Fungicide Resistance Management for Vineyard Powdery Mildew

By VitiScribe Editorial·May 25, 2025·Updated July 5, 2025

FRAC Group 11 resistance in California powdery mildew populations is real and well-documented. If you're running Abound or Flint as primary program materials in a California vineyard, you may be applying products that aren't working -- and you won't know it until your fruit is covered in mildew at harvest.

Resistance management isn't optional. It's what keeps your program working year after year.

TL;DR

FRAC Group 11 (QoI strobilurins -- Abound, Flint, Quadris) resistance is widespread in California and confirmed in other US wine regions -- using Group 11 as a primary program material in California is spending money without getting reliable disease control
Resistance is faster to develop under three conditions: sublethal rates (underapplication), frequent applications of the same FRAC group, and large pathogen populations during high disease pressure
Once resistance establishes in a vineyard population, it does not go away when you stop using that product class -- the resistant population persists
A 10-application rotation can cover Groups M2, 3, 7, U13, and U12 without using Group 11 once, cycling through genuinely different modes of action for each application
FRAC Group 7 (SDHIs -- Fontelis, Luna, Sercadis) currently has low resistance, but resistance potential is high -- use as a rotation partner with maximum 2-3 applications per season, not as a primary material
Sulfur (FRAC M2) has been used for 4,000 years without resistance development because it affects multiple biochemical targets simultaneously -- it belongs in every rotation as the multi-site backbone

How Fungicide Resistance Develops

Erysiphe necator populations are large and genetically diverse. Any time you apply a single-site fungicide (one that affects a specific biochemical target in the pathogen), you create selection pressure: sensitive individuals die, tolerant individuals survive and reproduce. Repeated applications of the same mode of action shift the population toward the resistant phenotype.

This process is faster with:

Sublethal rates (underapplication)
Frequent applications of the same FRAC group
Large pathogen populations (high disease pressure conditions)

Once resistance develops in your vineyard population, it doesn't go away when you stop using that product class. The resistant population persists.

For the complete FRAC group listing applicable to vineyard powdery mildew programs, see the fungicide FRAC groups guide.

FRAC Groups Used for Vineyard Powdery Mildew

FRAC Group 3 (DMI -- Demethylation Inhibitors)

Products: Rally (myclobutanil), Inspire Super (difenoconazole+cyprodinil), Tebuzol, Tilt

Resistance status: Reduced sensitivity documented in some California populations. Still effective at labeled rates in most regions but performance has declined over time with heavy use.

Maximum per season: 4 applications (check individual product labels)

FRAC Group 7 (SDHI -- Succinate Dehydrogenase Inhibitors)

Products: Fontelis (penthiopyrad), Luna Experience, Sercadis (fluxapyroxad)

Resistance status: Low resistance currently, but resistance potential is high. Use carefully as a rotation partner, not a primary material.

Maximum per season: 2-3 applications (check labels)

FRAC Group 11 (QoI -- Quinone Outside Inhibitors / Strobilurins)

Products: Abound (azoxystrobin), Flint (trifloxystrobin), Quadris

Resistance status: HIGH. Widespread resistance documented in California, increasingly in other states. Limit to 1-2 applications per season maximum, never consecutively. Do not use as primary program material in California.

FRAC Group U13 (Quinofumelin)

Products: Quintec

Resistance status: Low resistance currently. Unique mode of action makes it a valuable rotation partner.

Maximum per season: 2 applications

FRAC Group U12 (Metrafenone)

Products: Vivando

Resistance status: Low. Novel mode of action. Reserve for resistance management rotation.

Maximum per season: 2 applications (check label)

Multi-site (FRAC M2 -- Sulfur)

Resistance status: None. Sulfur has been used for 4,000 years and no resistance has developed because it affects multiple biochemical targets simultaneously.

Use freely within label guidelines; the backbone of any resistance-smart program.

Building a Resistance Management Rotation

The key rules:

Never apply the same FRAC group consecutively
Limit Group 11 to 2 applications per season maximum in California
Rotate between groups with different modes of action
Use multi-site materials (sulfur) as program backbones
Apply at full labeled rates -- sublethal rates accelerate resistance

A 10-application rotation example:

| Application | FRAC Group | Example Product |

|---|---|---|

| 1 (budbreak) | M2 | Sulfur |

| 2 (1-inch shoot) | 3 | Rally |

| 3 (pre-bloom) | M2 | Sulfur |

| 4 (bloom) | U13 | Quintec |

| 5 (post-bloom) | 7 | Fontelis |

| 6 (fruit set) | M2 | Sulfur |

| 7 (mid-season) | 3 | Inspire Super |

| 8 (pre-veraison) | U12 | Vivando |

| 9 (veraison) | M2 | Sulfur |

| 10 (post-veraison) | 7 | Luna Experience |

This rotation uses each FRAC group no more than twice and avoids consecutive applications from the same group. Group 11 doesn't appear -- because resistance makes it unreliable as a primary material in most California regions.

For the QoI-specific rotation adjustments needed in blocks with confirmed or likely Group 11 resistance, see the QoI resistance management guide.

Tracking FRAC Rotation in Your Records

You can't manage resistance without knowing what FRAC group you used last. If your spray records are in a paper journal, you're finding this out by flipping through pages and trying to remember which product is which group.

VitisScribe tracks FRAC groups per application per block. The spray history view shows your rotation sequence at a glance. If you're about to log a consecutive Group 3 application, the system flags it.

This is also the documentation your UC Farm Advisor or certified crop advisor will want to see when reviewing your program.

Signs That Resistance May Be Developing

Visible disease despite following labeled spray intervals and rates with a specific product class
Disease breakthrough in blocks treated with the same product repeatedly
Mildew recovery 3-4 days after an application that previously held for 10-14 days

If you see these signs, stop using that FRAC group in the affected block. Switch to a different mode of action. Notify your local UC Cooperative Extension -- documenting and reporting resistance observations helps track the regional picture.

FAQ

What FRAC groups should I rotate for powdery mildew in California vineyards?

In California, rotate among FRAC Groups 3 (DMI), 7 (SDHI), U13 (quinofumelin), and U12 (metrafenone), with sulfur (multi-site, FRAC M2) as the program backbone. Limit FRAC Group 11 (QoI strobilurins) to 1-2 applications per season maximum due to widespread resistance in California powdery mildew populations. Never apply the same FRAC group consecutively.

How many times can I apply Rally (myclobutanil) per season?

Rally's label allows a maximum of 4 applications per season for grapes. From a resistance management perspective, using it more than 2-3 times in a season -- especially consecutively -- creates resistance selection pressure. Rotate Rally with products from different FRAC groups (FRAC 7, U13, U12) and sulfur to reduce selection pressure.

Does powdery mildew resistance transfer between vineyards?

Yes. Powdery mildew spores travel by wind and can move resistant spores from one operation to another. This is why regional resistance patterns develop even in vineyards that haven't overused specific FRAC groups -- they receive resistant spores from neighboring operations. Regional resistance management programs, coordinated through UC Cooperative Extension and the wine industry, help slow the regional spread.

How should spray records document a FRAC rotation change made in response to suspected resistance, for a PCA review or winery sustainability certification?

The record sequence that documents a resistance response should include: the scouting observations that showed disease breakthrough (date, block, incidence level, product most recently applied to that block), the FRAC group history for the block in the current season (showing which groups had been used), the rotation change decision (which FRAC group was removed or limited), and the first application under the adjusted rotation. A note explaining the connection -- "Disease breakthrough in Block 3 Chardonnay following two consecutive Group 11 applications; Group 11 removed from Block 3 rotation for remainder of season" -- creates the IPM rationale that a PCA review or SIP/Lodi Rules certification inspection looks for. VitiScribe's block-level FRAC history and scouting module capture both sides of this documentation chain.

What happens to FRAC Group 7 (SDHI) efficacy if it replaces Group 11 as the primary rotation partner in California programs?

SDHI resistance in powdery mildew is beginning to emerge in some regions, which makes this question increasingly relevant as Group 11 has been deprioritized. FRAC Group 7 currently has low resistance in most US wine regions, but the resistance mechanism is similar to QoI resistance -- a single-site mode of action that creates strong selection pressure when used heavily. The appropriate response is to limit Group 7 to 2-3 applications per season and rotate it with Groups 3, U13, U12, and multisite materials rather than using it as a replacement primary material. FRAC's label guidelines for Group 7 products specify per-season application limits; following those limits and maintaining rotation discipline is what prevents Group 7 from repeating the Group 11 resistance pattern.

What is Fungicide Resistance Management for Vineyard Powdery Mildew?

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How much does Fungicide Resistance Management for Vineyard Powdery Mildew cost?

How does Fungicide Resistance Management for Vineyard Powdery Mildew work?

Sources

FRAC (Fungicide Resistance Action Committee)
UC Cooperative Extension Viticulture
UC Davis Plant Pathology
Cornell Cooperative Extension
American Vineyard Foundation

Get Started with VitiScribe

Resistance management requires knowing what FRAC group you applied last in each block -- not across your operation, but block by block, because resistance builds at the block level where selection pressure accumulates. VitisScribe tracks FRAC groups for every application in every block, flags consecutive same-group applications before you confirm them, and shows your FRAC rotation sequence at a glance across the full season. Try VitiScribe free and review your current season's FRAC rotation history today.