Managing Cercospora Resistance: An Outline of the Issue — And Recommendations — in Michigan Sugarbeets
By Greg Clark
Cercospora leafspot is among the most serious diseases of sugarbeets in Michigan, capable of inflicting significant tonnage and sucrose losses as well as increased impurities. Yield losses of two tons per acre and one-fourth point of sugar are common in our growing region, with some fields having lost upwards of several tons and a couple points of sugar.
Cercospora has been more difficult for some Michigan growers to control during the past two seasons — and it appears resistance issues with a couple strobilurin fungicides lie behind the problem. Typically, the region’s growers use Quadris® (a strobilurin) for Rhizoctonia control; then apply Proline®, Eminent®, Enable® or Inspire® (all triazoles) as the first leafspot spray. Headline® or Gem® (strobilurins) are generally the second leafspot spray, and that’s where it appears we are losing some control of Cercospora.
A sentinel plot was conducted near Elkton, Mich., in 2011 to evaluate the efficacy of fungicides for leafspot control. Results from that trial showed that Headline and Gem did not provide adequate leafspot control last year, compared to prior years. Eminent, Inspire, Proline and Super Tin did provide good Cercospora control.
Leaves from the 2011 sentinel plot were gathered and sent to Michigan State University and to North Dakota State University to analyze for Cercospora resistance. Results from both universities indicated that Cercospora spores from the sentinel plots were resistant to Headline and Gem.
Michigan Sugar Company agriculturists also sampled leaves from around the sugarbeet growing area, and a high percentage of those leaves tested positive for resistance to Headline and Gem. Analysis from both universities showed that more than 90% of the samples had an effective concentration (EC50) greater than 1.0 ppm, which indicates insensitivity (resistance).
“Why did resistance happen?” That question has been asked multiple times — and it’s a difficult one to answer, since many variables could contribute to the resistance issue. Possible reasons include:
• Increased use of susceptible varieties.
• Failing to rotate fungicide modes of action.
• Stretching spray intervals.
• Spraying other crops (e.g., corn, soybeans, dry beans, wheat) with Headline and Gem (strobilurin) and not rotating the mode of action.
• Late first leafspot application.
• Poor spraying techniques.
• Not tank mixing with other modes of action.
• Earlier planting dates.
• Not controlling leafspot to the end of the season.
There are two general types of fungicide resistance: “quantitative” and “qualitative.” With quantitative resistance, resistant fungus strains develop slowly; and initially, an increased dosage or shorter spray intervals will compensate for the decreased fungicide efficacy. Triazoles (e.g., Inspire, Eminent, Proline, Enable) can develop resistance in this way. Full-blown resistance with triazoles develops over a long period of time.
With qualitative resistance, the resistant fungus strains develop very rapidly, and an increased dose or shorter spray intervals will not compensate for the decreased efficacy. When a fungicide develops qualitative resistance, the effect of spraying that fungicide is akin to spraying with water. Strobilurins (e.g., Headline, Gem) develop resistance in this manner.
When different fungicides have the same mode of action, the fungus does not distinguish between the fungicides. This is called “cross-resistance.” For example, if a fungus is resistant to Headline, it also will be resistant to Gem since they have the same mode of action.
Management for Cercospora Leafspot Resistance
Cultural Practices — Since the Cercospora fungus overwinters on infected beet leaves and on some weed species, crop rotation is an important strategy in resistance management. A three- to four-year rotation is recommended for reducing carryover of the fungus.
Since spores can be blown several hundred feet, it’s important to check with surrounding growers to learn whether they had poor performance with a given fungicide the previous year. Under the “right” conditions, spores will germinate from last year’s sugarbeet fields and infect this year’s adjacent beet fields.
The spores break down more rapidly if buried, but they can survive 10 months or longer if buried eight inches or less. Burying sugarbeet litter via tillage helps reduce inoculum survival and dispersal. Fall tillage is most effective for reducing Cercospora populations, but it likewise may increase soil erosion.
Varieties — Referring to the Michigan sugarbeet region specifically, there are large differences in varietal susceptibility to Cercospora leafspot. The disease develops slowly on tolerant varieties, and normal control measures will effectively control Cercospora. However, leafspot is difficult to control with highly susceptible varieties, so an aggressive spray program then is required to protect the crop.
Fungicides — There currently are two general types of registered fungicides: protectants and systemics.
Protectant Fungicides -- Protectant fungicides such as Super Tin, the EBDCs (e.g., Dithane, Manzate, Penncozeb) and copper products kill fungi on the leaf surface and do not penetrate the leaves. These fungicides generally provide broad-spectrum disease control and have several modes of action, so that they do not develop resistance easily. Protectant-type fungicides do not provide long residual control and are susceptible to being washed off the leaves by rain. They should be tank-mixed with fungicides with higher risk for resistance development. Used in this manner, they will help slow resistance development.
Systemic Fungicides -- Systemics (e.g., Headline, Gem, Enable, Proline, Eminent) are absorbed by sugarbeet leaves and are rain-fast when dry. Systemic fungicides are active against Cercospora for a longer period of time, compared to protectants. They are highly susceptible to the development of resistance, however, and should always be tank-mixed with a protectant fungicide. The main value of protectants, for our sugarbeet situation, is for resistance management as tank-mix and rotation partners with the triazoles and strobilurins.
All of the fungicides used in sugarbeets should be applied preventively, as they do not “cure” an established Cercospora infection.
Unlike protectant fungicides, penetrant fungicides are rain-fast within a few hours of application and may require less-thorough application coverage to be effective. Both types of fungicides provide good disease control when applied prior to infection and are best applied on a preventive schedule.
`The following list of Cercospora resistance management recommendations was developed for Michigan growers in 2012. The first two points are obviously already in the “history book” as of this writing, but are still important to keep in mind for next year and beyond:
• Plant susceptible varieties only if you are willing to follow an aggressive spray recommendation.
• Use of more-tolerant varieties is especially important when planting next to a field that had Cercospora problems the previous year.
• Tank mix triazoles, strobilurins and Topsin fungicides with an EBDC, Super Tin or copper product.
• Never spray with the same modes of action back to back.
• Use Headline and Gem (strobilurins) and Topsin only once per season.
• Use the highest labeled rates of all fungicides, even in tank mixes.
• Apply fungicides in a manner that ensures maximum coverage, as enhanced coverage results in improved Cercospora leafspot control.
• Use 20-25 gallons of water with 90 psi or greater, as higher pressure and gallonage produce the best control. (A minimum of 20 gallons and 80 psi would be acceptable.)
• Use surfactants and additives as required by product labels.
• Do not delay your first leafspot application. Follow the BEETcast model; or, if scouting, don’t apply later than the first leafspot appearance in your area.
• When using Headline or Gem in other crops, always tank mix with a fungicide that has a different mode of action — or use available combination products.
• Longer crop rotation plays a key role in reducing the threat from Cercospora leafspot.
* Greg Clark is agronomist with Michigan Sugar Company. This article is based on a recent publication of MSC and Michigan Sugarbeet REACH.
Editor & General Manager of The Sugarbeet Grower