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.

3. First instance of fungicide resistance in grain crops in the United States was in West Tennessee in 2010.

4. FROGEYE LEAF SPOT STROBILURIN-SENSITIVITY Survey, 2011-12
Resistant isolates found in 15 of 19 counties surveyed. Resistant and sensitive isolates found in 9 counties (Ballard, Caldwell, Calloway, Henderson, Hopkins, Marshall, McLean, Union, Webster). Resistant only in 6 counties (Butler, Carlisle, Daviess, Graves, Hickman, Livingston). Sensitive only isolates were found in 4 of 19 counties surveyed (Christian, Crittenden, Hancock, Muhlenberg).
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

11. This illustrates how most spores are sensitive but a tiny fraction (less than shown) are resistant, purely by chance.

12. Outcome of having sprayed: Disease control
Outcome of having sprayed: Disease control. There are some escapes, but also surviving resistant spores.

13. The next infection cycle shows a buildup of resistance.

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).
I try to tie these principles to that series of slides showing the evolution of a resistant population.

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?