, USDA-ARS