Carol E. Windels, Associate Professor, Northwest Experiment Station, University of Minnesota, Crookston, 56716; H. Arthur Lamey, Extension Plant Pathologist, North Dakota State University, Fargo, 58105; Dave Hilde, General Agronomist, American Crystal Sugar Company, Moorhead, MN 56560; Jim Widner, Vice President of Agriculture, Southern Minnesota Beet Sugar Cooperative, Renville, 56284; and Tom Knudsen Vice President of Agriculture, Minn-Dak Farmers Cooperative, Wahpeton, ND 58075.

http://www.scisoc.org

Sugar beet (Beta vulgaris L.) is planted on nearly 268,000 acers in the Red River Valley (RRV) of Minnesota and North Dakota and in southern Minnesota. This geographic region is the leader in sugar beet production in the United States and represents about 45% of the acres planted (17) and accounts for 50% of the tonnage produced annually (28). The most recent economic analysis by Bangsund and Leistritz (1) estimated that the 1992 sugar crop in Minnesota and North Dakota had direct impacts (production and processing) at $575.5 million and secondary impacts at an additional $1.06 billion.

The sugar beet industry in Minnesota and North Dakota consists of three grower-owned cooperatives (Figure 1). American Crystal Sugar Company (186,810 ha) includes five districts, each with a factory where sugar is extracted. The Minn-Dak Farmers Cooperative (37,320 ha) and the Southern Minnesota Beet Sugar Cooperative (43,590 ha) are large districts, each with a single factory. The RRV includes the American Crystal and Minn-Dak cooperatives. Distance from the northern to southern borders of the three cooperatives spans about 560 km.

This article reviews the history of Cercospora leaf spot in Minnesota and North Dakota that culminated in development of a leaf spot model. It also covers implementation and evolution of the model during 10 years of application by the sugar beet industry. Limitations and benefits of the Cercospora leaf spot model are presented.

Epidemic of 1981

Leaf spot caused by Cercospora beticola Sacc. was not economically important to the sugar beet crop produced in the RRV and southern Minnesota before 1980. Several significant changes occurred earlier in the 1970s, however, that set the stage for an epidemic. In the fall of 1973, American Crystal Sugar Company (the only sugar company in Minnesota and North Dakota at that time) became a grower-owned cooperative. Policies previously set by agricultural management of the company now included input from sugar beet producers, since both groups were represented on the Board of Directors. Producers no longer wanted to plant ‘American 2 Hybrid B’, a widely planted "exclusive" cultivar that was moderately resistant to Cercospora leaf spot. Instead, they preferred an "open seed policy" that allowed commercial availability of high yielding cultivars introduced into the United States from Europe in 1973. These cultivars rapidly gained popularity and were widely planted as the American Crystal cooperative expanded in size and as additional areas were planted with the establishment of Minn-Dak Farmers Cooperative in 1974 and the Southern Minnesota Beet Sugar Cooperative in 1975. The most popular cultivars were highly susceptible (e.g., Beta 1345, Beta 1443, Hilleshog Mono 309) to C. beticola, although several cultivars had some resistance (ACS ACH 30, G.W. Mono Hy M-8) to the disease (27).

Despite a few reports of Cercospora leaf spot in the late 1970s in the RRV, the disease appeared to be a minor problem. In 1979, a few growers in the southern Minnesota cooperative applied the benzimidazole fungicides benomyl (Benlate) or thiabendazole (TBZ or Mertect) when the disease occurred late in the season. Cercospora leaf spot was an economic problem in a few sugar beet fields in the southern RRV and in southern Minnesota in August, 1980. These fields turned "tobacco brown" (Fig. 2) almost overnight and producers and agricultural staff wondered if this was a new disease. Some panic spraying of fungicides was done in August, but these applications were too late to stop the epidemic.

Starting with the 1981 season, the cooperatives instituted an aggressive "calendar spray schedule" policy because of uncertainty as to when Cercospora leaf spot would develop and because of abundant inoculum from the previous season. The first application of fungicide was to occur "7 days before you see the first spot", with subsequent applications continuing at 10- to 14- or at 21-day intervals (depending upon the fungicide).

Cercospora leaf spot developed early in 1981. It was first noticed by mid-June in southern Minnesota, mid-July in the southern RRV, and in August in the northern RRV (5). The first fungicide applications were made by July 10 by some producers in the RRV and earlier in southern Minnesota. Overall, fungicides resulted in inconsistent control because some producers didn’t start applications early enough, used low rates, and over-extended spray intervals. Protectant fungicides (ethylenebisdithiocarbamate [EBDCs] = maneb, mancozeb) were available but were not favored because they had to be applied every 10 to 14 days. The benzimidazole fungicides were widely used because their systemic activity stopped infections that had occurred during the previous 24-36 h and because application intervals could be extended from 14 to 21 days. Protection with systemic fungicides was generally good in the American Crystal cooperative (5), where at least one application resulted in an average yield increase of 2.5 Mg/ha. Overall, the Minn-Dak cooperative noted that their crop had a low sugar content and purity compared to previous years (5). In the southern Minnesota cooperative, Cercospora leaf spot affected 80% of the crop and loss was estimated from 4.5 to 6.7 Mg/ha; sugar content averaged 13% (compared to a normal of 15%); and economic losses were from $250 to $285/ha (5).

In some fields in southern Minnesota, two to three applications of systemic fungicides were ineffective in controlling disease (Fig. 2). Strains of C. beticola with resistance to the benzimidazole fungicides were confirmed in 1981 (2) and systemic fungicides were no longer recommended (5). Many producers continued to apply benzimidazole fungicides, however, and within the next couple of years, resistance to these fungicides became widespread across the entire sugar beet growing area of Minnesota and North Dakota (18). Substantial shifts in fungicide use occurred by 1983 when protectant fungicides, primarily triphenyltin hydroxide (TPTH or Supertin), were applied on nearly all of the sugar beet crop compared to benzimidazole use on 7% of the crop (8).

Leaf spot caused by C. beticola is the most common and destructive disease of sugar beet in Minnesota and North Dakota. Severity of Cercospora leaf spot varies from year to year, depending on weather conditions and effectiveness of disease control. If leaf spots cover at least 3% of the foliage by harvest, economic losses occur through reduced root tonnage and sucrose content and increased impurities. Also, roots of infected plants do not store as well as roots of healthy plants (15).

Individual leaf spots are circular, from 3 to 5 mm in diameter, and have tan to ash gray centers with dark brown to reddish purple borders (Fig. 3A). Stromata are visible in the necrotic spots as tiny black dots (Fig. 3B). During periods of warm, wet and humid weather, the lesions become steel blue to gray in color from the production of conidiophores and conidia on the stromata and spots may coalesce (Fig. 3C). Severely infected leaves can wither, die, and fall to the ground but usually remain attached to the crown. Disease begins on the older leaves and progresses to younger foliage. Cercospora leaf spot also occurs on petioles.

The fungus overwinters on infected beet residue as stromata (19). During humid weather, conidiophores and conidia form on stromata and conidia are spread by wind, water (irrigation and rain), and insects (16). Conidia and stromata also are carried on seed produced in regions were disease is severe, but this source is of minor importance (16). Several common weed hosts, such as redroot pigweed, lamb’s-quarters, mallow, and bindweed also serve as minor sources of inoculum (20).

Sporulation, germination, and infection by C. beticola are favored by day time temperatures of 25 to 35 ^oC, night temperatures above 16 ^oC, and extended periods of high humidity (90-95%) or free moisture on leaves (15, 19, 20). Conidia of C. beticola are produced most readily at temperatures from 20 to 26 ^oC and relative humidities from 90 to 100%, but do not form at temperatures less than 10 ^oC. Ideal conditions for germination and infection occur in free water on leaves when the temperature is 25 to 35 ^oC for at least 8.5 h. Conidia germinate on the leaf surface and then penetrate through stomata. Symptoms develop from 5 to 21 days after infection, depending upon weather conditions.

Integrated management of Cercospora leaf spot includes several cultural practices, cultivars that are moderately resistant, and the prediction model (scouting for disease and monitoring of weather conditions), which allows judicious application of foliar fungicides.

Cultural practices. Since C. beticola survives in infected beet leaves, cultural practices include deep tillage (to turn under residues) and a minimum rotation of beets every third year with nonhost crops. Also, beet fields are sown at least 100 m from fields planted to beets the previous year to avoid spread of conidia from infested debris (16).

Cultivar resistance. Following the epidemic of 1981, the cooperatives adopted a stringent selection policy for cultivars approved for sale to member producers. The most susceptible cultivars were not planted in 1982. New cultivars approved for commercial production required a leaf spot rating (averaged across five to seven evaluations from late July through late August) of 5.5 or less, based on the 1-9 Kleinwanzleber Saatzucht (KWS) scale (14). They also had to yield an average percentage of recoverable sucrose equal to, or greater than, the mean of all cultivars evaluated in 3 years of tests.

Evaluations for resistance to Cercospora leaf spot are performed annually at the Betaseed Cercospora Screening Nursery in Shakopee, Minnesota, where plots are inoculated with C. beticola (Fig. 4). Coded trials for sugar beet quality and yield are evaluated annually in commercial beet fields at multiple locations throughout Minnesota and North Dakota.

Because of increasingly severe disease pressure in recent years, the maximum KWS value for commercial cultivars has been lowered. The cooperative in southern Minnesota reduced their acceptance threshold of resistance to Cercospora leaf spot to a maximum KWS rating of 5.3 in the early 1990s and then lowered it to 5.0 in 1996. The Minn-Dak Farmers Cooperative is lowering their acceptance value to 5.2 in 1998. American Crystal Sugar Company is phasing in cultivars with a KWS rating of 5.3 or lower by 1999.

Fungicide management. Fields are monitored for disease and fungicides are applied at disease onset or when weather favors disease. When conditions favor disease, or if disease already is prevalent, fungicide applications are more frequent than when disease pressure is low. Use of protectant fungicides (TPTH, EBDCs, and fixed copper [which provides limited control]) sometimes requires reapplication on a 10- to 14-day schedule to prevent disease. Registered systemic fungicides include benomyl and thiophanate methyl (Topsin M).

In 1994, tolerance to TPTH was first confirmed in southern Minnesota (3). Tolerance to TPTH is defined as reduced linear mycelial growth of C. beticola on potato dextrose agar (PDA) amended with 0.2 or 1 ppm TPTH (concentrations that completely inhibit growth of sensitive strains) compared to an unamended PDA control (3). Subsequently, agriculturists employed by the cooperatives have submitted diseased sugar beet leaves collected from fields under their supervision to the USDA, Agricultural Research Service, Northern Crop Science Laboratory at North Dakota State University, Fargo. Cultures of C. beticola isolated from these diseased leaves are assessed for tolerance to TPTH and for resistance to thiophanate methyl. Tolerance to TPTH is prevalent in southern Minnesota and in the southern RRV and is fairly prevalent in the central and northern RRV (26). Isolates of C. beticola with resistance to benomyl (3) and thiophanate methyl are widespread in southern Minnesota and in southern RRV and are uncommon in the central and northern RRV (26).

Cross-resistance of C. beticola to thiophanate methyl and TPTH was detected in 1996 and 1997 (L.G. Campbell, G.A. Smith, H.A. Lamey, unpublished). Plots treated with thiophanate methyl (three applications at 14-day intervals) resulted in a significant increase in isolates of C. beticola resistant to thiophanate methyl and tolerant to TPTH compared to the untreated control. There is no obvious explanation for this phenomenon since thiophanate methyl is a benzimidazole fungicide and TPTH is an organometallic fungicide.

Tolerance to TPTH and resistance to the benzimidazole fungicides has limited options for management of Cercospora leaf spot. One strategy is to tank mix TPTH with an EBDC fungicide (producers usually select mancozeb). Another approach is to select an EBDC fungicide (full label rate) for the first one or two applications, followed by a tank mix of an EBDC with TPTH, and then mancozeb at the end of the season, if needed. Benzimidazole fungicides are recommended only in the Hillsboro district and north (Fig. 1) as a single early-season application in a tank mix with mancozeb.

From 1982 to 1985, the Sugarbeet Research and Education Board of Minnesota and North Dakota funded the research of William W. Shane and Paul S. Teng, Department of Plant Pathology, University of Minnesota, St. Paul, to develop a Cercospora leaf spot prediction model. The intent of the prediction model was to provide information for timing applications of protectant fungicides so a producer could tailor fungicide application programs to specific fields and years, rather than rely on a fixed-calendar schedule. Information on the development and use of the model was published in a series of annual reports distributed to producers and the sugar industry in 1983-1986 (21, 22, 23, 25) and was summarized in 1991 (12).

The Cercospora leaf spot prediction model developed by Shane and Teng (21, 22, 23, 25) consisted of two integrally related components: percent severity of disease based on field monitoring and a Cercospora Advisory based on weather information. Fungicide applications were recommended at certain ratings of disease severity when weather was favorable for disease development.

Disease severity assessment. Disease monitoring began at canopy closure (usually late June to early July) and continued weekly until mid-September. Disease severity was assessed by inspecting plants throughout the field and along two sides bordering last year’s beet field, windbreaks, ditches, or other protected areas (12). Cercospora-free plants were recorded as zeros. If leaf spot was present, a lower leaf was randomly selected and its category of damage was estimated and recorded on an assessment form (Fig. 5). A total of 100 plants were rated according to a spot-percentage scale and assigned to a category (1-10) based on disease severity: 1-5 spots per leaf = 0.10% (Category 1), 6-12 spots = 0.35% (Category 2), 13-25 spots = 0.75% (Category 3), 26-50 spots = 1.50% (Category 4), and 51-75 spots = 2.50% (Category 5). At higher disease incidences, the average affected area per leaf was estimated from standard area diagrams, and Categories 6 through 10 represented 3, 6, 12, 25, and 50% severity, respectively (12, 21). An average severity value was calculated and compared to Action Zones for Cercospora leaf spot (25) (Fig. 6). As long as the average percent disease severity remained within the safety zone for a given calendar date, fungicide applications were not economical. As soon as the average percent severity value entered the caution zone, an initial fungicide application was made.

Cercospora Advisory. The Advisory (22) described the potential for infection by C. beticola that existed the previous 48 h as a single whole number between 0 and 14 based on a Daily Infection Value (DIV). The DIVs ranged from zero to seven and were calculated from the number of hours per day (midnight to midnight) with relative humidity >90% and the average temperature during those hours (Table 1). Shane and Teng (22) developed the DIV table from greenhouse trials (sugar beet plants inoculated with C. beticola at various temperatures and humidities, as measured by a hygrothermograph) in combination with published data (29). In the field, infections may take longer than 1 day, so the Cercospora Advisory consisted of adding the DIVs for the two preceding 24-hr periods. If the sum of two adjacent days was less than six, the likelihood of infection was low; a sum of six was marginal; and sums of seven to 14 indicated conditions favorable for infection.

After the first fungicide application, subsequent applications differed with location (12). In the northern RRV, second and subsequent fungicide applications were based on average percent severity (determined by field monitoring), Cercospora Action Zones (Fig. 6), and the Cercospora Advisory. In the southern RRV and especially in southern Minnesota, where weather conditions tend to be favorable for leaf spot development, the second fungicide application was made at the end of first spray interval based on the product label. Then, field monitoring was resumed to determine percent disease severity and these values were used in conjunction with the Cercospora Action Zones and Cercospora Advisory to determine further fungicide applications.

Disease monitoring. Early in implementation of the model, producers, crop scouts, and agricultural staff employed by the cooperatives found that assessment of percent disease severity was time-consuming and difficult. Although the percent disease severity value was noted as accurate, assessment of a typical 65 ha field took from 1.5 to 2 h per inspection. This was too time-consuming for producers because when disease assessment was needed (July through mid-September), they were harvesting grain and managing other crops. Crop consultants lacked the time to assess each sugar beet field as proposed in the model because they also monitored other crops for various pests each week. Agricultural staff employed by the cooperatives did not have time to assess fields for their producers, since they each were responsible for an average of 6,800 ha of sugar beet. Thus, assessment of disease severity and use of the Cercospora Action Zones (Fig. 6) were impractical and discontinued.

Instead of assessing fields for percent disease severity, fields now are inspected for the onset of Cercospora leaf spot starting in mid-June in the southern Minnesota cooperative and in late June to early July in the Minn-Dak and American Crystal cooperatives. These dates correspond to the onset of row closure - when the canopy provides a suitable microclimate and weather (temperature and moisture) favors development of disease. Many field scouts, consultants, and agriculturists engage in friendly competition for the first sighting of Cercospora leaf spot each season. Word of confirmed cases of leaf spot travels quickly by electronic means, newsletters (industry and the extension service), radio announcements, and in conversations at local coffee shops. Field monitoring for the occurrence and increases in Cercospora leaf spot occur weekly and usually ends by mid-September.

According to a 1996 survey of producers (566 respondents), 58% monitor their fields for initial Cercospora leaf spot; 23% hire a consultant or field scout; and 19% rely on agricultural field staff from their cooperative (7). When producers monitor their own fields for leaf spot, 74% walk into the field; 31% inspect the edges of fields adjacent to last year’s beet field; and 26% check along tree lines, near water, and other protected areas (7) (total is more than 100% because some respondents select more than one method). Agricultural staff employed by the sugar cooperatives select representative fields and inspect them once or twice a week as indicators of disease development in their area, but do not calculate the percent disease severity as designated in the original prediction model.

DIVs and the Cercospora Advisory. The DIVs used to calculate the Cercospora Advisory have proved most beneficial after a fungicide spray program is initiated. If producers rely solely on the DIVs and the Cercospora Advisory to decide the first fungicide application, they can be lulled into a false sense of security by a series of zero values because microclimates vary within and between fields and data from weather stations cannot accurately be extrapolated to fields throughout a factory district. Also, amounts of inoculum vary among fields. When conditions for Cercospora leaf spot are marginal, but inoculum is readily available, a low incidence of successful infections can result in development of considerable disease.

According to agriculturists (33 of 38) surveyed in November 1996, rationale for recommending the first fungicide application varied with the cooperative (Table 2). The most frequent response from agriculturists at the American Crystal and Minn-Dak cooperatives was when Cercospora leaf spot was detected in a field or in nearby fields (61 and 86%, respectively); the most popular determinant in the southern Minnesota cooperative (75%) was when the canopy closed the row (Table 2). Other important criteria included: weather conditions (recent, current, predicted), DIVs (often used together with current and predicted weather), susceptibility of cultivar, and field location or history (proximity to sheltered areas, crop development). Prioritization of these criteria, however, varied with the cooperative. Calendar date (application of fungicide at a set time each season) was identified as having limited value in the American Crystal and southern Minnesota cooperatives, but was not noted by agricultural staff at Minn-Dak.

Over the last 10 years, calculation of DIVs and employment of the Cercospora Advisory have been modified. The DIVs, when calculated at 90% RH, did not account for the incidence of Cercospora leaf spot observed in the field, particularly in southern Minnesota and the southern RRV. To reduce the risk of untreated disease, the southern Minnesota cooperative began calculating DIVs based on 87% relative humidity readings starting in 1988. This change was a conservative "educated guess" to more closely match field observations. The same change in calculation of the DIV was adopted by American Crystal in 1993 and by Minn-Dak in 1995.

The southern Minnesota and Minn-Dak cooperatives have encountered conditions extremely favorable for Cercospora leaf spot in the last few seasons. In southern Minnesota, the DIV is interpreted in a very conservative manner (and the Cercospora Advisory usually is not implemented). The agriculture staff notes DIVs over a 5-day period. If DIVs are one or two, there is a low potential for disease development. When DIVs increase to three or four for 1 day, conditions are favorable for Cercospora leaf spot. During these periods, sugar beet leaves often remain wet until noon (although relative humidity readings may be low); a fungicide application is advised if the interval of the previously applied fungicide is starting to lapse. In 1996, the Minn-Dak Farmers Cooperative also stopped using the Cercospora Advisory and started providing their producers with DIVs for the preceding 7 days. DIVs that fall between zero to four are interpreted as indicating low disease potential, five and six indicate medium potential, and seven indicates high potential. To use DIVs successfully, however, producers also are advised to monitor fields for leaf spot and to consult with agricultural staff.

Criteria used as a basis to decide when to apply the second (and subsequent) fungicide application varied with the cooperative, according to a survey of agriculturists in November, 1996. The most popular criterion was the length of interval recommended on the fungicide label by the manufacturer (Table 2). The DIV was ranked as the second most important criterion at American Crystal (56%), but was of less importance among agriculturists at the Minn-Dak (29%) and southern Minnesota (25%) cooperatives. Criteria listed as more important than DIVs by agriculturists at Minn-Dak were weather and cultivar susceptibility to C. beticola and at the southern Minnesota cooperative were weather and incidence of disease. Among the least important factors were density of canopy, time before harvest, and recommendations from chemical company representatives.

Implementation and evaluation of the Cercospora leaf spot prediction model commenced as it was being developed and involved cooperation of the sugar beet industry, producers, and university personnel. Two key ingredients promoted adoption of the model: 1) education programs for the potential advocates (agricultural staff of the cooperatives) and producers and 2) establishment of weather monitoring networks to provide data on relative humidity (RH) and temperature on a 24-h basis for calculation of the DIV (to determine the Cercospora Advisory).

Education programs. Education efforts initiated in the 1980s continue today. Extension and research faculty from the University of Minnesota and North Dakota State University participate in annual seminars for producers and other personnel involved in sugar beet production. They present information on the biology, identification, and control of Cercospora leaf spot and explain the leaf spot prediction model. This information also is distributed to all sugar beet producers through research and extension publications. In addition, the sugar beet cooperatives hold training sessions for their agricultural staff and producers. They also distribute newsletters to producers to emphasize and explain field monitoring, DIVs, and the Cercospora Advisory.

Early response by the sugar beet industry and producers to the original prediction model was mixed because of uncertainty as to how it worked and if it worked. Many producers thought the model should not entail extra work but instead, provide a single numerical value indicating whether a fungicide application was needed. Problems with weather monitoring stations that resulted in inaccurate DIVs and Cercospora Advisories also raised questions about credibility of the model. Some agriculturists had many of the same concerns. The southern Minnesota and Minn-Dak cooperatives encouraged adoption of the prediction model. The American Crystal cooperative left this option to the discretion of each producer, but on June 17, 1987, it departed from the traditional calendar preventative spray program and announced a policy "to implement the Cercospora prediction model as a guide to starting the first fungicide spray as well as when succeeding sprays are needed." Acceptance of this policy had improved by then, especially in the northern RRV where Cercospora leaf spot had been a minor problem in previous years and validity of the calendar spray program was questioned.

Weather monitoring networks. Because the DIVs and Cercospora Advisory depend on the availability of accurate weather data, particularly RH and temperature, the cooperatives established weather stations in commercial sugar beet fields. The model was originally designed and tested with hygrothermographs installed in standard meteorological enclosures and placed just above the ground between rows (24). In 1983, these units were installed at pilot sites in the southern Minnesota, Minn-Dak, and American Crystal cooperatives. Several problems developed that compromised accuracy of the data. Periodic maintenance and quality control of the units was difficult because soil, water, rodents, etc., interfered with operation of the instruments. Some weather stations did not function for long periods of time, resulting in major gaps of data. Some units consistently peaked at night at apparent RH values as low as 85% or as high as 110%. This meant that the DIV was underestimated if the RH reading was too low or overestimated if the RH reading was too high. Also, considerable time was required to travel to and from the weather stations and to calculate DIVs.

In 1985, the cooperatives installed automated weather stations (Fig. 7A) (Models CR-21, CR-21X, and later, CR-10; Campbell Scientific Co., Logan, UT). By 1988, seven stations were located in commercial fields throughout the RRV and three were in southern Minnesota. The weather stations were moved to different fields each season. When leaves covered the row, humidity and temperature probes were set 5 cm below the top of the leaf canopy (Fig. 7B and C). Data were recorded every minute for 24 h (midnight to midnight), downloaded daily to each cooperative, and used to calculate the DIV. The agricultural staff at each factory prepared a daily Cercospora Advisory.

Automatic weather stations have greatly improved accuracy of the Cercospora Advisory, but problems still occur when sensors for RH and temperature are not placed in the canopy or if they are dirty. Currently, agricultural staff at the Minn-Dak and southern Minnesota cooperatives maintain two and four automated weather stations, respectively. Since 1995, The American Crystal cooperative has maintained six factory weather stations as part of the North Dakota Agricultural Weather Network (NDAWN) through North Dakota State University. NDAWN weather stations are solar powered and data are downloaded daily by phone. The top of the shield for the RH sensor is placed 40 cm above the soil and the sensing element is 30.5 cm above the soil surface. Every spring, the sensors are calibrated with a laboratory standard dew point generator at 25, 50, and 90% RH; accuracy of the sensor is +2% when RH is below 90% and +3% when RH is above 90% (J.W. Enz, North Dakota State University, personal communication). Without a full leaf canopy, however, RH in the canopy is significantly underestimated (9).

All weather stations currently maintained by the three cooperatives are located at permanent sites in commercial fields that represent the macroclimate of the area as much as possible. Cooperatives establish a sugar beet crop near or around the weather stations so sensors can be placed in the plant canopy. This requires good husbandry of the crop. Most sites have been well maintained but weather data at other sites are not reliable because of weeds and low populations of sugar beet plants. Also, planting continuous beets at permanent sites is not always possible; therefore, the Minn-Dak cooperative has started to grow rhubarb at the weather station sites, because rhubarb simulates a sugar beet canopy and is easier to maintain.

Initially, DIVs and the Cercospora Advisory were accessible to producers and crop consultants through dedicated telephone hot lines and by contact with sugar beet industry agricultural staff. Later, DIVs and the Cercospora Advisory were expanded to a special page on the Data Transmission Network (DTN, Omaha, NE) by the cooperatives (Minn-Dak in 1987; American Crystal, 1992; and southern Minnesota, 1993). A survey of sugar beet producers in October, 1996, revealed that 71% (of 566 respondents) accessed DIVs and the Cercospora Advisory by the DTN while 17% contacted agricultural staff, and 12% used the telephone hot line (7).

Fungicide Application Problems

The timing of the initial application, as well as application intervals, is critical to control Cercospora leaf spot. If disease is not detected early at onset, then the first fungicide application may be too late and a large population of conidia develops to infect new, unprotected foliage before the next fungicide application. The greater the disease pressure, as indicated by the DIV and severity of leaf spot, the shorter the application interval required. Inclement weather and the unavailability of commercial aerial applicators when needed can delay timely application of fungicides. When wet weather persists, it is impossible to apply fungicides by ground and aerial applicators often have a backlog of orders to apply fungicides to beets. This situation can delay the first application, or extend intervals between applications, either of which results in reduced disease control.

Uniform coverage of foliage with fungicide, either by air (Fig. 8A) or ground application (Fig. 8B), is vital for control of leaf spot. Careless application of fungicide, or defective equipment, can result in strips and patches of foliage with severe Cercospora leaf spot (Fig. 9). Some aerial applicators rely on the Global Positioning System to assure thorough application of pesticides over a field. Coverage under power lines, along shelter belts, and near trees, however, is difficult for aerial applicators. Operators must pull up their aircraft over such obstacles and thus do not provide adequate coverage near these obstacles. Many aerial applicators routinely side-dress along power lines and shelter belts to assure adequate coverage. In 1997, 62% of fungicide treatments were applied by airplane and 38% by ground equipment (6).

Effectiveness of fungicide application is reduced by improper calibration of spray equipment, improper spacing of nozzles, and worn and plugged nozzles. The type of application equipment, operating pressure, and amount of water carrier also affects efficacy of fungicides (4, 11). Coverage of foliage with fungicides applied by ground equipment is most effective when the pressure is more than 6.9 x 10⁵ Pa, but 1.4 x 10⁶ Pa or greater does not enhance coverage (4). The best coverage for ground application ranges from 187 to 374 L/ha; amounts of less than 187 L/ha do not cover leaves adequately and 561 L/ha results in run off (4).

Inadequate control of Cercospora leaf spot usually occurs because of problems related to fungicide application, according to a survey of agriculturists in November, 1996 (Table 3). Agriculturists also identified other fungicide-related problems including selection of tolerance to the tin fungicides and early cessation of fungicide programs. Inadequate control of Cercospora leaf spot also was attributed to planting the most susceptible cultivars available, prolonged weather conditions favorable for disease, delay of the first fungicide application until disease severity was too high to regain control of the disease, and close rotation of sugar beet crops.

Economic benefits gained from the Cercospora leaf spot model are difficult to determine directly. In annual surveys of 14-33 sugar beet fields in the southern Minnesota and Minn-Dak cooperatives, the average number of fungicide applications was 3.7 in 1982, 4.0 in 1983, 3.7 in 1984 and then dropped to 2.1 when the model was implemented in 1985 (24). When fungicide use from 1986 through 1988 was evaluated across the fields of the American Crystal cooperative, the number of fungicide applications was reduced by 1.5, representing a $4.2 million annual savings to producers (10). These years were unseasonably dry, but many producers would have continued to apply fungicides on a calendar spray program without the prediction model. The prediction model was designed for non-irrigated beets in Minnesota and North Dakota, but when evaluated on irrigated sugar beets in the panhandle of Nebraska, it resulted in reduction of fungicide applications (13).

A variable pattern in number of fungicide applications per season has prevailed across the sugar beet-growing regions of Minnesota and North Dakota (Fig. 10) since these data were first collected in 1983 (A.G. Dexter, North Dakota State University, personal communication). Every year, the lowest fungicide use generally has occurred in the northern RRV compared to intermediate use in the southern RRV and highest use in southern Minnesota. These trends reflect increasingly favorable temperature and moisture conditions for development of Cercospora leaf spot from the northern to the southern beet-growing areas (Table 4).

Since 1991, weather conditions compared to previous seasons have been more favorable for development of Cercospora leaf spot (especially in the southern RRV and in southern Minnesota), and the number of fungicide applications has increased (Fig. 10). In these seasons, adoption of the model has been especially important because it takes only a modest return in yield to offset the costs of fungicide and application. Average costs per hectare for each fungicide application are about $13.30 for TPTH and $14.80 for an EBDC fungicide, with an additional expense to apply the chemical by custom airplane ($9.90), custom ground equipment ($8.65), or with producers ground equipment ($3.70). When use of the model extends the length of a spray interval and results in one less fungicide application on 50% of the hectares, savings in input costs (fungicide and application) are about $3.2 M annually.

Fewer fungicide applications are made when cultivars that are least susceptible to Cercospora leaf spot are planted, but they usually yield lower amounts of recoverable sucrose compared to more susceptible cultivars. Consequently, cultivars with moderate susceptibility to Cercospora leaf spot are preferred, because even with additional fungicide applications, they are more profitable than the less susceptible cultivars.

The Cercospora leaf spot model has been integrated with other control options for more than 10 years. When the model was developed, it was evaluated in select fields with moderate levels of disease but when implemented on a wide scale, several uncertainty factors emerged. For instance, the model was untested under a wide range of concentrations of inoculum, under prolonged environmental conditions favorable for disease, and when DIVs varied from 0 to 6 on adjacent days. When seasons very favorable for development of Cercospora leaf spot were encountered, and adherence to the model resulted in inadequate disease control, the industry made several modifications in the model: adjustment of RH values from 90 to 87% to calculate DIVs; employment of DIVs and the Cercospora Advisory to determine the second and subsequent fungicide applications but not the first; and use of DIVs rather than the Cercospora Advisory in geographic regions where inoculum concentrations are high and weather conditions are extremely favorable for disease. These changes have improved reliability of forecasting Cercospora leaf spot and reduced risk of disease, but should be confirmed or fine-tuned by scientific investigation. Also, reliable environmental data are needed as well as information on the geographic area reasonably covered by a weather station. Perhaps weather data currently collected from distant specific sites is robust and useful in many fields, but may be inadequate when disease is sporadic or where disease pressure is low but cumulative.

Two factors will allow for broader adoption of the Cercospora model: a simpler and easier means to monitor and assess sugar beet fields for disease severity and a reliable measurement of RH and temperature. These factors define the limits of the model, and thus affect disease control and economic benefits. The possibility of site specific weather data appears promising but current resolution (1 km²) by satellite is inadequate to predict where hot spots of disease might develop. Availability of site specific weather data on a field scale, directly or by simulation, would open exciting new possibilities for Cercospora leaf spot management by the sugar beet industry. Regardless of how weather data are collected, each producer needs to know what happens in his/her fields - whether it be an early morning shower or a dew that remains until noon - to make an intelligent decision on leaf spot control.

Despite some limitations and modifications in the Cercospora leaf spot model, it has increased grower awareness of environmental conditions that favor disease. This knowledge has signaled a logical basis for departure from fungicide applications based on calendar dates, and resulted in judicious application of fungicides, improved leaf spot control, and frequently, reduced production costs.

We thank the agriculturists who responded to our survey and colleagues who critically reviewed the manuscript and/or provided photographs.

Literature Cited

1. Bangsund, D.A., and Leistritz, F.L. 1993. Economic contribution of the sugarbeet industry to the economy of North Dakota and Minnesota. North Dakota State Univ., Agric. Econ. Rept. No. 305-S. 8 pp.

2. Bugbee, W.M., 1982. Sugar beet disease research - 1981. 1981 Sugarbeet Res. Ext. Rept. 12:155.

3. Bugbee, W.M. 1995. Cercospora beticola tolerant to triphenyltin hydroxide. J. Sugar Beet Res. 32:167-174.

4. Cattanach, A., Dahl, K., and Smith, L. 1983. Fluorescent dye and photography comparisons of selected fungicide application equipment. 1982 Sugarbeet Res. Ext. Rept. 13:179-182.

5. Cattanach, A., Dexter, A., Bissonnette, H., Bugbee, W., and Lamey, A. 1982. Cercospora leaf spot management meeting and resultant recommendations. 1981 Sugarbeet Res. Ext. Rept. 12:165-169.

6. Dexter, A.G., Luecke, J.L., and Cattanach, A.W. 1998. Survey of fungicide use in sugarbeet in eastern North Dakota and Minnesota - 1997. 1997 Sugarbeet Res. Ext. Rept. 28:263-267.

7. Dexter, A.G., Luecke, J.L., Windels, C.E., Lamey, H.A., and Cattanach, A.W. 1997. Survey of fungicide use in sugarbeet in eastern North Dakota and Minnesota - 1996. 1996 Sugarbeet Res. Ext. Rept. 27:219-224.

8. Dexter, A.G., Reynolds, D.A., and Cattanach, A.W. 1984. Survey of fungicide use in sugarbeets - 1983. 1983 Sugarbeet Res. Ext. Rept. 14:179-180.

9. Enz, J.W., Brenk, P.C., and Carcoana, R. 1996. Humidity and temperature measurements in the sugarbeet canopy. 1995 Sugarbeet Res. Ext. Rept. 26:193-196.

10. Hilde, D.J., Holle, M., and Ellingson, R. 1989. The commercial application of Cercospora advisory system in the Red River Valley. (Abstr.) J. Sugar Beet Res. 26:A11.

11. Hofman, and Panigrahi, S. 1997. Spray coverage in a sugarbeet plant canopy. 1996 Sugarbeet Res. Ext. Rept. 27:168-171.

12. Jones, R.K., and Windels, C.E. 1991. A management model for Cercospora leaf spot of sugarbeets. Minnesota Ext. Serv., Univ. Minnesota. AG-FO-5643-E. 8 pp.

13. Kerr, E.D., and Weiss, A. 1990. Fungicide efficacy and yield responses to fungicide treatments based on predictions of Cercospora leaf spot of sugar beet. J. Sugar Beet Res. 27:58-71.

14. Kleinwanzleber Saatzucht Ag. Einbeck. 1970. Cercospora Tafel. Kleinwanzleber Saatzucht Ag. Einbeck Rabbethge and Giesecke. 14 pp.

15. Lamey, H.A., Cattanach, A.W., Bugbee, W.M., and Windels, C.E. 1996. Cercopsora leafspot of sugarbeet. North Dakota State Univ. Ext. Circ. PP- 764. 4 pp.

16. McKay, M.B., and Pool, V.W. 1918. Field studies of Cercospora beticola. Phytopathology 8:119-136.

17. Minnesota Agricultural Statistics Service. 1996 (January). Annual Crop Summary. St. Paul, MN.

18. Percich, J.A., Hotchkiss, M.W., and Nickelson, L.J. 1986. Survey and screening of benomyl-resistant strains of Cercospora beticola in Minnesota and North Dakota. J. Sugar Beet Res. 23:148-153.

19. Pool, V.W., and McKay, M.B. 1916. Climatic conditions as related to Cercospora beticola. J. Agric. Res. 6:21-60.

20. Ruppel, E.G. 1986. Cercospora leaf spot. Pages 8-9 in: Compendium of Beet Diseases and Insects. E.D. Whitney and J.E. Duffus, eds. American Phytopathological Society, St. Paul, MN.

21. Shane, W.W., and Teng, P.S. 1983. Epidemiology of Cercospora leafspot. 1982 Sugarbeet Res. Ext. Rept. 13:201-213.

22. Shane, W.W., and Teng, P.S. 1984. Cercospora beticola infection prediction model. 1983 Sugarbeet Res. Ext. Rept. 14:174-179.

23. Shane, W.W., and Teng, P.S. 1985. Evaluation and implementation of the Cercospora leafspot prediction model. 1984 Sugarbeet Res. Ext. Rept. 15:129-138.

24. Shane, W.W., Teng, P.S., Lamey, A., and Cattanach, A. 1985. Management of Cercospora leaf spot of sugarbeets: Decision aids. North Dakota Farm Res. 43 (6):3-5.

25. Shane, W.W., Teng, P.S., Lamey, H.A.,and Holen, C. 1986. Field evaluation of the Cercospora leaf spot control scheme. 1985 Sugarbeet Res. Ext. Rept. 16:144-156.

26. Smith, G.A., Campbell, L.G., and Lamey, H.A. 1997. A survey for the prevalence and distribution of Cercospora beticola tolerant to triphenyltin hydroxide and resistant to thiophanate methyl in 1995-1996. 1996 Sugarbeet Res. Ext. Rept. 27:261-262.

27. Steen, R.A. 1981. Results of American Crystal’s coded tests. 1980 Sugarbeet Res. Ext. Rept. 11:161-185.

28. United States Beet Sugar Association. 1997. Directory of American Beet Sugar Companies. Washington, D.C.

29. Wallin, J.R., and Loonan, D.V. 1971. Effect of leaf wetness duration and air temperature on Cercospora beticola infection of sugarbeet. Phytopathology 61:546-549.

Dr. Carol E. Windels is an Associate Professor at the University of Minnesota, Northwest Experiment Station, Crookston, and the Department of Plant Pathology, St. Paul. She has a B.A. in Biology from St. Cloud State University, St. Cloud, MN and M.S. and Ph.D. degrees in Plant Pathology from the University of Minnesota. Her research program focuses on the etiology and integrated management of sugar beet diseases, with an emphasis on soilborne fungi.

Dr. H. Arthur Lamey is an Extension Plant Pathologist at North Dakota State University, Fargo. He has a B.A. in Botany from Ohio Wesleyan University, Delaware, Ohio, and a Ph.D. in Plant Pathology from the University of Wisconsin, Madison. His program includes development of disease management strategies for dry beans, canola, lentil, potato, soybean, sugar beet, and sunflower and grower education.

Mr. Dave Hilde has B.S. and M.S. degrees in Soil Science and Agronomy from the University of Minnesota. In June, 1997 he retired from American Crystal Sugar Company after more than 36 years as General Agronomist. He has worked on increasing sugar beet quality through improved crop management practices, including implementation of the Cercospora prediction model. Mr. Hilde also was editor of Ag Notes, a producer newsletter that discusses results of research from universities and American Crystal.

Dr. Jim Widner has a B.S. degree in Agronomy from New Mexico State University, Las Cruces and a Ph.D. in Crop Science - Genetics with a minor in Plant Pathology from North Dakota State University, Fargo. He was a plant breeder (1968-1983) and manager of agricultural research (1972-1975) for The Great Western Sugar Company. In 1883 he joined the Southern Minnesota Beet Sugar Cooperative as Agricultural Manager and in 1987 was promoted to Vice President of Agriculture.

Mr. Tom Knudsen has a B.S. in Horticulture from North Dakota State University, Fargo. He started working for Minn-Dak Farmers Cooperative in 1997 as an agriculturist and in 1986 was promoted to Vice President of Agriculture.

Table 1. Daily Infection Values (DIVs) for Cercospora leaf spot on sugar beet calculated from the number of hours per day (24 h) with relative humidity (RH) > 90% and the average temperature during those hours (22)

Table 2. Factors identified by agriculturists employed by three sugar cooperatives as the basis for recommending fungicide applications to control Cercospora leaf spot

Agriculturists identifying each factor^a(%)

Table 3. Factors identified by agriculturists employed by three sugar cooperatives as responsible for inadequate control of Cercospora leaf spot of sugar beet

% Agriculturists identifying each factor

Table 4. Average 30-year (1967-1996) precipitation and maximum and minimum temperatures during summer months at three sugar beet factory locations in the Red River Valley (RRV) and southern Minnesota

Fig. 1. Location of sugar beet cooperatives and factory districts in Minnesota (MN) and North Dakota (ND).

Fig. 2. Severe leaf spot in a sugar beet field where Cercospora beticola is resistant to the fungicide benomyl (Benlate).

Fig. 3. Cercospora beticola on sugar beet: A) individual leaf spots, B) stromata, and C) leaf spots coalescing and killing large areas of leaf tissue.

Fig. 4. Screening germplasm for resistance to Cercospora leaf spot at the Betaseed Nursery in Shakopee, Minnesota; a resistant variety (right) and susceptible variety (left).

Fig. 5. Assessment form for calculating average severity of Cercospora leaf spot based on examination of 100 sugar beet plants. Damage categories (0-10) correspond to % disease severity: 0 = no leaf spot, 1-5 spots per leaf = 0.10% (Category 1), 6-12 spots = 0.35% (Category 2), 13-25 spots = 0.75% (Category 3), 26-50 spots = 1.50% (Category 4), and 51-75 spots = 2.50% (Category 5); at higher disease incidences, the average affected area per leaf is estimated from standard area diagrams and Categories 6 through 10 represent 3, 6, 12, 25, and 50% severity, respectively (12, 21).

Fig. 6. Action Zones (A = Danger, B = Caution, C = Safety) for control of Cercospora leaf spot of sugar beet by fungicide application are based on disease severity and time of season (25).

Fig. 7. A) A CR-10 automated weather station; B) stakes show position of temperature and humidity probes (noted by an arrow) placed incorrectly above the canopy and C) placed properly within the canopy.

Fig. 8. Fungicides are applied by A) airplane and B) ground spray equipment to control Cercospora leaf spot of sugar beet.

Fig. 9. Severe Cercospora leaf spot on a few rows of sugar beet plants that were missed when fungicide was applied by ground equipment.

Fig. 10. Mean number of fungicide applications per season in Renville county and surrounding counties in the Southern Minnesota Beet Sugar Cooperative (So. MN); Richland County in the Minn-Dak Farmers Cooperative, southern portion of the Red River Valley (So. RRV); and Pembina county in American Crystal Sugar Co., northern portion of the Red River Valley (No. RRV). Figure compiled from annual surveys conducted by A.G. Dexter (North Dakota State University, personal communication).