Update on Fungicide Resistance Research and New Implications for Scab Management Katherine L. Stevenson
Fungicides for Pecan Disease Management Class FRAC Resistance Risk group MBCs 1 High DMIs 3 Medium QoIs 11 High Organotins 30 Low to Medium Phosphonates 33 Low Guanidines U12 Low to Medium
UGA Plant Pathologist, R. H. Littrell, reports benomyl resistance Pecan South, March 1976 Pecan South, June 1977
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 15 1995 Orchard 5 n=50 M ean RG=62.1% 10 5 s e t a 0 l o s 2003 Orchard 5 i f n=59 o M ean RG=90.0% r 10 e b m u N 5 0 0 10 20 30 40 50 60 70 80 90 100 110 120 Mean relative growth on 0.2 μ g/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 Log EC 50 Value ( μ g/ml) more sensitive less sensitive
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 Concentration code “At-risk” Fungicide ( μ g/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
Testing for Sensitivity to Fungicides 95 μ m % spore germination % micro-colony growth
Insensitivity to Fentin Hydroxide 2008 2014 2015 100 90 80 % of samples 70 60 50 40 30 20 10 0 None Low Medium High RG 0% 0 – 40% 40 – 70% ≥ 70% Level of insensitivity
Insensitivity to Propiconazole 2008 2014 2015 100 90 80 % of samples 70 60 50 40 30 20 10 0 None Low Medium High RG 0% 0 – 40% 40 – 70% ≥ 70% Level of insensitivity
Insensitivity to DMIs Propiconazole Difenoconazole 100 90 80 % of samples 70 60 50 40 30 20 10 0 None Low Medium High RG 0% 0 – 40% 40 – 70% ≥ 70% Level of insensitivity
Insensitivity to Dodine 2008 2014 2015 100 90 80 % of samples 70 60 50 40 30 20 10 0 None Low Medium High RG 0% 0 – 40% 40 – 70% ≥ 70% Level of insensitivity
Insensitivity to Thiophanate-methyl 2008 2014 2015 100 90 80 % of samples 70 60 50 40 30 20 10 0 None Low Medium High RG 0% 0 – 40% 40 – 70% ≥ 70% Level of insensitivity
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 a Max. leaf scab incidence (%) ab bc cd d d
Nut Scab Severity* a 100 a Nut scab severity (%) 80 b 60 bc 40 cd d 20 0 * 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 of 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!
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