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Summary of Cercospora Leaf Spot (CLS) Epidemiology Recent Research Findings

Dr. Wyatt, Dr. Bolton, Dr. Secor, and Viviana Rivera-Vara’s work at Fargo USDA & NDSU has shown ground-breaking advancements in understanding Cercospora Leaf Spot (CLS) epidemiology. I thought it was very important to provide a summary of these CLS research findings that were presented at this winter’s meetings. Leaf samples from throughout the RRV have been supplied by American Crystal Ag Staff over the years to assist in this research. This research has identified: early season CLS spore dispersal timing from previous year’s beet fields; CLS presence in leaves prior to visual detection; dynamic in-season Cercospora resistant population shifts in response to the fungicide modes of action applied. Also, it has been found that resistance for a location is not absolute in that there are surprisingly high levels of susceptible isolates within a sampled population. Therefore, efficacy and CLS control can be maintained by tank mixing and rotating fungicide modes of action (MOA’s) as well as starting early to treat primary CLS infection and delay CLS on-set in the field.

The slides contained in this Ag Notes issue were created by and are credited to Dr. Wyatt, Fargo USDA Epidemiologist.

Latent CLS Infection - Initial Infection Prior to Visual Symptoms/Spots

Latent CLS infection period is the timeframe between the initial infection of the CLS spore in the leaf and the visual detection of the actual Cercospora Leaf Spot. Through early season sampling, Dr. Wyatt’s research has shown that by the first week of July every year virtually all locations 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, late June to the first week of July.

Initial fungicide application timing is the most critical component to get correct in the Cercospora fungicide program for control and to optimize revenue potential. If it is too late, the entire fungicide program is not as effective and put in jeopardy as you are already “behind”.

In-Season CLS Resistance Population Shifts In Response to Fungicide Modes of Action Applied

In 2022 a research program was launched with Dr. Wyatt, Fargo USDA Epidemiologist, to study how In-Season Cercospora Leaf Spot (CLS) populations shift in susceptibility/resistance levels based on what fungicide is applied. Because CLS is polycyclic in its nature (multiple generations in a growing season), resistant populations decrease or increase in response to the fungicide mode of action (MOA) that is applied. Example: If a DMI/Triazole fungicide is applied, the resulting subsequent CLS population increases in resistance towards that particular DMI/Triazole, but it increases the population’s degree of susceptibility to the other fungicide MOA’s (Ex. Headline, Tin, & Topsin). Then when Tin is applied in a subsequent application it reduces the DMI/Triazole resistant CLS population and raises the resistant Tin population. And so on and so forth. This interaction occurs between every fungicide application so that within the CLS season there are many susceptible CLS spores controlled by this reaction of the CLS population which allows the effective use of multiple MOA’s throughout the Cercospora control program.

Fungicide Cross-Resistance

The lower the cross-resistance factor, the greater the probability that if one fungicide mode of action won’t control Cercospora, the other will. This is why we tank mix. When comparing Headline to Headline you have a fungicide cross resistance factor of “1.00” as you are comparing the likelihood of probability of cross resistance to itself. When looking at the cross-resistance factor of Tin to Topsin it is very low at 0.21, making this an excellent tank-mix combination. Another pairing to highlight is Headline to Tin with a cross resistance of 0.40. This shows why these tank-mix partners have performed very well together. As well, there has not been any documented resistance to EBDC’s making them an ideal tank-mix partner with the Triazole (DMI) modes of action (Domark/Eminent, Proline, Inspire XT, Provysol). Tank-mixing and rotating these modes of action provides strength to the fungicide program limiting escapes in control & lowers resistance levels across modes of action.

% Resistance Population vs. % of Locations with Resistance

The traditional end-of-season sugarbeet leaf samples used over the last few decades have shown that there have been increases in the % of field locations that contain fungicide resistance across all modes of action. This was measured in a way that if there was 1 out of 100 isolates from a location sample that were positive for resistance, that location was marked as entirely positive for resistance to that fungicide to the degree it was expressed. This interpretation is what was shown in the resistance maps that were developed in previous years. Because of new genetic evaluation technology being used, isolates can be studied on an individual basis (quantitatively) and not as a whole general population for a location (qualitatively).

Now it can be detected that even though many sample locations have isolates with fungicide resistance, the CLS isolate population demographic in those fields actually have a higher percentage of susceptibility to all fungicide modes of action than what could have previously been detected. In 2024 samples, only 25% of the Cercospora isolates were resistant to the strobilurin Headline, 15% for Topsin, 30% for Tin, 53% for Inspire/Provysol, and 62% for Domark/Eminent/Proline. This new information is groundbreaking in restoring confidence in using and tank-mixing all of our available fungicide modes of action for Cercospora control and continued resistance management.

Cercospora Population Genetic Variation - Adaption to CR+

Since before the introduction of the CR+ trait, there were naturally occurring resistant isolates to it in the environment. When the CR+ trait was introduced, it created selection pressure towards the proliferation of these resistant isolates. This is expressed in the population divergence in Cercospora population mutations that became apparent in 2021 and even more confirming in 2023 (see chart). This is why it is important to maintain a fungicide program with properly timed application intervals of 12 days to limit the propagation of these isolates. Through using proper initial application timing, tank-mixing and rotating fungicide modes of action, and high tolerance genetics (another mode of action for control) the efficacy of all tools can be optimized and maintained.

General Comments

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 recommended 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. This website also has detailed information. Another useful website is www.sbreb.org for the latest in North Dakota State University and University of Minnesota research information.