Fungicide Resistance in Sugarbeets

Sugarbeet growers in Kent County continue to struggle with potential resistance to group 3 fungicides in fields affected by Cercospora Leaf Spot (CLS) (Figure 1). The causal agent of CLS, the fungal pathogen Cercospora beticola, has a long history of fungicide resistance in North America, which has been narrowing the control options for the disease.

Chemical control options for CLS management available in Canada (Table 1) primarily include fungicides from the following FRAC (Fungicide Resistance Action Committee) groups:

• Group 1: Methyl benzimidazole carbamates (BMC)
• Group 3: Demethylation inhibitors (DMI)
• Group 11: Quinone outside inhibitors (QoI)
• Group M1: Copper hydroxide and copper octanoates
• Group M3: Dithiocarbamates

In Ontario, C. beticola populations have confirmed resistance to group 11 and group 1 fungicides. Therefore, farmers have mostly been relying on group 3 triazoles, such as Proline 480 SC (Bayer Crop Science). However, overreliance on triazoles is now driving resistance to group 3 products as well.

Triazole-resistant C. beticola strains have also been detected in other sugarbeet-growing regions worldwide, including Japan, Greece, Czech Republic, Serbia, and in the U.S., in Michigan, North Dakota, and Minnesota.

Product	Active compound	Group	Resistance Risk
Senator 50 SC	thiophanate-methyl	FRAC 1	High
Proline 480 SC	prothioconazole	FRAC 3	Medium
Mettle 210 ME	tetraconazole	FRAC 3	Medium
Quadris Top	difenoconazole + azoxystrobin	FRAC 3,11	Medium, High*
Miravis Duo	difenoconozole + pydiflumetofen	FRAC 3,7	Medium, Medium-High*
Priaxor	pyraclostrobin + fluxapyroxad	FRAC 7,11	Medium-High, High*
Acapela	picoxystrobin	FRAC 11	High
Headline EC	pyraclostrobin	FRAC 11	High
Coppercide WP	copper hydroxide	FRAC M1	Low
Kocide 2000	copper hydroxide	FRAC M1	Low
Parasol WG	copper hydroxide	FRAC M1	Low
Cueva	copper octanoate	FRAC M1	Low
Manzate Pro-stick	mancozeb	FRAC M3	Low
Serenade OPTI	Bacillus subtilis strain QST 713	FRAC BM2	Unknown

How resistance develops?

Resistance to a particular fungicide develops when a new mutation occurs in a pathogen population, allowing certain strains to survive fungicide treatments that would normally kill them. When a particular product or different products with the same mode of action is used repeatedly, these resistant strains have a better chance to thrive, as they no longer face competition from the sensitive strains that have been eliminated. Over time, resistant strains become more common, making the fungicide less effective.

Certain practices can speed up resistance development. For example, if the same fungicide is applied repeatedly within or between seasons, or if off-label rates are applied, the selective pressure on the pathogen population increases, encouraging the survival of resistant strains. Additionally, using a single fungicide group throughout the growing season without rotating to different modes of action further promotes resistance.

Resistance management for sugarbeets

While the search for alternative fungicides is ongoing, efficient methods for controlling CLS remain limited. To help slow down resistance, it is crucial to rotate or tank-mix fungicides from different FRAC groups, apply them at recommended rates, and integrate non-chemical strategies such as crop rotation and resistant varieties.

• Use sugarbeet varieties with good tolerance to Cercospora Leaf Spot.
• Do not wait until an outbreak starts to spray. Use BEETcast to determine best spray date. Generally, 50 DSV (disease severity value) is a good starting point.
• Always include either copper or mancozeb in your triazole fungicide tank-mix.