Botrytis Fungicide Resistance Management for Vineyards

By VitiScribe Editorial·July 29, 2025·Updated September 27, 2025

FRAC Group 17 (anilinopyrimidines) resistance in botrytis is widespread in high-use vineyards -- that's the lead fact about where botrytis resistance management stands today, and it's not isolated to one region. Products like Vangard and Scala, which were highly effective tools when introduced, have faced growing resistance pressure in vineyards where they've been used intensively over multiple seasons. If your botrytis program leans heavily on a few products without active rotation, you're likely managing resistance without knowing it.

TL;DR

FRAC Group 17 (anilinopyrimidines: Vangard, Scala, cyprodinil in Switch) resistance is widespread in high-use US vineyards -- do not rely on this group as a primary workhorse without rotation
Group 7 (SDHI fungicides: Luna, Sercadis) resistance in botrytis is emerging; limit to one application per season and rotate with non-SDHI modes
No FRAC group should appear more than once per season in your primary botrytis rotation -- that is the baseline rule for resistance management
Biological controls (Serenade Optimum, Botector) at the pre-harvest window are sound resistance management as well as 0-day PHI compliance tools since they do not share modes of action with any synthetic group
Resistance bioassays for local botrytis populations are available through UC Davis Plant Pathology, Cornell PPDL, and OSU Plant Clinic
Removing a resistant group from rotation for a full season reduces selection pressure but resistance does not permanently disappear -- plan for possible re-introduction of that group in future seasons under tight rotation

VitiScribe tracks FRAC group history by block and flags consecutive same-mode botrytis spray applications before they become a resistance problem. Resistance doesn't announce itself until your program fails during a wet harvest week when you needed it most.

How Botrytis Develops Fungicide Resistance

Botrytis cinerea is one of the fastest fungi to develop fungicide resistance because it produces enormous numbers of spores, has a short generation time, and its population consists of many genetically diverse individuals. When you apply a fungicide that targets a specific mechanism -- say, inhibiting an enzyme in the fungal cell -- resistant individuals in the population that carry a mutation at that target site survive while susceptible individuals die. Those survivors reproduce, and within a few seasons the resistant genotype can dominate your vineyard's population.

The resistance risk is real for multiple FRAC groups used in botrytis programs. Group 1 (MBC fungicides like Topsin), Group 2 (dicarboximides like Rovral), Group 7 (SDHI fungicides like Luna), Group 9+12 (Switch, a premix of Groups 9 and 12), and Group 17 (anilinopyrimidines) have all shown documented resistance in botrytis populations in US vineyards.

This doesn't mean these products no longer work in your vineyard -- it means you need to rotate them deliberately rather than relying on any single group repeatedly.

Highest-Risk FRAC Groups for Botrytis Resistance

Group 1 (MBC fungicides -- thiophanate-methyl): Resistance is widespread. Products like Topsin-M have limited solo use as botrytis management tools in most US wine regions. Still useful in tank mixes for multi-site resistance management.

Group 2 (dicarboximides -- iprodione): Rovral and related products have documented resistance in many regions. Less commonly used now due to both resistance and regulatory restrictions on iprodione.

Group 9+12 (Switch): Switch is a premix of fludioxonil (Group 12) and cyprodinil (Group 9). Both individual modes have documented resistance in some populations. Using Switch as a regular rotation partner rather than a primary workhorse helps preserve its efficacy.

Group 17 (anilinopyrimidines -- pyrimethanil, cyprodinil): Vangard, Scala, and the Group 9 component of Switch. Widespread resistance in high-use areas. Still effective where resistance hasn't established, but monitoring is essential.

Group 7 (SDHI -- fluopyram, fluxapyroxad): Luna products and Sercadis. Newer SDHI materials have been effective against botrytis, but SDHI resistance in botrytis is emerging in some regions. Limit applications per season and rotate with non-SDHI modes.

Building a Botrytis FRAC Rotation

A well-structured botrytis rotation for a Pinot Noir program might look like this across the four key application windows:

Pre-bloom (50% capfall): This is usually the most important single botrytis application. Apply Switch (Groups 9+12) at this window. Its effectiveness against petal debris infection makes it well suited to pre-bloom.

Post-bloom/fruit set: Rotate to a different group. Elevate (Group 17 -- fenhexamid) if resistance testing shows susceptibility, or Miravis Prime (Groups 7+12) for SDHI coverage.

Veraison: Apply Luna Experience (Groups 3+7) or Scala (Group 17) if resistance levels are acceptable. In high-pressure seasons, shorten the interval from veraison to the next application rather than skipping.

Pre-harvest: Use 0-day PHI materials. Elevate (0-day PHI, Group 17), Miravis Prime (0-day PHI), or biological controls like Serenade Optimum (0-day PHI) or Botector (0-day PHI). Biological botrytis products at harvest are a sound resistance management choice because they don't share modes of action with synthetic fungicides.

No more than one application per FRAC group per season in your botrytis program. That's the baseline rule -- though in high-pressure seasons with more than four applications, you may need to return to a group with careful timing.

Monitoring for Resistance Breakdown

You can't see fungicide resistance -- you infer it from program performance. Signs that your botrytis program may be facing resistance issues:

Visible botrytis infection in clusters despite timely applications at the correct rates
Higher incidence in blocks or varieties where the same products have been used for many consecutive seasons
Botrytis development beginning unusually soon after a recent application

If you suspect resistance, contact your local Cooperative Extension or a plant pathology diagnostic lab. Some labs offer resistance bioassays that test local botrytis populations for sensitivity to specific fungicide groups. This testing is available through UC Davis Plant Pathology, Cornell PPDL, and OSU Plant Clinic, among others.

When resistance is confirmed in a specific group, remove that group from your rotation for at least one full season. Resistance doesn't disappear permanently, but population pressure can shift when selection pressure is removed.

Documenting Botrytis Rotation in VitiScribe

VitiScribe's botrytis tracking connects to your IPM hub so you can review FRAC group sequence by block at the end of each season. If you applied two consecutive SDHI applications in a wet August, that shows up in your rotation report as a flag -- a signal to adjust the following season's program.

Your pesticide resistance rotation planner in VitiScribe tracks group sequence across fungicide classes simultaneously. A season where you managed both powdery mildew FRAC rotation and botrytis FRAC rotation without any consecutive same-mode applications is a season where you've done the core work of resistance management correctly.

At audit time for sustainable certification programs, your FRAC rotation records demonstrate that your botrytis program is managed with resistance awareness -- which is an explicit documentation requirement for SIP Certified and an implicit expectation in most sustainable viticulture programs.

Frequently Asked Questions

Which FRAC groups are at highest resistance risk for botrytis in vineyards?

Groups 1 (MBC fungicides -- thiophanate-methyl), 2 (dicarboximides -- iprodione), 9 (cyprodinil), 12 (fludioxonil, in some populations), and 17 (anilinopyrimidines -- pyrimethanil, fenhexamid) have the most documented resistance in US vineyard botrytis populations. Group 7 (SDHI) resistance is emerging in some regions. Groups 1 and 2 have such widespread resistance in high-use areas that most programs treat them as having limited solo efficacy and use them only in tank mixes or as rotational partners with other modes.

How do I build a resistance management rotation for botrytis fungicides?

Plan four application windows: pre-bloom, fruit set, veraison, and pre-harvest. Assign a different FRAC group to each window. No group appears more than once per season in your primary botrytis rotation. Include at least one biological product (Serenade, Botector, or Prestop) in your program, especially at the pre-harvest window where 0-day PHI is required. Use the Switch premix (Groups 9+12) at one window rather than two -- it covers two groups but the components are still subject to resistance. Review your rotation each year against any regional resistance monitoring data from extension services.

What are signs of fungicide resistance in vineyard botrytis?

The most common signal is botrytis appearing in clusters at levels inconsistent with your application timing and coverage. If you applied a botrytis fungicide 7-10 days ago at the correct rate with adequate coverage and you're finding early cluster infections, that's worth investigating. Compare incidence in blocks that received different fungicide groups -- if botrytis is consistently worse in blocks where a particular group was used more frequently, that pattern suggests reduced sensitivity. Resistance bioassays from a plant pathology lab provide definitive confirmation, but program performance over multiple seasons is often the first practical signal.

Should I test my local botrytis population for resistance before changing my program?

Resistance bioassay testing is most valuable when you already have reason to suspect reduced efficacy -- visible program failures, unexpectedly high incidence despite timely applications, or reports from neighbors of the same issue. For most growers, a conservative rotation program that avoids consecutive same-group applications is a sufficient baseline without individual testing. Testing becomes more important if you're managing a high-value block with a history of heavy botryticide use and you want to know specifically which groups still have full activity before committing to a season-long program.

How does tank mixing botryticides affect resistance management?

Mixing two fungicides with different modes of action is sometimes recommended as a resistance management strategy, but its effectiveness depends on both components having activity against the target pathogen. For botrytis, tank mixing a Group 9 material with a Group 7 material applies two different selection pressures simultaneously. If a resistant individual escapes one mode, the other may still control it. This is more effective early in the resistance process when only single-mode resistance exists; in populations where multi-mode resistance has developed, tank mixing provides less protection. A rotation-based program is generally more sustainable than relying on tank mixing as the primary resistance management tool.

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Sources

California Department of Pesticide Regulation (CDPR)
UC Cooperative Extension Viticulture
Oregon Department of Agriculture (ODA)
American Society for Enology and Viticulture (ASEV)
American Vineyard Foundation

Get Started with VitiScribe

Tracking botrytis FRAC group rotation by block across multiple seasons is exactly the kind of analysis that falls through the cracks in paper records and spreadsheets. VitiScribe's FRAC tracking shows you the group sequence for every block so you can see before you apply whether you're about to repeat a mode that's already been used this season. End-of-season rotation reports give you the data to adjust next year's program before resistance pressure forces the change. Try VitiScribe free and review your botrytis rotation history from last season as your first step.