Don HershmanExtension Plant Pathologist University of KY, Princeton

Fungicide Resistance in Field Crops

Fungicide Resistance:Fungicide Resistance: There are many instances of fungicide There are many instances of fungicide

resistance developing in horticultural crops resistance developing in horticultural crops in the US in both horticultural and grain in the US in both horticultural and grain crops in Europe.crops in Europe.

Strobilurin fungicides like Headline and Strobilurin fungicides like Headline and Quadris are the biggest culprits.Quadris are the biggest culprits.– More than 20 instances of resistance to this class More than 20 instances of resistance to this class

of fungicides has been reported internationally.of fungicides has been reported internationally. Resistance in triazoles, like Domark, much Resistance in triazoles, like Domark, much

rarer.rarer.

FROGEYE LEAF SPOT STROBILURIN-SENSITIVITYSurvey, 2011-12

1. 85 isolates tested.2. Resistant isolates (purple) found in 15 of 19

counties.3. Resistant and sensitive isolates found in 9

counties.4. Resistant isolates only in 6 counties.5. Sensitive only isolates found in 4 counties

(red)

Fungicide resistance – Fungicide resistance – how does it occur?how does it occur?

Fungicide resistance development is Fungicide resistance development is due to two factors:due to two factors:– Pathogen variabilityPathogen variability– Selection pressureSelection pressure

Fungicide resistance – Fungicide resistance – how does it occur?how does it occur?

Pathogen variabilityPathogen variability– Greatest in fungi with high reproductive Greatest in fungi with high reproductive

rates (produce lots of spores in multiple rates (produce lots of spores in multiple disease cycles each year).disease cycles each year). Examples: Examples: Rusts, frogeye leaf spot, grey Rusts, frogeye leaf spot, grey

leaf spot.leaf spot.

Fungicide resistance – Fungicide resistance – how does it occur?how does it occur?

Selection pressure depends on the Selection pressure depends on the fungicide applied:fungicide applied:– Single site of action vs. multi-site Single site of action vs. multi-site

activity.activity.– Numbers of applications over time.Numbers of applications over time.– Rates used.Rates used.

Strobilurin (QoI) fungicidesStrobilurin (QoI) fungicides Quinone outside inhibitorsQuinone outside inhibitors Also referred to as “strobilurins”Also referred to as “strobilurins” Mode of action = respiration inhibitorMode of action = respiration inhibitor High efficacy of inhibiting spore germinationHigh efficacy of inhibiting spore germination Examples of products used in field cropsExamples of products used in field crops

– Headline, Quadris, Evito (solo a.i.)Headline, Quadris, Evito (solo a.i.)– Stratego YLD, Quilt Xcel, Headline AMP Stratego YLD, Quilt Xcel, Headline AMP

(combinations of strobilurin + triazole (combinations of strobilurin + triazole fungicides)fungicides)

FRAC CodeFRAC Code

Selection pressure (Every time you spray)

Conidia (spores)

Selection pressure

How Much Selection How Much Selection Pressure Exists in KY Grain Pressure Exists in KY Grain

Crops?Crops? Prior to 2002, almost no fungicides were Prior to 2002, almost no fungicides were

applied to corn and soybeans; a moderate applied to corn and soybeans; a moderate amount of fungicides were used in wheatamount of fungicides were used in wheat

Currently, about 35% of corn acres, 20% of Currently, about 35% of corn acres, 20% of soybean, and 80% of wheat acres are soybean, and 80% of wheat acres are sprayed with a fungicide.sprayed with a fungicide.

Most acres receive 1 fungicide application at Most acres receive 1 fungicide application at most in each crop, but this is changing.most in each crop, but this is changing.

Reduced use rates are common in early Reduced use rates are common in early applications.applications.

Strategies for reducing riskStrategies for reducing risk Minimize fungicide useMinimize fungicide use Mix or alternate fungicide classes (FRAC Mix or alternate fungicide classes (FRAC

groups)groups) Avoid “plant health” usesAvoid “plant health” uses If resistance to strobilurins is confirmed If resistance to strobilurins is confirmed

or highly suspected, do not apply or highly suspected, do not apply strobilurins as solo applications.strobilurins as solo applications.– Use premixes or fungicides from a sensitive Use premixes or fungicides from a sensitive

fungicide class (e.g., triazoles).fungicide class (e.g., triazoles).

The Fungicide Resistance The Fungicide Resistance Risk is:Risk is:

Real and increasing.Real and increasing. No new fungicide classes will be in No new fungicide classes will be in

the pipeline into the foreseeable the pipeline into the foreseeable future.future.

In other words, do not expect In other words, do not expect technology to bail us out of fungicide technology to bail us out of fungicide resistance problems.resistance problems.

QUESTIONS?