2023 Cercospora Resistance Maps

Comments on Cercospora Leafspot (CLS) Resistance Maps from 2023 Growing Season

Dr. Wyatt, Dr. Secor, and Viviana Rivera’s work has shown exciting advancements in understanding Cercospora Leaf Spot (CLS) epidemiology. This identified early season CLS spore dispersal from previous year’s beet fields, CLS presence in leaves prior to visual detection, and that within the growing season there are dynamic resistant population shifts in response to fungicide applications. Therefore, efficacy and CLS control can be maintained by tank mixing and rotating fungicide MOA’s as well as starting early to treat primary CLS infection and delay CLS on-set in the field.

Dr. Wyatt Latent & In-Season Cercospora Sample Research 

Latent CLS infection period (pages 3-6) is the timeframe between the initial infection of the CLS spore in the leaf and visual detection of the Cercospora Leaf Spot. Through early season sampling, Dr. Wyatt’s research has shown that by the first week of July every year virtually all samples are positive for the presence of CLS. This coincides with when the sugarbeet is at or near canopy closure. This research finding strengthens the practice of a timely initial CLS fungicide application near row closure.

In 2022 a research program was launched with Dr. Wyatt Fargo USDA Epidemiologist to study how In-Season Cercospora Leaf Spot (CLS) resistant populations shift based on what fungicide is applied.(pages 12-17) Resistant populations decrease or increase in response to the fungicide mode of action (MOA) that is applied. Example: If a DMI fungicide is applied, the resistant population towards that particular DMI rises, but it decreases the resistant population of the other fungicide MOA’s, (Tin, Headline) and those spores susceptible to the DMI. When Tin is applied in a subsequent application it reduces the DMI resistant CLS population and raises the resistant Tin population. This process goes on between every application until we get to the End-of-Season. The End-of-Season sample results show the “filtering” that has occurred throughout the fungicide application season and only shows the end result, the few spores that are left are a highly resistant CLS population to most fungicide MOA’s. But within the CLS season there were many susceptible CLS spores controlled by the use of multiple MOA’s.

The traditional End-of-Season Cercospora Leafspot (CLS) Resistance maps are presented as the average resistance level for the township based on individual samples collected and analyzed by Dr. Secor and Viviana Rivera-Varas at NDSU. These samples are collected at the end of the growing season, in mid-September, and represent the Cercospora spore population present at that time.

In 2021 – 2023 there were 17 sample locations/Agriculturist. This is approximately half the number of locations were sampled as previous years.

Starting in 2019 for the DMI fungicides, (Eminent, Inspire, Proline & Provysol) Dr. Secor’s research lab provided measurements of resistance levels greater than 10ppm for EC50 values. Being able to have these observations provided greater resolution in the strength of resistance to the DMI fungicide populations. Prior to 2019 the map only showed EC50’s as greater than 10ppm was preserved but a new map was created to show the addition of the detailed data of greater than 10ppm measurements and this map will be referenced in the below comments. This map was segmented for all DMI’s by adding “Very High Level” categories of: 10.0001 – 25.0 ppm (VH1); 25.0001 – 50.0 ppm (VH2); >50.0 ppm (VH3). Creating these segments allows the use of the actual ppm measurements greater than 10ppm in creating the township averages.

To Note: The maps as well as the descriptions below are based on the average township severity rating based on all samples taken in that township. Individual samples within the township may be below or above the average level of resistance for the township.

Map 1– Eminent/Minerva by township
The map and below sample information represents End-of-Season Cercospora resistance populations. See Dr. Wyatt’s research above on In-Season resistant population shifts. Eminent/Minerva resistance levels remain mostly in the very high categories throughout the RRV. Map 1 shows that the Very High Level 3 resistant level (>50.0 ppm) is averaged in the majority of townships at 66% followed by Very High Level 2 (25.0001- 50.0 ppm) at 18% of the townships. From 2020 – 2023, the Very High 3 resistance levels increased from 17% to 66% respectively. In 2023, 8% of the township are in the less than 10ppm average resistance level categories. Eminent/Minerva is strongly recommended to only be used as a tank mix partner with another non-Triazole fungicide. Not more than 50% of total applications per year should include triazoles. Contact your Agriculturist for information on a section by section basis.

Map 6 – Headline/Priaxor by township
The map and below sample information represents End-of-Season Cercospora resistance populations. See Dr. Wyatt’s research above on In-Season resistant population shifts. Headline/Priaxor is reported in terms of % of sensitive/resistant isolates. The percent of isolates resistant to Headline/Priaxor are very high overall. 96% of townships averaged 100% resistance in 2023 in the End-of-Season samples which are taken typically after Headline has been applied as the last fungicide of the year. In 2023, 3% of townships averaged some level of sensitivity to Headline/Priaxor. Ideal timing for this application remains to be the last week of August through the first week of September for sugarbeet root frost protection/recovery and plant health/storage benefits. Headline/Priaxor MUST be tank mixed with a full rate of each product in the tank mix, NEVER use these products without a tank mix partner. Contact your Agriculturist for information on a section by section basis.

Map 8 – Inspire by township
The map and below sample information represents End-of-Season Cercospora resistance populations. See Dr. Wyatt’s research above on In-Season resistant population shifts. Inspire resistance levels are mostly in the Very High resistant level ranges. In 2023 the majority of township averaged in the Very High level 3 (>50ppm) range at 50% followed by Very High level 2 (25.0001 – 50 ppm) at 23%. For the Very High level 3 range this remained fairly stable from 2022 levels of 47%. There was a slight reduction in townships with less than 10ppm average resistance level which is at 19%. Inspire is strongly recommended to only be used as a tank mix partner with another non-Triazole fungicide. Not more than 50% of total applications per year should include triazoles. Contact your Agriculturist for information on a section by section basis.

Map 13 – Proline by township
The map and below sample information represents End-of-Season Cercospora resistance populations. See Dr. Wyatt’s research above on In-Season resistant population shifts. Proline resistance levels remain mostly Very High throughout the RRV. In 2023, the Very High level 3 (>50 ppm) has the highest percentage of township averages at 90%, similar to 2022. The percent of townships at the less than 10 ppm average resistance level which is at 6%. Proline is strongly recommended to only be used as a tank mix partner with another non-Triazole fungicide. Not more than 50% of total applications per year should include triazoles. Contact your Agriculturist for information on a section by section basis.

Map 18 – Provysol by township
The map and below sample information represents End-of-Season Cercospora resistance populations. See Dr. Wyatt’s research above on In-Season resistant population shifts. Provysol resistance levels remain mostly Very High. In 2023, Provysol resistance levels in the majority of townships averaged at the Very High Level 3 (>50 ppm) category at 61% remaining consistent to 2022. The percent of townships at the less than 10 ppm average resistance level is at 19.%, again similar to 2022 levels. Provysol is strongly recommended to only be used as a tank mix partner with another non-Triazole fungicide. Not more than 50% of total applications per year should include triazoles. Contact your Agriculturist for information on a section by section basis.

Map 23 – TPTH (Tin) by township
The map and below sample information represents End-of-Season Cercospora resistance populations. See Dr. Wyatt’s research above on In-Season resistant population shifts. There was a major reduction in the level of resistance to Tin in 2023. From 2022 to 2023, resistance levels dropped from 72% down to 0.5% for the percent of townships in the category of 51-75% of spores being resistance to 1 ppm of TPTH. In 2023, the majority of townships (54%) are now in the category of 0% of spores being resistance to 1 ppm of TPTH. Tin should be utilized as a tank mix partner with other chemistries, at full rates of each tank mix partner. Contact your Agriculturist for information on a section by section basis.

Topsin by township – see 2019 Map 25
The map and below sample information represents End-of-Season Cercospora resistance populations. See Dr. Wyatt’s research above on In-Season resistant population shifts. Through Dr. Wyatt’s research it has been found that Topsin has the lowest cross resistance of any of the fungicides. Therefore it maintains value as a tankmix partner. To note: Topsin cross resistance is lowest to Tin, so what Tin doesn’t control Topsin is the fungicide that can. Samples for End-of-Season Topsin resistance have not been run since 2019. Please refer to Map 25 – Topsin by Township from 2019. The 2019 Topsin map is reported in terms of % of sensitive/resistant isolates. The percent of isolates resistant to Topsin are very high overall, but Topsin remains quite effective as a tank-mix partner and helps to preserve other fungicides (reference TPTH). Topsin is NEVER recommended to be applied alone. Topsin with a suitable tank mix partner with TPTH in the 1st or 2nd fungicide application and remains to be a very good CLS management option. Remember Topsin PHI is 21 days, so plan harvest accordingly around your spray rotation.

General Comments 
Even though 2023 growing season was droughty with low Cercospora pressure, proper timing of the initial fungicide application and subsequent one paid off in reducing CLS pressure and optimizing recoverable sugar/acre. In 2022 the Agriculturists began sampling sugarbeet leaves weekly in June for the presence of Cercospora DNA in the absence of visual spots. By the 1st week of July, 100% of the sample locations in the RRV were positive for Cercospora DNA in a late planted dry year. This followed suit to what’s been witnessed past couple years since early sampling began. In 2023, again the first visual Cercospora spots were found the first week of July. Cercospora remained under control for much of the growing season due to timely initial and subsequent fungicide applications in which fungicides were tank-mixed and rotated. A late onset of Cercospora infections occurred however in September but was not considered as severe as prior years. Growers in 2023 had an average of 3 CLS fungicide applications across our growing region, compared to 3.2 in 2022. The 3 fungicide applications are the 5th highest average in our data base dating back to 1980. It is extremely important to have a well-timed initial fungicide application, prior to or at row closure, to keep Cercospora from becoming established in fields. Continue diligent CLS management by tank-mixing and rotating fungicide modes of action is highly recommended to aid in reducing overall CLS infections and inoculum. Using 15-20 gallons per acre water at 80+ psi, starting spray program earlier and 12 day maximum spray intervals are all essential practices that need to implemented. It is recommend that glyphosate and other herbicide applications be made separate from fungicide applications as water volumes vary by targeted pest.

For more information on CLS management, contact your Agriculturist. Another useful website is www.sbreb.org for the latest in North Dakota State University and University of Minnesota research information.