Planning for the 2020 Season: Resistance Management Guidelines for Fungicides Used for Downy Mildew of Grape

By Andy Muza, Penn State Extension – Erie County

Downy mildew has been a major problem for many grape growers in Pennsylvania (outside of Erie County) over the last few years. This is no surprise, considering the amount of rainfall that has occurred throughout areas of PA during the past few seasons. As a result, downy mildew inoculum levels may be high in many vineyards at the start of the 2020 growing season.

Therefore, in preparation for planning your downy mildew management program (and reducing the risk of fungicide resistance) for the 2020 season, the following information from the 2019 New York and Pennsylvania Pest Management Guidelines for Grapes is provided (1). 

Fungicide Resistance Risks 

Fungicides that have similar chemical structures and share common modes of action are classified together into Groups. Resistance risk for each fungicide group is considered as either: LOWMEDIUMHIGH or Resistance not known (2). “The likelihood and speed of resistance development largely depends upon whether the fungicide affects a single metabolic site (single-site) within the fungus or multiple sites (multi-site). High-risk products have a single site of action or those for which disease resistance populations have been discovered. Medium-risk products are associated with fungicides where resistance is seen with the mutation of more than one target site or resistance formation is less frequent than that of high risk. Low-risk fungicides are characterized by a very rare or undocumented occurrence of resistance after many years of use” (3).

FRAC Codes

The Fungicide Resistance Action Committee (FRAC) developed a Code, consisting of letters and/or numbers, to distinguish different fungicide groups based on their mode of action (2). Each fungicide product includes a FRAC Code on the fungicide label.

Eleven different modes of action groups [FRAC Codes – M 01, M 03, M 04, 4, 11, 21, 22, 33, 40, 43, 45] are included in Tables 3.2.1 and 3.2.2 on pages 46 – 48 of the 2019 New York and Pennsylvania Pest Management Guidelines for Grapes  that are rated excellent (++++) to good (+++) for management of downy mildew (1).
NOTE: PRESIDIO (fluopicolide – FRAC Code 43) is not included in the information below. The manufacturer has pulled the grape use from the PRESIDIO label, and any new product will not be legal for use on grapes. However, grape growers will be able to legally use up old stock of PRESIDIO with the grape use pattern on the label. 

Multi-Site Fungicides

  • FRAC Code M 01 – The common name of the active ingredient in this group is copper.
  • FRAC Code M 03 – The common names of active ingredients in this group include mancozeb and ziram.
  • FRAC Code M 04 – The common name of an active ingredient in this group includes captan.

Resistance Risk: LOW. No signs of resistance developing.

Fungicide Products containing active ingredients listed within FRAC Codes: M 01, M 03, M 04

  • COPPER products (several formulations) – copper
  • DITHANE, MANZATE and PENNCOZEB products – mancozeb
  • ZIRAM – ziram
  • CAPTAN products – captan

Resistance Management Guideline: Multi-site fungicides are important tools in a resistance management program (4). Copper, mancozeb and captan products provide good control of downy mildew while ziram provides moderate control. These fungicides can also be used, as either a tank mix partner or as a co-formulation product, with fungicides that are designated as HIGH or MEDIUM risk for downy mildew resistance. Use of multi-site fungicides, as either a co-formulation product or as a tank mix partner, can: improve disease control; reduce the risk of resistance development; or provide a measure of control if resistance is already present in a vineyard (5).

Be sure to read the label of the products used for information such as: maximum allowable rates/A/season, compatibility and phytotoxicity precautions, preharvest and reentry intervals. 

FRAC Code 4 – The common name of an active ingredient in this group includes mefenoxam.

Resistance Risk: HIGH

Fungicide Products containing an active ingredient listed within FRAC Code 4

  • RIDOMIL GOLD/COPPER – a co-formulation product containing mefanoxam + copper hydroxide.
  • RIDOMIL GOLD MZ WG – a co-formulation product containing mefanoxam + mancozeb.

RESISTANCE WARNINGRidomil Gold is an outstanding fungicide against downy mildew, but the causal organism (Plasmopara viticola) can develop resistance to it very quickly when the product is used intensively. This fungicide became ineffective in the humid viticultural regions of Europe soon after its introduction many years ago.

Resistance Management Guideline: To reduce the risk of developing resistance:

  • DO NOT make more than two applications of RIDOMIL GOLD per season (MZ and copper formulations combined). The conservative (safer) strategy is only one application per season. 
  • DO NOT make two consecutive applications of RIDOMIL GOLD (MZ and/or copper formulations) per season.
  • ROTATE RIDOMIL GOLD products with an unrelated fungicide (different Frac Code) having efficacy against downy mildew. 
  • DO NOT attempt “rescue” treatments with RIDOMIL GOLD if an epidemic is in progress.

FRAC Code 11 – Common names of active ingredients in this group include: azoxystrobin, kresoxim-methyl, mandestrobin, pyraclostrobin, trifloxystrobin and fenamidone.

Resistance Risk: HIGH

Fungicide Products containing active ingredients listed within FRAC Code 11

  • ABOUND, AZAKA 2.08 SC – azoxystrobin
  • QUADRIS TOP – a co-formulation product containing azoxystrobin + difenoconazole
  • TOPGUARD EQ – a co-formulation product containing azoxystrobin + flutriafol
  • DEXTER MAX – a co-formulation product containing azoxystrobin + mancozeb
  • SOVRAN 50WG – kresoxim-methyl
  • INTUITY 4SC- mandestrobin
  • PRISTINE 38WG – a co-formulation product containing pyraclostrobin + boscalid 
  • FLINT, FLINT EXTRA – trifloxystrobin
  • LUNA SENSATION – a co-formulation product containing trifloxystrobin + fluopyram
  • REASON – fenamidone. Although not a strobilurin, fenamidone has the same biochemical mode of activity. Cross resistance has been shown between all FRAC Code 11 fungicides.

RESISTANCE WARNINGDowny mildew resistance to the strobilurin (FRAC Code 11) fungicides has occurred in multiple vineyards throughout New York, various mid-Atlantic regions, and probably Pennsylvania. It is now risky to rely on FRAC Code 11 fungicides for control of either downy mildew or powdery mildew. When such resistance occurs, none of the FRAC Code 11 fungicides will provide commercial control of downy mildew, and they must be combined with an effective rate of an unrelated fungicide to avoid potential crop loss. 

Resistance Management Guideline: 

  • DO NOT make more than two applications per season of all FRAC Code 11 products (combined).
  • DO NOT make two consecutive applications of a FRAC Code 11 product. 
  • ROTATE FRAC Code 11 products with an unrelated fungicide (different Frac Code) having efficacy against downy mildew. 

FRAC Code 21 – The common name of an active ingredient in this group includes cyazofamid.

Resistance Risk: Unknown but assumed to be MEDIUM to HIGH. 

Fungicide Product containing an active ingredient listed within FRAC Code 21

  • RANMAN – cyazofamid

Resistance Management Guideline: 

  • DO NOT make more than two applications per season of RANMAN.
  • DO NOT make two consecutive applications of RANMAN. 
  • ROTATE RANMAN with an unrelated fungicide (different Frac Code) having efficacy against downy mildew. 
  • Tank Mixing RANMAN with a phosphorus acid product (e.g., ProPhyt, Phostrol) has provided excellent control of downy mildew.

FRAC Code 22 – The common name of an active ingredient in this group includes zoxamide.

Resistance Risk: MEDIUM

Fungicide Product containing an active ingredient listed within FRAC Code 22.

  • GAVEL 75DF – a co-formulation product containing zoxamide + mancozeb. The resistance risk for zoxamideis MEDIUM and the resistance risk for mancozeb is LOW.

This product when applied at the labeled rate of 2.0 – 2.5 lbs/A provides the same amount of mancozeb as 1.8 – 2.2 lbs of standard 75DF formulations of other mancozeb products (e.g., Dithane, Penncozeb). For control of diseases other than downy mildew, GAVEL 75DF should be applied with sufficient quantities of another mancozeb product to provide a dosage equivalent to 3 – 4 lbs/A of the 75DF formulations of a solo mancozeb product.

Resistance Management Guideline: 

  • DO NOT make more than three applications per season of GAVEL 75DF.
  • DO NOT make more than two consecutive applications of GAVEL 75DF. 
  • ROTATE GAVEL 75DF with an unrelated fungicide (different Frac Code) having efficacy against downy mildew. 

FRAC Code 33 – The common name of an active ingredient in this group includes phosphorous acid (various formulations). A number of products containing phosphorous acid (also called “phosphite” or “phosphonate”) are sold as nutritional supplements and “plant conditioners,” but only a few are registered for disease control on grapes; two that have proven efficacy in NY spray trials are ProPhyt and Phostrol, although others (e.g., Rampart, Reveille, Fosphite) have been effective in commercial use.

Resistance Risk: MEDIUM

Fungicide Products containing active ingredients listed within FRAC Code 33

  • PHOSTROL, PROPHYT, RAMPART, etc. – phosphorous acid

RESISTANCE WARNINGDowny Mildew resistance to phosphorous acid products has occurred when they have been used intensively on other crops, and a reduction in performance has been noted in several New York vineyards over the past few years, although resistance has not been proven conclusively. 

Resistance Management Guideline:

  • DO NOT make more than three applications per season of phosphorous acid products.
  • DO NOT make more than two consecutive applications of a phosphorous acid product. 
  • ROTATE phosphorous acid products with an unrelated fungicide (different Frac Code) having efficacy against downy mildew.

FRAC Code 40 – Common names of active ingredients in this group include dimethomorph and mandipropamid.

Resistance Risk: MEDIUM

Fungicide Products containing active ingredients listed within FRAC Code 40.

  • ZAMPRO – a co-formulation product containing dimethomorph + ametoctradin
  • REVUS 2SC – mandipropamid
  • REVUS TOP 4SC – a co-formulation product containing mandipropamid + difenoconazole

RESISTANCE WARNINGDowny mildew resistance to FRAC Code 40 fungicides has been found in three vineyards in Virginia and one in North Carolina in 2018. 

Resistance Management Guideline: 

  • DO NOT make more than three applications per season of Frac 40 products (ZAMPRO, REVUS/REVUS TOP) combined.
  • DO NOT make more than two consecutive applications of a Frac 40 product. 
  • ROTATE Frac 40 products with an unrelated fungicide (different Frac Code) having efficacy against downy mildew. 

FRAC Code 45 – The common name of an active ingredient in this group includes ametoctradin.

Resistance Risk: assumed to be MEDIUM to HIGH. 

Fungicide Product containing an active ingredient listed within FRAC Code 45:  

Zampro – a co-formulation product containing ametoctradin + dimethomorph 

Resistance Management Guideline: Same as listed for ZAMPRO under FRAC 40 above.

References:

  1. 2019 New York and Pennsylvania Pest Management Guidelines for Grapes. Weigle, T. H., and A. J. Muza. Cornell and Penn State Extension. 168 pp. https://cropandpestguides.cce.cornell.edu/Guidelines/2019/Grapes/index1.aspx
  2. Frac Code List 2019: Fungal control agents sorted by cross resistance pattern and mode of action (including FRAC Code numbering).https://www.frac.info/docs/default-source/publications/frac-code-list/frac-code-list-2019.pdf
  3. Raised Resistance Risks.https://pesticidestewardship.org/resistance/fungicide-resistance/raised-resistance-risks/
  4. Importance of multisite fungicides in managing pathogen resistance. https://www.frac.info/docs/default-source/publications/frac-recommendations-for-multisites/frac-statement-on-multisite-fungicides-2018.pdf?sfvrsn=19544b9a_2
  5. FRAC recommendations for fungicide mixtures designed to delay resistance evolution. https://www.frac.info/docs/default-source/publications/frac-recommendations-for-fungicide-mixtures/frac-recommendations-for-fungicide-mixtures—january-2010.pdf?sfvrsn=7e9d419a_4

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