Update on Fungicide Resistance Research and New Implications for Scab Management. Katherine L. Stevenson

Transcription

1 Update on Fungicide Resistance Research and New Implications for Scab Management Katherine L. Stevenson

2 Fungicides for Pecan Disease Management Class FRAC group MBCs 1 High Resistance Risk DMIs 3 Medium QoIs 11 High Organotins 30 Low to Medium Phosphonates 33 Low Guanidines U12 Low to Medium

3 UGA Plant Pathologist, R. H. Littrell, reports benomyl resistance Pecan South, March 1976 Pecan South, June 1977

4 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

5 History of Fungicide Resistance in Pecan Scab DMI fungicides significant decrease in sensitivity to DMI fungicides propiconazole (Orbit) and fenbuconazole (Enable) associated with reduced scab control Orchard 5 n= Mean RG=62.1% 5 Number of isolates Orchard 5 n= Mean RG=90.0% Mean relative growth on 0.2 μg/ml propiconazole (%)

6 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 Log EC 50 Value (μg/ml) less sensitive

7 Testing for Sensitivity to Fungicides

8 Fungicide Sensitivity Monitoring 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 Year Samples tested

9 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

10 Testing for Sensitivity to Fungicides 95 μm % spore germination % micro-colony growth

11 Insensitivity to Fentin Hydroxide % of samples None Low Medium High RG 0% 0 40% 40 70% 70% Level of insensitivity

12 Insensitivity to Propiconazole % of samples None Low Medium High RG 0% 0 40% 40 70% 70% Level of insensitivity

13 Insensitivity to DMIs % of samples Propiconazole Difenoconazole None Low Medium High RG 0% 0 40% 40 70% 70% Level of insensitivity

14 Insensitivity to Dodine % of samples None Low Medium High RG 0% 0 40% 40 70% 70% Level of insensitivity

15 Insensitivity to Thiophanate-methyl % of samples None Low Medium High RG 0% 0 40% 40 70% 70% Level of insensitivity

16 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 F129L G137R common, complete resistance to QoIs not common, partial resistance to QoIs not common, partial resistance to QoIs * azoxystrobin (Abound), pyraclostrobin (Headline), kresoximmethyl (Sovran)

17 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

18 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!!)

19 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

20 Maximum Leaf Scab Incidence Max. leaf scab incidence (%) ab bc d d cd a

21 Nut Scab Severity* 100 a Nut scab severity (%) b a d cd bc 0 * on Sep

22 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

23 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

24 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

25 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

26 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.)

27 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

28 Thank you!