QoI Fungicide Resistance in Vineyard Powdery Mildew

By VitiScribe Editorial·February 8, 2025·Updated March 13, 2025

QoI resistance in powdery mildew is confirmed in California, Oregon, Washington, and other states -- that's the baseline fact that should shape every powdery mildew spray program in the US. If you're still treating QoI fungicides (the strobilurin group -- products like Flint, Abound, Pristine's pyraclostrobin component) as full-efficacy tools without accounting for resistance, you're likely getting less protection than your label rate suggests and potentially creating the selection pressure that will make resistance worse.

VitiScribe flags QoI product applications and tracks rotation against confirmed-resistance FRAC groups, giving you a block-level record of how much selection pressure you're applying season over season. Understanding what QoI resistance is, why it spread so quickly in powdery mildew, and what to do about it in your program is the foundation of effective resistance management.

TL;DR

The G143A mutation in the cytochrome b gene confers near-complete resistance to all QoI fungicides -- this is not reduced sensitivity but essentially eliminates binding of QoI compounds to the target site in resistant Erysiphe necator isolates
QoI resistance carries no fitness penalty for E. necator -- resistant isolates grow and sporulate as well as susceptible ones, which is why once established, the resistant population persists
Pristine (boscalid + pyraclostrobin, Groups 7+11) and other QoI premixes remain effective because the SDHI component (Group 7) still provides disease control -- when using Pristine in a resistance-documented region, Group 7 is providing the efficacy, not the QoI component
Standalone QoI products (Flint Extra, Abound, Cabrio) where the strobilurin is the sole active ingredient may provide little or no powdery mildew control in confirmed-resistance regions
Testing for QoI resistance in your specific blocks is available through UC Davis, Cornell, Oregon State, and WSU Plant Diagnostic Labs -- in practice, most US wine regions already have confirmed resistance, making the adjustment to treat Group 11 as a rotation partner rather than primary material appropriate regardless
Resistance to QoI compounds does not extend to unrelated modes of action -- sulfur, potassium bicarbonate, and Group 3 DMI materials remain fully effective against QoI-resistant populations

What QoI Resistance Is and Why It Spread

QoI fungicides -- FRAC Group 11 -- inhibit mitochondrial respiration in fungi by binding to the Qo site of cytochrome bc1. They were highly effective when introduced in the late 1990s and early 2000s, and their systemic, curative activity made them popular in powdery mildew programs.

The problem is that Erysiphe necator, the powdery mildew pathogen in grapes, has a specific single-nucleotide mutation (the G143A mutation in the cytochrome b gene) that confers near-complete resistance to all QoI fungicides. This mutation doesn't just reduce sensitivity -- it essentially eliminates the binding of QoI compounds at the Qo site. An E. necator isolate carrying this mutation is fully resistant to Flint, Abound, Cabrio, and the QoI components of premix products like Pristine and Compass.

Resistance spread quickly because:

The G143A mutation carries no fitness penalty for E. necator -- resistant isolates grow and sporulate as well as susceptible ones
Powdery mildew produces enormous numbers of asexual spores (conidia), which disperses resistant genotypes rapidly within and between vineyards
QoI use was widespread and sometimes in consecutive applications within seasons

By the mid-2000s, QoI resistance was documented in California wine regions. By the early 2010s, it was confirmed in the Pacific Northwest. Current monitoring indicates that QoI-resistant E. necator populations are present throughout most US wine regions.

For the full resistance management rotation framework that responds to QoI resistance, see the powdery mildew resistance management guide.

What QoI Resistance Means for Your Program

The confirmation of QoI resistance in your region doesn't mean you have to stop using QoI-containing products -- it means you need to use them strategically rather than as primary efficacy tools.

Group 11 premixes still provide value. Pristine (boscalid + pyraclostrobin -- FRAC Groups 7+11) and Compass (kresoxim-methyl + boscalid -- Groups 11+7) remain effective for powdery mildew because their SDHI component (Group 7) still has efficacy. The QoI component in these premixes may have reduced or no efficacy against resistant populations, but the FRAC Group 7 component provides meaningful protection. When you use Pristine, you're essentially relying on Group 7 for powdery mildew efficacy.

Limit standalone QoI applications. Products where the QoI compound is the sole active ingredient (Flint Extra, Abound, Cabrio) may provide little or no powdery mildew control in regions with confirmed resistance. Using them as primary powdery mildew tools in high-resistance areas is spending money without getting full return.

Maintain QoI limitations. FRAC recommends no more than 1-2 Group 11 applications per season as part of a rotation, not as a program cornerstone. This limitation applies regardless of whether resistance is confirmed in your specific blocks -- the practice is designed to prevent further resistance selection.

Rotate to non-QoI modes. Your primary powdery mildew rotation should draw on FRAC Groups 3 (DMIs), 7 (SDHIs), 13 (quinoxyfen), U8 (cyflufenamid), and multisite materials (sulfur). These groups remain effective, though SDHI resistance is beginning to emerge in some regions and warrants monitoring.

Testing for QoI Resistance in Your Vineyard

If you've had powdery mildew breakthrough despite timely QoI applications, resistance testing can confirm whether resistance is the cause.

UC Cooperative Extension and some state extension services offer resistance bioassays where collected E. necator samples from your vineyard are tested for sensitivity to QoI compounds. The G143A mutation can also be detected through molecular testing using PCR methods. Contact your local farm advisor, PCA, or university extension plant pathologist for sampling protocols and laboratory options in your region.

In practice, most US wine regions already have confirmed resistance populations, and the decision to deprioritize QoI products in primary powdery mildew programs is appropriate regardless of whether you've tested your specific block. The regional data is clear enough that testing is less important than adjusting your rotation.

Building a QoI-Adjusted Rotation

A powdery mildew rotation that accounts for QoI resistance might look like this for a 10-application season:

| Application | FRAC Group | Example Product |

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

| 1 (budbreak) | M2 | Sulfur |

| 2 (4-6 inch shoot) | 3 | Rally |

| 3 (pre-bloom) | 7 | Luna Privilege |

| 4 (early bloom) | 13 | Quintec |

| 5 (full bloom) | 3+M2 | Elite + sulfur |

| 6 (post-set) | U8 | Vivando |

| 7 (berry development) | 7 | Sercadis |

| 8 (cluster) | 3 | Tebuzol |

| 9 (veraison) | M2 | Sulfur |

| 10 (pre-harvest) | 7+11 | Pristine (0-day PHI) |

Group 11 appears once in this rotation, at pre-harvest in a premix where Group 7 provides the primary efficacy. This approach gets the 0-day PHI benefit of Pristine without relying on the QoI component for disease control.

VitiScribe's pesticide resistance rotation planner tracks Group 11 applications by block and flags if your season shows more QoI applications than recommended.

Frequently Asked Questions

Is QoI fungicide resistance a problem in my vineyard's powdery mildew population?

In most US wine regions, the answer is yes -- or at least, you should assume yes unless you have recent resistance monitoring data indicating otherwise. QoI resistance in Erysiphe necator is confirmed in California, Oregon, Washington, New York, Virginia, and other states. The G143A mutation that confers resistance doesn't create any fitness disadvantage for the pathogen, so once it establishes in a population, it tends to persist. If you're in a region with confirmed QoI resistance and your program has relied heavily on Group 11 products, the practical adjustment is to treat them as rotation partners rather than primary efficacy tools.

How do I know if powdery mildew in my vineyard is QoI-resistant?

Resistance bioassays and PCR-based molecular testing can confirm whether QoI-resistant genotypes are present in your vineyard. UC Davis Plant Pathology, Cornell PPDL, Oregon State University Plant Clinic, and WSU Plant and Insect Diagnostic Lab all offer or can refer you to diagnostic services. In practice, if you're in a region with documented QoI resistance and you've had powdery mildew breakthrough despite timely Group 11 applications at correct rates, resistance is a likely contributing factor. You can request testing through your PCA or extension farm advisor with a simple sample collection protocol.

What fungicides should I use if my vineyard has QoI-resistant powdery mildew?

Build your rotation around FRAC groups that don't rely on the QoI mechanism: Group 3 (DMI fungicides -- Rally, Elite, Tebuzol), Group 7 (SDHI -- Luna Privilege, Sercadis), Group 13 (quinoxyfen -- Quintec), Group U8 (cyflufenamid -- Vivando), and multisite materials like sulfur. Limit Group 11 applications to no more than 1-2 per season, and use them in premixes where a second active ingredient provides the primary efficacy. Organic programs in regions with confirmed QoI resistance rely on sulfur, potassium bicarbonate, and biofungicide options -- which remain fully effective since resistance to QoI compounds doesn't extend to these unrelated modes of action.

What scouting and spray record documentation should accompany a resistance bioassay sample submission to demonstrate that QoI resistance is suspected based on program performance, not just submitted speculatively?

The documentation package that provides context for a resistance bioassay should include: the FRAC group history for the block in the current and prior seasons (showing QoI application frequency and placement in the rotation), the spray records for the applications that preceded disease breakthrough (product, rate, interval, application date), the scouting record that identified the breakthrough disease (date, incidence level, tissue affected), and the weather conditions during the spray window preceding the breakthrough (to rule out application failure due to conditions rather than resistance). This record set allows the diagnostic lab and your PCA to distinguish resistance from other failure causes (missed coverage, adverse conditions, interval gaps) and supports the appropriate program adjustment decision.

If Group 11 resistance is confirmed in a block, how should the spray records going forward document that the program has been adjusted?

The first spray record after a resistance confirmation should explicitly note the rotation change: which FRAC group was removed from or limited in the block rotation, what group replaced it, and the basis for the change (resistance confirmed in this block or confirmed regionally). Subsequent records should show the adjusted rotation consistently. If Group 11 still appears in the rotation (in a premix for PHI reasons, for example), the record should note that the Group 11 component is not relied upon for primary efficacy and identify which other FRAC group in the premix provides disease control. This documentation shows that the program adapted to a confirmed resistance situation -- a record that supports PCA review and sustainability certification inspection.

What is QoI Fungicide Resistance in Vineyard Powdery Mildew?

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Sources

FRAC (Fungicide Resistance Action Committee)
UC Davis Plant Pathology
UC Cooperative Extension Viticulture
Cornell Cooperative Extension
Oregon State University Extension

Get Started with VitiScribe

Managing QoI resistance requires knowing exactly how many Group 11 applications have been made in each block this season, whether any were consecutive, and whether breakthrough disease coincided with Group 11 applications -- block-level data that paper spray logs and generic apps don't surface without manual review. VitiScribe tracks FRAC Group 11 applications by block, flags QoI overuse against FRAC recommendations, and connects scouting observations to the spray events that preceded them so disease breakthrough patterns are visible. Try VitiScribe free and review your current season's Group 11 application history by block today.