December 1988

Sugarbeet Powdery Mildew

H. Arthur Lamey
Plant Pathologist

The first observation of powdery mildew of sugarbeet in Minnesota and North Dakota was in the Renville area in southern Minnesota on Sept. 4, 1975.Within two weeks, it was found as far north as Grand Forks. The late appearance of powdery mildew as well as timely application of sulfur fungicide to affected fields prevented any serious yield losses in1975. Powdery mildew has occurred sporadically since 1975 in southern Minnesota and the Red River Valley. Often it has appeared too late in the season to cause losses, but not always.

In 1985 powdery mildew was first observed on Aug. 22 in the Hillsboro area. On that date mildew,which appears first on the lower leaves, was already on the upper leaves in some fields. Weather favored powdery mildew, and the disease spread rapidly to other fields and areas in 1985. By early September,powdery mildew was widespread in the southern half of the Red River Valley. Numerous fields were sprayed with sulfur fungicide.

Some powdery mildew occurred in 1986, primarily in the Hillsboro area. In the drought year of 1988, powdery mildew was observed in several areas by late July and became widespread and serious in late August and early September.

Powdery mildew has been studied extensively in other areas, but not in the Minnesota-North Dakota region due to its sporadic nature. The disease has occurred often enough and has been serious enough that information is needed on the biology and control of this disease. This circular will address those issues. In the absence of local data, data on control will be used from California studies and data on the biology of the fungus will be drawn from extensive studies in Germany.

Some local studies are in progress, but reliable local data may not be available for several years.

Symptoms_______________________________________

Powdery mildew f irst shows up on the lower leaves. The first symptoms are a wispy growth of white to light gray threadlike filaments, often radiating from a central point (Fig. 1). Early symptoms of powdery mildew are not detected easily. Experience of Red River Valley sugar cooperative field staff indicate that early symptoms are most readily detected under full sunlight, with the sun to your back.Once the early symptoms of mildew show up, the disease may progress very rapidly in favorable weather. A powdery white or gray-white growth may cover a leaf within a week, and the mildew may begin to show up on the middle leaves. Mildew may even show up on the upper leaves that have not completely expanded when disease is severe. A characteristic odor similar to that of a musty basement may be noted in fields with severe mildew.

Sugarbeet Powdery Mildew

Powdery mildew is less severe where nitrogen fertility is high. When inspecting a field for powdery mildew, look in the areas of the field that are beginning to turn a light green or yellow green due to depletion of available nitrogen. Powdery mildew is not likely to be found in the rest of the field if it cannot be found in the yellowing areas.

Severely mildewed leaves may begin to turn yellow within a month of initial symptoms. Observations made near harvest time in North Dakota indicate that mildewed leaves are killed by light freezes that do not kill healthy leaves.

Losses___________________________________________

Yield losses from powdery mildew result from losses in tonnage, in percent sucrose and increased impurities. Total sucrose per acre was reduced 24.5percent in trials at Davis, California in 1974; 24 percent at Davis in 1975; 22 percent in 1976; and 27.8percent in the Salinas Valley of California in 1974.Losses in percent sucrose varied from 0.7 to 1.5 percent in these trials. Root impurities also were higher in the 1974 Salinas Valley trials.

Studies in Germany indicate that losses from powdery mildew are most severe in extended periods of drought, due to the premature death of many severely mildewed leaves.

No data are available on losses in southern Minnesota or the Red River Valley. However, yields were not up to expectations in some fields that were severely mildewed in 1985. The worst fields did not receive a sulfur spray until mildew was already severe in the entire field.

Biology of the Fungus_______________________________

Powdery mildew is caused by the fungus Erysiphe betae. Most of the biology of the fungus and factors affecting infection were defined by Drandarevski's extensive studies in Germany, published in 1969.

The disease is favored by long periods of drought,warm days, cool nights, and a wide fluctuation in day-night temperatures. The most favorable temperatures for production of conidia(the fungus spores)are day/night temperatures of 81/54 degrees Fahrenheit, although 86/50 F is nearly as good and 90/45 F also favors the fungus.

The conidia are formed in the morning and usually are released about noon. Germination of conidia occurs in the afternoon and early evening, depending on temperature, and the formation of appresoria (infection cushions) occurs in the evening. Conidia germinate best in the light at 86 F and nearly as well at77 F. Appresoria form best at 59-68 F and nearly as well at 50 F and 77 F. Weather data from the Red River Valley in 1985 and 1988 and mildew outbreaks compare favorably with Drandarevski's published data from Germany. Comparing maximum and minimum temperatures for the Red River Valley, periods of major infection appeared to occur when Drandarevski's data indicated that temperatures were highly favorable to favorable for periods of several days to a week at a time.

Once infection occurs, the incubation period (period from infection to development of a spore producing lesion) can be as little as five days if temperatures are around 77 F.

At the present time, local data on overwintering studies are inconclusive: it is not known with certainty whether the powdery mildew fungus survives the winter in Minnesota and North Dakota or not. The severe outbreak at Hillsboro in 1985 occurred before mildew was severe in many Great Plains areas farther to the south; this suggests that the mildew fungus may have overwintered the previous winter. The fact that mildew was common in the same area the next year also suggests this, but provides no proof. Overwintering studies of W.M. Bugbee, done for several years at North Dakota State University, have provided conflicting data: overwintering appeared to have occurred one winter and not the next. If overwintering of the powdery mildew fungus proves to be sporadic, this might help explain the sporadic occurrence of mildew outbreaks.

Control____________________________________________

Studies from 1974 to 1976 at several locations in California have shown that sulfur is a relatively inexpensive and effective control for powdery mildew. Although much of this early work was done with sulfur dust, some studies also were done with wettable powders or flowable formulations. A rate of 8 to 10 pounds active ingredient per acre (ai/A) gave good control in trials at several locations. Rates of 2 to 4 pounds ai/A generally gave moderate control. A single application of 10 pounds ai/A gave control for up to four weeks, and a single application of 2 pounds ai/A gave partial control for four weeks.

A single application of 10 pounds ai/A applied at first sign of mildew in the Salinas Valley gave half of the yield increase of four applications begun when mildew first appeared. A single application of 7.9 pounds ai/A wettable sulfur applied at first sign of mildew in 1974 at Davis gave 92 percent of the sucrose per acre of five biweekly applications applied beginning at first sign of mildew. The Davis studies stress the importance of applying sulfur at the first sign of mildew. If five applications beginning at the first sign of mildew is taken as the standard, then a delay of two weeks before biweekly applications were started resulted in a 16.9 percent reduction in sucrose per acre, and a four-week delay resulted in a 23.3 percent loss, nearly the same as the untreated (Table 1). Another study done at Hemet, southeast of Los Angeles, in 1974 showed that yield was not increased if the application of sulfur was delayed until 50 percent of the plants showed some signs of disease. Thus, all data indicate that application of sulfur at the first sign of mildew is essential and delay may result in little yield response.

Bayleton also is registered for control of powdery mildew, but at rates which make this a much more expensive treatment than sulfur. Data from California demonstrate that Bayleton gives very effective control.

Other fungicides commonly used on sugarbeets are not registered for control of powdery mildew and are only marginally effective to ineffective. Benlate, no longer recommended for use in our area for control of Cercospora, has given variable results in California. Control was poor in some trials and only fair in others. Good control was obtained only when application of Benlate was initiated before the disease was observed. Triphenyltin hydroxide at 3.66 ounces ai/A (equals 7.32 fluid ounces per acre of a 4pounds per gallon flowable) applied at first sign of mildew and repeated every three weeks for a total of three applications gave a slight reduction in mildew and a slight increase in yield, but was less effective than sulfur. A copper-sulfur fungicide also gave moderate control. A.W. Cattanach obtained similar results at Fargo in 1988: triphenyltin hydroxide at 3.5 ounces ai/A provided fair control. Other rates of triphenyltin hydroxide and copper fungicides provided fair to poor control. Mancozeb provided almost no control.

The data from California indicate that application of sulfur should be made as soon as mildew is observed if losses are to be prevented. However, the California data leave two unanswered questions for our area.

  1. How late in the fall should sulfur be applied if disease appears late (late August or September)? The data indicate that mildew can increase rapidly and cause losses in as little as a month. Experience in our area has shown that weather can favor mildew through the middle of September, with occasional favorable periods later in the season. Moreover, severely mildewed leaves can be severely injured by light freezes. Based on these facts, it seems that control measures should be initiated if mildew occurs before September. Moreover, it may be prudent to use sulfur if the infection starts as late as Sept. 15.
  2. How much sulfur is needed for control? All tests agree that 8 to 10 pounds ai/A gives excellent control, and that this rate will continue to give control for up to four weeks. Lower rates give partial control for up to four weeks but may not be adequate when disease pressure is severe. Possibly the rate might be varied depending on the date of mildew appearance, with higher rates being applied in August and lower rates in September. Rate and timing studies are needed in the Minnesota-North Dakota area to clarify these questions.

Table 1

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