Update on Fungicide Resistance Research and New Implications for - - PowerPoint PPT Presentation

Update on Fungicide Resistance Research and New Implications for Scab Management

Katherine L. Stevenson

Fungicides for Pecan Disease Management

Class FRAC group Resistance Risk MBCs 1 High DMIs 3 Medium QoIs 11 High Organotins 30 Low to Medium Phosphonates 33 Low Guanidines U12 Low to Medium

Pecan South, March 1976 Pecan South, June 1977

UGA Plant Pathologist, R. H. Littrell, reports benomyl resistance

History of Fungicide Resistance in Pecan Scab

MBC fungicides

• resistance to benomyl (Benlate) reported in mid-

1970s in Georgia

• use of thiophanate-methyl (Topsin M), has increased

in the last 7-8 years

• both products share the same mode of action and

have a high risk of resistance

• cross resistance is common

History of Fungicide Resistance in Pecan Scab

DMI fungicides

• significant decrease in sensitivity to DMI fungicides

propiconazole (Orbit) and fenbuconazole (Enable) 1995-2003 associated with reduced scab control

5 10

N u m b e r

• f

i s

• l

a t e s

5 10 15 10 20 30 40 50 60 70 80 90 100 110 120 Orchard 5 n=59 Orchard 5 n=50 1995 M ean RG=62.1% 2003 M ean RG=90.0%

Mean relative growth on 0.2 μg/ml propiconazole (%)

History of Fungicide Resistance in Pecan Scab

Organotin fungicides

• significant shifts in sensitivity to fentin hydroxide

(TPTH) detected in 1998, reduced efficacy reported, but no control failures

more sensitive less sensitive Log EC50 Value (μg/ml)

Testing for Sensitivity to Fungicides

Fungicide Sensitivity Monitoring 2008-2015

• Fungicide sensitivity monitoring was offered as a

service to growers in Georgia

• Growers submitted samples for testing and received a

report of the results by email Year Samples tested 2008 33 2014 179 2015 78

Fungicides Included in the Sensitivity Tests

FRAC code “At-risk” Fungicide Concentration (μg/ml)

30 Fentin hydroxide (Super Tin, Agri Tin) 30 1 Thiophanate-methyl (Topsin M) 1 U12 Dodine (Elast) 3 3 Propiconazole (Orbit, Propimax, etc.) 1 3 Tebuconazole (Orius, Tebuzol, etc.) 1 3 Difenoconazole (Quadris Top*) 1

* mixture of azoxystrobin and difenoconazole

% spore germination % micro-colony growth

95 μm

Testing for Sensitivity to Fungicides

10 20 30 40 50 60 70 80 90 100 None Low Medium High

% of samples Level of insensitivity

2008 2014 2015 RG 0% 0 – 40% 40 – 70% ≥ 70%

Insensitivity to Fentin Hydroxide

10 20 30 40 50 60 70 80 90 100 None Low Medium High

% of samples Level of insensitivity

2008 2014 2015 RG 0% 0 – 40% 40 – 70% ≥ 70%

Insensitivity to Propiconazole

10 20 30 40 50 60 70 80 90 100 None Low Medium High

% of samples Level of insensitivity

Propiconazole Difenoconazole RG 0% 0 – 40% 40 – 70% ≥ 70%

Insensitivity to DMIs

10 20 30 40 50 60 70 80 90 100 None Low Medium High

% of samples Level of insensitivity

2008 2014 2015 RG 0% 0 – 40% 40 – 70% ≥ 70%

Insensitivity to Dodine

10 20 30 40 50 60 70 80 90 100 None Low Medium High

% of samples Level of insensitivity

2008 2014 2015 RG 0% 0 – 40% 40 – 70% ≥ 70%

Insensitivity to Thiophanate-methyl

Risk of Resistance to QoI Fungicides*

• The rapid assay method is not accurate for QoI

fungicides (strobilurins)

• Cytochrome b is essential for fungal respiration and is

the target of QoI fungicide activity

• Mutations in the cytochrome b gene are known

mechanisms of QoI resistance G143A common, complete resistance to QoIs F129L not common, partial resistance to QoIs G137R not common, partial resistance to QoIs

* azoxystrobin (Abound), pyraclostrobin (Headline), kresoxim- methyl (Sovran)

Risk of Resistance to QoI Fungicides

• a test was designed to look for mutations in a small

section of the cytochrome b gene that included the known mutation sites

• 125 isolates of the scab fungus were tested
• no mutations found at positions 129 or 143
• a new mutation found at position 137 in 7 isolates; the

effect of this mutation on sensitivity to QoIs is unknown, but is currently being investigated

Risk of Resistance to QoI Fungicides

• two non-coding regions of DNA (introns) were detected

in all 125 isolates

• location of the large intron next to position 143 renders

a mutation at this position lethal, thus preventing complete resistance to QoIs

• presence of this intron in all 125 isolates suggests that

the risk of complete resistance to QoIs is low for the pecan scab fungus (very good news!!)

What Does It Mean for Scab Management?

• Field study to determine effect of reduced sensitivity on

fungicide efficacy for scab control

• 10 consecutive applications, 14-day intervals

Maximum Leaf Scab Incidence

• Max. leaf scab incidence (%)

ab bc d d cd a

Nut Scab Severity*

Nut scab severity (%)

100 80 60 40 20

b a d cd bc a

* on Sep 15 2016

TPTH – Scab Incidence vs. Insensitivity*

Incidence or insensitivity (%)

May 19 May 29 Jun 8 Jun 18 Jun 28 Jul 8 Jul 18 Jul 28 * insensitivity to TPTH at 30 µg/ml

Tebuconazole – Scab Incidence vs. Insensitivity*

Incidence or insensitivity (%)

May 19 May 29 Jun 8 Jun 18 Jun 28 Jul 8 Jul 18 Jul 28 * insensitivity to tebuconazole at 1.0 µg/ml

Implications for Scab Management

• Insensitivity to TPTH, propiconazole and tebuconazole is

common in Georgia pecan orchards

• Some insensitivity to dodine, but increasing slowly
• Among the DMIs, difenoconazole remains effective,

even in locations where reduced sensitivity to propiconazole or tebuconazole has been detected

• Thiophanate-methyl (Topsin) should be used very

cautiously; no more than 1 or 2 applications per season; high levels of resistance can build up very quickly

• Low risk of complete resistance to QoI fungicides in the

scab fungus

Implications for Scab Management

• When insensitivity to TPTH was 0-15%, no significant

control of scab with TPTH

• When insensitivity to tebuconazole was 50-70%, control
• f scab with tebuconazole was inadequate
• Tank mix of TPTH + tebuconazole provided significantly

better scab control than either product alone

• Azoxystrobin provided consistently good scab control,

significantly better than other treatments; consistent with predicted low resistance risk for the scab fungus

Resistance Management More Important Than Ever!

• Alternate or tank-mix fungicides with different modes of

action (different FRAC codes: 1, 3, 7, 11, 30, 33, U12)

• Select fungicides and time applications to maximize

effectiveness

• Use non-chemical disease management practices to

reduce need for fungicides (tree thinning, limb pruning, hedging, scab-resistant cultivars, etc.)

Acknowledgments

• Tim Brenneman
• Jason Brock
• Clive Bock
• Murat Seyran
• Jeff Standish
• Lara Lee Hickman
• Ethan McBrayer
• Auburn Diffie
• Faith Anderson
• Andrew McInnes
• Tyler Snow
• Marianna Kastberg-Leonard

The Georgia Agricultural Commodity Commission for Pecans

Thank you!