By: Kristi Waterworth
Beets and their colorful cousins, the chards, are beautiful and nutritious additions to your homegrown dinner table, but things don’t always go as planned with this family of root vegetables. Sometimes, the weather isn’t on your side and instead favors beet Cercospora spot, a fungal pathogen that can cause both foliar damage and reduce yields considerably. Whether you’ve had beets with Cercospora spot in the past or suspect it in this year’s crop, we can help you tame it!
Cercospora spot on beets can be pretty frightening to see in your crop, especially if you didn’t realize what it was at first and let the small spots spread before making a move. Fortunately, your crop should be able to weather this storm, but you must start by making a positive identification today. You’ll know beet Cercospora spot by the small, pale, circular to oval spots with purple or brown borders.
As these little spots spread, they can grow together to form larger, misshapen areas of dead tissue. More mature spots will also have visible black reproductive structures known as pseudostromata at their centers, though you may need a magnifying glass to be sure. When these spots fruit, they’re covered in colorless, fuzzy spores, which can then infect healthy plants. Heavily infected leaves may turn yellow or simply wither and die.
Noticing Cercospora spot symptoms early can mean the difference between successful treatment and another year of lost beet crops.
If your beets are just now showing signs of Cercospora spot, you’re in a fortunate place because treatment can help them dramatically. There are two important things to keep in mind when treating Cercospora spot, however.
First, you need to read the package insert for your chosen fungicide(s) so you’re aware of how long to wait before harvesting your bounty.
Secondly, it’s important to rotate fungicides since many strains of Cercospora spot have developed resistance. However, rotating different types of fungicide, like pyraclostrobin, triphenyltin hydroxide, and tetraconazole throughout the growing season can help overcome this resistance. Keep in mind that treating your beets with a fungicide will not correct any damage that has already occurred, but it can prevent new spots from erupting.
Over the long term, you can reduce your risk of Cercospora spot by practicing a 3-year crop rotation, removing or plowing under all old or dead vegetation both during the growing season and post-harvest, and using more Cercospora spot resistant varieties. Trying a few different varieties of beets next season will not only provide a lot more eye-catching color in your beet garden, but allow you to test various beets for their resistance in your local climate.
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The symptoms, factors favoring infection, prediction and control measures for Cercospora leaf spot of sugar beet is described in this NebGuide.
Robert M. Harveson, Extension Plant Pathologist
Cercospora leaf spot (CLS) is the most serious and destructive foliar disease of sugar beet in the central High Plains of western Nebraska, northeastern Colorado, and southeastern Wyoming. This disease is caused by the airborne fungus Cercospora beticola. CLS has a long history and has played a shaping role in sugar beet cultivation throughout the central and eastern production areas of the United States. This disease became a major limiting factor in early Nebraska production areas years ago, and was a primary reason for the shift of sugar beet production from eastern portions of the state to the west in the 1920s. The disease was of minor importance in western Nebraska until the mid-1980s, when it began to cause significant yield and sugar reductions throughout the North Platte River Valley.
Losses due to this disease can approach 40 percent, and are represented by both root tonnage and sugar percentage in roots. Beets with low sugar levels do not store well, and losses in storage result from increased storage decay. Profitable yields are additionally reduced due to greater levels of impurities in roots and increased sugar loss to molasses during processing.
Individual leaf spots initially occur on older leaves and then progress to younger leaves. Individual lesions are approximately one-eighth inch in diameter with ash-colored centers and purple to brown borders, and are circular to oval shaped (Figure 1A). Cercospora leaf spot is distinguished from other leaf diseases (Alternaria, Phoma and bacterial leaf spots) by their smaller size and shape (Figure 2), and the presence of black spore-bearing structures, called pseudostromata, that form in the center of the lesions (Figure 3). These structures are easily seen as black dots with the aid of a hand lens (10X magnification) (Figure 4). During periods of high humidity the black dots will be covered with colorless fuzzy masses of spores resembling cobwebs (Figure 5), which serve as the source for secondary infections within fields that season.
As disease progresses, heavily infected leaves initially turn yellow (Figure 6). Individual spots may coalesce and form larger areas of dead tissue (Figure 1B), causing severely infected leaves to wither and die (Figure 7). Severely diseased plants can be viewed at a distance when dead or dying leaves appear above the canopy, giving a burned or scorched appearance (Figure 8). Disease often is unevenly distributed in fields, usually being more severe in protected areas adjacent to windbreaks formed by trees or taller crops, or other areas that may result in higher levels of humidity.
The development of disease is highly dependent upon the presence of susceptible cultivars, adequate inoculum and environmental conditions characterized by periods of high humidity or leaf wetness periods longer than 11 hours and warm temperatures (> 60°F). Since leaf wetness is not routinely measured, relative humidity above 90 percent humidity can be used as a substitute. All of these factors must be present simultaneously for disease to begin and progress. For example, if a susceptible cultivar and adequate inoculum are present, but temperatures are too cold or warm or no moisture is on leaf surfaces, then infection cannot occur.
Generally, temperatures at night in western Nebraska are near the lower limits favoring disease development, but leaf wetness does occur (particularly with sprinkler irrigation). Very little infection will occur below 60°F or during periods of less than 11 hours of leaf wetness. Greater spore germination and leaf infection generally occurs when night temperatures exceed 60°F and day temperatures are between 80° and 90°F.
Initial inoculum potential depends on the survival of the fungus spores and spore-bearing structures (psuedostromata) from the previous year’s infected crop residue. Plants related to sugar beets such as weeds (lamb’s quarters and pigweed), and vegetable crops (chard, spinach, table beets) also may be a source of inoculum for infection in sugar beet. New spores produced under humid conditions on surviving pseudostromata can be carried by wind or splashing water to infect adjacent leaves and plants. Under favorable conditions in mid-summer, inoculum levels continue to increase and the life cycle of the disease may be completed within 10 days. Whenever leaf spots are observed, infection has taken place sometime in the previous 7-10 days, thus total infection present during favorable conditions is greater than what would be visible for up to seven days.
Because of the strict environmental conditions needed for infection to occur, the disease is well suited to prediction of time periods when outbreaks would most likely be favorable. The prediction system is an estimate of the potential for disease development based on the relative humidity and temperature measured within fields. This system was developed in the late 1980s by UNL scientists, Albert Weiss and Eric Kerr, and still is being used today at the University of Nebraska Panhandle Research and Extension Center. Over the last 15 years the forecasting system has utilized up to 14 sites per season located in Nebraska, Colorado, Wyoming, and Montana. Results are then collated and disseminated to more than 40 sources, including consultants, researchers and media (Web, print, television, and radio).
This system assumes that a susceptible host and sufficient inoculum are present. Based on hours of leaf wetness or high relative humidity (> 90 percent) and temperature during this period, a daily infection value (DIV) is determined (Table I). If the two-day sum of the DIVs is seven or greater, there is a strong potential for infection and further disease development. If the sum is less than six, there is little likelihood of infection.
The following example will illustrate how to use the information in Table I. Assume that on Day 1 there were 13 hours of leaf wetness and the mean temperature during this period was 63°F. On Day 2, there were 15 hours of leaf wetness with a mean temperature of 65°F. The DIV for Day 1 was three while on Day 2 it was four. The sum of these two days was seven, resulting in conditions that would favor infection. If no symptoms were observed on leaves, then the DIV sum indicates that careful scouting is advised, and if symptoms were present, then a fungicide application would be warranted. If on Day 3, there were 12 hours of leaf wetness and the temperature during this period was 62°F, then the DIV sum for Days and 2 and 3 is 4+0=4, and no action would be necessary.
Hosts of Cercospora beticola include sugar beets (Beta vulgaris), swiss chard (Beta vulgaris L. subsp. cicla) and other leafy greens. Symptoms include the random distribution of spots with brownish red rings which eventually cause leaf collapse. Older leaves will have spots of larger diameters as rings grow outward. Conidia are not observable by the unaided eye   
Stromata (a sclerotia-like survival structure containing conidia when made) in field debris starts the life cycle. Under favorable wet conditions, conidia are rain-splashed and insect-carried to new hosts, where, under humid and wet conditions, they germinate and penetrate through stomata. These conidia germinate in polycyclic microcycles until the end of the growing season. At the end of the growing season, C. beticola produces stromata again as a survival structure. Microcycles like the one used by C. beticola are very effective at producing many conidia. Because these conidia are effective at penetrating the host, mycelium is not necessary, and conidia produce their own conidia at each new infection (microcycle). There have been no direct observations of sexual spores in C. beticola.    
Copper was historically used to control C. beticola in the field, though today fungicides are more common. C. beticola has been shown to have some resistance to benzimidazole and thiophanate class fungicides. As a result, experts often have recommended fungicide rotation to kill any potential fungicide resistant strains. Some varieties of sugar beet also show resistance to C. beticola, unfortunately they have all had low yields in lab tests. Today the most common fungicides used are QoI, Headline, Proline, Inspire SB, Eminent and Super Tin or Agri Tin   
Spots on a spinach leaf caused by Cercospora beticola
Once infected, plants can be treated if it’s done early. Fungicides work well with this blight at this stage. If the plant is more than 20% infested, then destruction may be the only choice to keep other plants from succumbing.
Raymond Parenteau says
What is some good fungicides for cercospora leaf spots? My damage is already done but I want to prepare for next year.
This disease was observed in a home garden recently on table beet (Figure 1) . I was surprised considering how dry it has been. Cercospora leaf spot also affects swiss chard. Symptoms include circular leaf spots that may have a reddish margin. The center of the lesions may start off a light brown and turn to gray after the fungus ( Cercospora beticola ) begins to sporulate. Under conditions conducive to disease, the lesions can coalesce and result in loss of foliage. Yield and quality of the crop can be reduced.
Figure 1. Cercospora leaf spot of beet.
Cercospora leaf spot is favored by rainy weather or overhead irrigation and temperatures from 77 to 95°F. The spores are readily dispersed in rainy, windy weather. One reference I found said that Cercccospora leaf spot can start with 90% relative humidity—that is, leaf wetness may not be necessary. That might be w hy this disease is present in this dry spell.
Cercospora may in survive crop residue over winter. The fungus may also be s eed borne.
Resistant cultivars are available. Fall tillage and crop rotations of 2 to 3 years should help to lessen disease severity.
Several fungicides are listed in the 20 20 Midwest Vegetable Production Guide for Commercial Growers including copper compounds, some of which may be allowed in organic certifications.