Managing Fungicide Resistance

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Managing Fungicide Resistance

• Anne DeMarsay
• Maryland Wine Grape Industry Meeting
• February 28, 2009

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Fungicide Resistance 101
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What Is Fungicide Resistance?

• Resistance a stable, heritable trait that
  results in a reduction in sensitivity to a
  fungicide by an individual fungus
• Practical resistance labeled rates of a
  fungicide no longer provide commercially
  acceptable control of a disease

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Why Monitor Resistance?

• Resistance ? control failures ? crop losses
• Use of ineffective fungicides may increase
  resistant strains of fungi

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How Does Resistance Occur?

• Origin rare genetic mutation(s) that alter the
  target site in the fungus to block the action of
  the fungicide
• Natural selection fungicide causes selection of
  the fittest (resistant) individuals

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Fungicides select resistant individuals
A. B. Baudoin, VPI SU
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After the fungicide wears off...
A. B. Baudoin, VPI SU
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How Does Resistance Occur?

• Origin rare genetic mutations that alter the
  target site(s) in the fungus to block the action
  of the fungicide
• Natural selection fungicide causes selection of
  the fittest (resistant) individuals
• Resistant individuals are more likely to survive
  to reproduce
• When the fungus reproduces, resistant individuals
  pass on the mutation

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Types of Resistance

• Qualitative resistance sudden loss of control
• Resistance results from a single mutation in one
  gene
• Quantitative resistance gradual reduction in
  control
• Resistance results from mutations in several
  genes that interact

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Target site mutation
Stepwise, small changes
One-step, large changes
A. B. Baudoin, VPI SU
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Resistance to More Than One Fungicide

• Cross resistance when a pathogen resistant to
  one fungicide exhibits resistance to other
  fungicides in the same chemical class, even
  without exposure
• Multiple resistance when a pathogen
  independently develops resistance to fungicides
  in different chemical classes

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What Fungicides Are at Risk?
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Fungicide categories
A. B. Baudoin, VPI SU
Protectant
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Contrasts protectant vs. penetrant

• Protectant fungicides tend to...
• have a multi-site mode of action
• have few problems with resistant strains of
  target fungi

• Penetrant fungicides tend to...
• have a single-site mode of action
• allow target fungi to develop strains with
  resistance to the fungicide (risk varies)

A. B. Baudoin, VPI SU
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Multi-site inhibitor
A. B. Baudoin, VPI SU
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Protectants cannot penetrate plant tissue

A. B. Baudoin, VPI SU
Cuticle
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Penetrants and systemics DO penetrate plant
tissue.

Target site differs single-site
Cuticle
A. B. Baudoin, VPI SU
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Fungicides at Risk

• Most newer fungicides are single-site inhibitors
  (penetrant or systemic)
• Single-site inhibitors are more prone to
  resistance development (qualitative and
  quantitative)

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FRAC Class/Code Risk Common name Trade name
Benzimidazoles (1) High thiophanate-methyl Topsin-M
Phenylamides (4) High mefenoxam Ridomil
Strobilurins (QoI) (11) High azoxystrobin, kresoxim-methyl, pyraclostrobin, trifloxystrobin Abound, Sovran, Pristine (boscalid), Flint
Dicarboximides (2) Med High iprodione Rovral
Sterol biosynthesis inhibitors (SBI) (3) Med fenarimol, myclobutanil, tebuconazole, triflumizole Rubigan, Rally, Elite, Procure
Carboximides (7) Med boscalid Endura, Pristine ( pyraclostrobin)
Anilinopyrimidines (9) Med cyprodinil, pyrimethanil Vangard, Scala
Quinolines (13) Med quinoxyfen Quintec
Hydroxyanilid (17) Med fenhexamid Elevate, CaptEvate ( captan)
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Single-site inhibitors are much more prone to
resistance development than multi-site inhibitors
HOW much more? Depends on...

1. Fungicide and its target site
2. Characteristics of target fungus and disease

A. B. Baudoin, VPI SU
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HOW much more? Factor 1 fungicide
and target site

• Nature of target site, how essential?
• Nature of changes possible
• How easily does site mutate?
• Are changes detrimental to organism?

A. B. Baudoin, VPI SU
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Factor 2 Characteristics of target fungus and
disease

• How fast does fungus reproduce?
• How easily does it spread?
• How much variability in population?

A. B. Baudoin, VPI SU
J. Verreet
H. Couch
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Fungicide/target site risk
A. B. Baudoin, VPI SU
Hot zone!
High
Benzimidazoles Strobilurins
Mancozeb Chlorothalonil
Powdery mildew
Rhizoctonia
Low
Pathogen risk
Low
High
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Fungicide Resistance Two Case Studies
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Case 1 Strobilurins (QoIs)

• Used to control grape powdery mildew (PM) and
  grape downy mildew (DM)
• First sold in 1996 in widespread use by 1998
• Azoxystrobin (Abound) labeled for grapes in 1997
• Others introduced between 2001 and 2008
• High risk of resistance development
• Mode of action single-site inhibitor of energy
  production in fungal mitochondria
• Qualitative resistance (one mutation) sudden
• Cross resistance among all strobilurins

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Case 1 Strobilurins (QoIs)

• Resistant grape PM strains detected
• In NY and PA in 2002 (1st cases in U.S.)
• In VA, MD, NC, and PA in 200507
• In Europe in 2006
• Resistant grape DM strains detected
• In Europe in 2000
• In VA in 2005 (1st case in U.S.)
• In MD, NC, and PA in 200507

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Map J. F Colcol, A. B. Baudoin
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QoI sensitive
Majority QoI resistant
Map J. F Colcol, A. B. Baudoin
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Case 1 Strobilurins (QoIs)

• When resistance was first detected in Virginia
• Vineyards with resistant PM/DM had been sprayed
  from 0 to 17 times with strobilurins
• Sprays per season averaged 2 to 3.4
• FRAC recommends 3 sprays max. per year labels
  allow 4 sprays
• Current resistance management guidelines not
  adequate to prevent RAPID development of
  resistance

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Case 2 Sterol Biosynthesis Inhibitors (SBIs)

• In use since 1982 for grape PM
• 1982 triadimefon (Bayleton)
• 1989 fenarimol (Rubigan), myclobutanil
  (Nova/Rally)
• 1990s tebuconazole (Elite), triflumizole
  (Procure)
• Medium risk of resistance
• Mode of action single-site inhibitor of
  ergosterol synthesis
• Quantitative resistance (several mutations in
  interacting genes) gradual loss of sensitivity
• Partial cross resistance

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Case 2 Sterol Biosynthesis Inhibitors (SBIs)

• Loss of sensitivity in grape PM strains detected
• In CA in 198586 (triadimefon)
• In NY in 1995 (triadimefon, fenarimol,
  myclobutanil)
• In Canada in 19992000 (myclobutanil)
• In VA, MD, NC, and PA in 200507 (triadimefon,
  fenarimol, myclobutanil, tebuconazole,
  triflumizole)

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Case 2 Sterol Biosynthesis Inhibitors (SBIs)

• Mid-Atlantic samples of grape PM
• Greater loss of sensitivity to myclobutanil
  (Nova/Rally) and tebuconazole (Elite) than other
  SBIs
• Multiple resistance isolates that were resistant
  to strobilurins were more resistant to SBIs
• A possible reason correlated selection pressure

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Case 2 Sterol Biosynthesis Inhibitors (SBIs)

• Spray history at Upper Marlboro, MD farm
• Vineyard planted in 2001
• Reduced sensitivity to SBIs detected in 2006
• Myclobutanil (Nova) applied 11 times 200307
• Sprays per season 1 or 2 once 3
• FRAC recommends 4 sprays max. per year
  Nova/Rally label allows 5 sprays
• Current resistance management guidelines not
  adequate to prevent loss of sensitivity

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Managing Resistance
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Other Reasons for Control Failures

• Poor timing
• Starting protectant applications too late
• Spray intervals too long
• Missed sprays
• Spotty coverage
• Sprayer not calibrated or nozzles not adjusted
• Inadequate spray volume for canopy

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Coverage
A. B. Baudoin, VPI SU
Variable
More uniform
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Other Reasons for Control Failures

• Wrong rate of fungicide
• Rate too low for the diseaseread the label!
• In a tank mix, use at least the minimum rate on
  the label for each fungicide
• Canopy too dense
• Too windy during spraying

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Delaying Resistance Development

• Practice integrated disease management
• Plant disease-resistant varieties
• Use cultural controls first (good canopy
  management, sanitation)
• Use protectant fungicides to prevent disease
  buildup
• Spray effectively
• Calibrate your sprayer every year. Check coverage
  with water-sensitive spray cards
• Use appropriate spray intervals and volume for
  the fungicide and time of the season

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A. B. Baudoin, VPI SU
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After the fungicide wears off...
A. B. Baudoin, VPI SU
Poor coverage
Better coverage
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Delaying Resistance Development

• Plan your spray program
• Limit the number of applications of high- and
  medium-risk fungicides
• Use the right rate for the disease
• Rotate among different classes of fungicides
• Tank-mix fungicides from different classes that
  are effective against the same disease
• Add 25 lb sulfur to a strobilurin or SBI spray
  for powdery mildew
• Add captan or a phosphorous acid to Pristine for
  downy mildew

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Summary

• Resistance is the evolutionary response of a
  fungus to a threat to its survival the fungicide
• Fungicides that act at a single site are more
  prone to resistance development
• How quickly resistance will develop, and how much
  control will be lost, depend on both the
  fungicide and the target fungus
• Resistance to most newer fungicides will occur,
  but we can prolong their effective life by
  careful use

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Any Questions?