I. Seedling Diseases
When done properly, seed treatment protects the seed from fungi associated with the seed. Spores of these fungi adhere to the seed. When the seed is planted, these fungi begin to grow, invade the seed or seedling, and cause a seedling blight. Other fungi live in the soil and may cause seedling problems. Soil-borne Pythium, Aphanomyces and Rhizoctonia fungi can cause serious stand loss when the soil is moist or wet.
Tachigaren seed pelleting is highly effective against Pythium at lower rates and Aphanomyces at higher rates. It persists for only 3-4 weeks and will provide protection only for the emerging seedling; it does not provide protection against mid-season infection. For season-long management of Aphanomyces, the best approach is to apply Tachigaren to varieties with partial resistance to Aphanomyces. Early planting and cultivation to help dry out the soil may also help reduce early season losses from Aphanomyces seedling disease.
Tachigaren is not effective against Rhizoctonia, so an Aphanomyces soil test is important to determine what management practices to use.
II. Leaf Spots
There are various leaf spot diseases of sugarbeets. Cercospora leaf spot is the most common and destructive disease in this area. The severity of Cercospora varies from year to year depending on weather conditions and varietal resistance. Cercospora can cause losses in susceptible varieties through reduced tonnage, reduced percent sucrose, increased impurities and poorer storage after harvest when the beets are in the pile. Even fairly low levels of leaf spot may cause these effects. Cercospora levels were very high in 1998. Bacterial leafspot may develop in wet weather; no fungicide is registered for its control.
Leafspot Management. Management of Cercospora requires an integrated approach which includes crop rotation, use of varieties that are less susceptible, disease scouting, timely application of fungicide, adherence to appropriate application intervals and more frequent applications when disease conditions are favorable. Avoid planting next to last year's beets. This is especially important if last year's fields had high levels of Cercospora. In high risk situations, select approved varieties that are less susceptible than the average. Begin checking for Cercospora in late June or early July, making sure to check near last year's fields or shelterbelts. The first fungicide application should occur when conditions first favor disease or at disease onset. If the first application is late, control will be difficult all season, even if shorter than normal application intervals are used once applications start. When conditions favor disease, or disease is already prevalent, fungicide applications must be more frequent than when disease pressure is low.
Resistance and Tolerance to Fungicides. The terms "resistance" and "tolerance" are often used interchangeably. However, in the following discussion they are used with specific different meanings. Resistance is used to indicate that the Cercospora fungus is unaffected by a level of fungicide that previously prevented growth in the laboratory. Tolerance is used to indicate that growth of the Cercospora fungus is reduced in the laboratory by a level of fungicide that previously prevented growth in the laboratory. Resistant strains of Cercospora are not controlled by field applications of a fungicide. If tolerant strains are present, a reduced level of control will occur.
The systemic fungicides benomyl (Benlate) and thiophanate methyl (Topsin M) have federal registration for Cercospora control, and are in the benzimidazole class of fungicides. They are not recommended for use as stand-alone fungicides in 1999 because the Cercospora fungus has developed resistance to this class of fungicide throughout all of North Dakota and Minnesota. These fungicides can be used in a tank mix with mancozeb, but only once in a season. If used, the benzimidazole-mancozeb tank mix should be used as the first or second spray at the beginning of the season and be followed by an application of TPTH (Super Tin or Agri Tin) or mancozeb no more than 10 days later in southern Minnesota, 10-12 days in the southern Red River Valley and 10-14 days later in the northern Red River Valley.
Benzimidazole (Benlate or Topsin M) resistant strains, grow normally in the laboratory in the presence of 5 ppm of benzimidazole fungicide. Sensitive strains do not grow at all in the presence of 5 ppm of benzimidazole fungicide. Benzimidazole resistant strains were common and widespread in 1998. Some strains of the Cercospora fungus have been found that were resistant to both the benzimidazole fungicides (Benlate and Topsin M) and tolerant to TPTH. Continuous use of the benzimidazole fungicides in research plots at Breckenridge, MN in 1996 and Crookston, MN in 1997 resulted in the development of resistance to the benzimidazole fungicides and tolerance toTPTH.
Experience has shown that resistant strains of Cercospora increase in number and this renders systemics fungicides ineffective where applied. The resistant strains may compete well with sensitive strains, so the population of benzimidazole-resistant strains may not diminish readily following use of an alternative fungicide.
Strains of Cercospora with tolerance to TPTH were confirmed for the first time in southern Minnesota and the southern Red River Valley in 1994. Tolerance was detected in fields where control was not as good as expected. Such tolerance is difficult to distinguish from inadequate application technique or a late start in application. Tolerance is best defined as an ability of the fungus to grow in the laboratory in the presence of TPTH at 0.2 ppm (part per million) or at 1 ppm. Sensitive strains do not grow at all when subjected to these levels of Super Tin, but tolerant strains grow at a reduced rate compared to growth in the absence of TPTH. Tin tolerance is incremental and progressive, and the percent of fields with tolerance to both 0.2 and 1 ppm in 1998 was very high in southern Minnesota and the southern Red River Valley and high in the northern Red River Valley, making Cercospora management more difficult.
Management of Tolerance. Tolerance to TPTH occurs in most fields in southern Minnesota and the southern Red River Valley, and is common farther north. Tolerance management should be practiced in all factory districts with the assumption that tolerance is or will be present in 1999. Since Super Tin remains important for Cercospora management, steps are necessary to manage tin tolerance. Timeliness of the first fungicide application is extremely important, especially in areas where leafspot starts early and is a major limiting factor in production. Several management strategies are suggested.
One management strategy is to apply mancozeb or maneb at full label rate for the first one or two applications at the beginning of the season, followed by use of TPTH, then mancozeb at full label rate again at the end of the season, if needed.
Another strategy is to alternate mancozeb and TPTH applications. Data from Europe indicate that tin tolerant strains do not compete well with sensitive strains. Thus, an application or two of mancozeb at the beginning of the season or mancozeb in alternation with TPTH might help suppress tolerant strains early in the season.
A third management strategy is to use a tank mix of benzimidazole fungicide (Benlate or Topsin M) + mancozeb once in the season. This tank mix should be the first application of the season due to the high prevalence of benzimidazole resistance. Benzimidazole fungicides should never be used alone.
A fourth management strategy is to use the full 5 oz rate of TPTH. The 5 oz rate permits use of only 3 applications of TPTH. The full rate may help to provide control in the presence of low level of tin tolerance. The best strategy would be to alternate the use of the 5 oz rate of TPTH with mancozeb. This would help to reduce selection pressure on the fungus; in contrast, repeated use of TPTH would increase selection pressure and the chances of increased tin tolerance.
A similar strategy that was successful in southern Minnesota was to alternate mancozeb and TPTH on a 7 day schedule.
If aerial application is made, make sure that areas around power lines and trees are side-dressed by the aerial applicator or by use of ground equipment. Aerial applicators should use a minimum of 5 gal/A; 7-10 gal/A gives better coverage. Improperly sprayed areas become focal points for Cercospora spread.
Preharvest Intervals. The preharvest interval (PHI) or waiting period before harvest is 21 days for Super Tin and the benzimidazole fungicides Benlate and Topsin M, 14 days for mancozeb and maneb, and 0 days (no waiting period) for the copper fungicides. Fungicides may be needed well into September in many years; stopping application of fungicides before this time may result in late-season damage that can reduce tonnage, sucrose and quality. Do not allow PHI to be an excuse for missing an application late in the season. Cercospora continued to advance through September and into October in 1998, and has done so in other years as well. The PHI may necessitate finishing the season with mancozeb or even copper in order to maintain disease control when it is too late to apply Super Tin. It may be preferable to spray a field leaving the headlands and a strip in the middle unsprayed, allowing pre-pile harvest in these areas.
Variety Selection and Cercospora Management. There are differences in Cercospora susceptibility among approved varieties. Cercospora may be somewhat easier to manage on varieties with higher than average tolerance to Cercospora. Conversely, varieties that are more susceptible than the average may need an extra spray in years that are highly favorable for Cercospora. Use of more tolerant varieties can be an important part of an integrated disease management plan.
Fungicide Rates. The benzimidazole fungicides Benlate and Topsin M should be applied at 3/8 lb/A in a tank mix with mancozeb at 2 lb/A. The higher rate (1/2 lb) of Benlate or Topsin M should not be needed and will only increase costs. Mancozeb or maneb should be applied at 2 lb/A. Super Tin should be applied at 3.75 oz/A from the Crookston factory district and north. Super Tin can be applied at the 3.75 oz or the 5 oz rate in the Hillsboro factory district and south. The higher rate of Super Tin may increase effectiveness where Cercospora strains are tolerant to low levels of tin but sensitive to higher levels.
Application Intervals. Tank mixes of the benzimidazole fungicides (Benlate or Topsin M) with mancozeb should be followed by an application of mancozeb or Super Tin 10 days after the tank mix has been applied in southern Minnesota, 10-12 days in the southern Red River Valley and 10-14 days in the northern Red river Valley. A fungicide application is needed 7-10 days (7 days in southern Minnesota) after an application of mancozeb or maneb. A fungicide application is needed 10-14 days (7-10 days in southern Minnesota) after an application of 3.75 oz of Super Tin and 14 days (10-12 days in southern Minnesota) after an application of 5 oz of Super Tin. The shorter intervals stated should be used when weather favors disease, when Cercospora levels are already high or when heavy rainfall washes off product and reduces fungicide residual and activity.
Spray Coverage. Total coverage of sugarbeet plants with fungicide usually provides the best protection from diseases. A technique has been developed to measure the spray coverage on plant leaves with the use of fluorescent dye, a CCD camera, and a computer programed with the appropriate software to determine area of coverage. This method is used to compare the coverage efficiency of various types of spray equipment. Several applicators have been studied including conventional sprayers, air assist sprayers and spray planes. Air-assist sprayers have shown slightly improved application efficiency (1-2%) as compared to conventional sprayers at equal application rates. Most ground sprayers show improved coverage over aircraft of about 16-17% but the application rate for ground sprayers is usually much higher than for aircraft.
The concept of redistribution of spray deposit on the leaf surface indicates that moisture condensation (dew) on the leaf surface resuspends some of the deposit and causes the fungicide to spread across the leaf. Also, condensation occurring during future evenings dissolves or suspends minute amounts of fungicide, spreading it further. This concept is probably more helpful for spray planes as their application rate is usually less.
III. Powdery Mildew
Powdery mildew may occur first in the southern Minnesota beet acreage, or it may develop first in the Red River Valley. The spread may be spotty, depending on the time of infection and weather conditions. When mildew occurs in late July or early August, we might expect some crop loss. Late occurring mildew (September) would not be expected to cause a measurable crop loss. The sulfur fungicides give very good, quick control. Apply sulfur as soon as mildew appears, if it develops prior to mid-September.
There are differences in the labeled amounts of active ingredients per acre (ai/A) for the various sulfur products. The ai/A is noted in the table. Although North Dakota or Minnesota data are not available on how much sulfur is needed for powdery mildew control, data from California indicates that 8-10 lb ai/A gives excellent control for 4 weeks; 2-4 lb ai/A gives moderate or partial control for 4 weeks. If powdery mildew appears by mid- to late-August, 8-10 lb ai/A is advisable and a product should be selected with a label that allows these rates. If powdery mildew appears after mid- to late-August, but before mid-September, then lower rates may provide satisfactory control at reduced costs.
IV. Note
Other fungicides may be registered or currently recommended products be dropped from registration. Therefore, be sure to check the product label for registered use on sugarbeets. For rates and intervals of application, follow the directions on the label. More detailed information is available in past Sugarbeet Research and Extension Reports.
For leafspot control, use higher label rates and shorter label application intervals when disease is severe or weather favors rapid disease buildup on susceptible varieties. Use 5-10 gals. water by airplane. Best results with ground equipment are obtained by using high pressure (at least 150 psi) and high volume. Label rates vary considerably for recommended gallonage with ground equipment. Research trials indicate that 20-40 gal. is adequate when high pressure is used.