Fungicide Resistance in Field Crops

1. Fungicide Resistance in Field Crops Don Hershman Extension Plant Pathologist University of KY, Princeton

2. Fungicide Resistance: • There are many instances of fungicide resistance developing in horticultural crops in the US in both horticultural and grain crops in Europe. • Strobilurin fungicides like Headline and Quadris are the biggest culprits. • More than 20 instances of resistance to this class of fungicides has been reported internationally. • Resistance in triazoles, like Domark, much rarer.

4. FROGEYE LEAF SPOT STROBILURIN-SENSITIVITY Survey, 2011-12 • 85 isolates tested. • Resistant isolates (purple) found in 15 of 19 counties. • Resistant and sensitive isolates found in 9 counties. • Resistant isolates only in 6 counties. • Sensitive only isolates found in 4 counties (red)

5. Fungicide resistance – how does it occur? • Fungicide resistance development is due to two factors: • Pathogen variability • Selection pressure

6. Fungicide resistance – how does it occur? • Pathogen variability • Greatest in fungi with high reproductive rates (produce lots of spores in multiple disease cycles each year). • Examples: Rusts, frogeye leaf spot, grey leaf spot.

7. Fungicide resistance – how does it occur? • Selection pressure depends on the fungicide applied: • Single site of action vs. multi-site activity. • Numbers of applications over time. • Rates used.

8. Strobilurin (QoI) fungicides • Quinone outside inhibitors • Also referred to as “strobilurins” • Mode of action = respiration inhibitor • High efficacy of inhibiting spore germination • Examples of products used in field crops • Headline, Quadris, Evito (solo a.i.) • Stratego YLD, Quilt Xcel, Headline AMP (combinations of strobilurin + triazole fungicides)

9. FRAC Code

10. Selection pressure (Every time you spray) Conidia (spores) Selection pressure

14. How Much Selection Pressure Exists in KY Grain Crops? • Prior to 2002, almost no fungicides were applied to corn and soybeans; a moderate amount of fungicides were used in wheat • Currently, about 35% of corn acres, 20% of soybean, and 80% of wheat acres are sprayed with a fungicide. • Most acres receive 1 fungicide application at most in each crop, but this is changing. • Reduced use rates are common in early applications.

15. Strategies for reducing risk • Minimize fungicide use • Mix or alternate fungicide classes (FRAC groups) • Avoid “plant health” uses • If resistance to strobilurins is confirmed or highly suspected, do not apply strobilurins as solo applications. • Use premixes or fungicides from a sensitive fungicide class (e.g., triazoles).

16. The Fungicide Resistance Risk is: • Real and increasing. • No new fungicide classes will be in the pipeline into the foreseeable future. • In other words, do not expect technology to bail us out of fungicide resistance problems.

17. QUESTIONS?