1) SUMMARY

Sugarbeet powdery mildew was unknown in the United States until the pathogen was reported in a single field in California in 1934.  The disease was not again reported in this country until it became epidemic in 1974.  In April of that year, the disease became widespread in the Imperial Valley of California.  By September, the malady had spread all the way to Sidney, Montana, and by the following year, the disease occurred in all sugarbeet production areas of the United States.

Powdery mildew is a fungal disease.  The causal fungus is called Erysiphe polygoni.  It is related to the fungi that cause powdery mildews on grain and other crops.  The fungus produces spores called conidia.  The conidia blow in the wind, and some will land on sugarbeet foliage.  The conidia then germinate and start growing.  The fungus grows vegetatively by producing strands called hyphae.  The hyphae first appear in small circular areas of the leaf, growing superficially on the surface.  As the hyphae grow, these areas will coalesce, and they eventually cover the surface of the leaf.  Usually, the upper surface is more affected than the lower surface.  The fungus derives its nutrition by adsorption through specialized hyphae called haustoria.  The hyphae will grow through the cell wall of the plant and form haustoria.  The haustoria invaginate the cell membrane; nutrients defuse through the membrane and are absorbed by the haustoria.

Soon after infection, the fungus will begin to produce conidia.  The conidia are formed on other specialized hyphae called conidiophores.  The conidia form on the ends of the conidiophores in short chains.  The conidia are each composed of a single cell, measure approximately 2/1000 inch long by 1/1000 inch wide, and are transparent.

When the hyphae and conidia form, the leaf will take on a white, powdery appearance.  As the infection progresses, the leaf tissue becomes chlorotic, then brown.  These symptoms usually form first on the older leaves.  Eventually, all leaves may become infected.

Like most powdery mildew fungi, the sugarbeet pathogen has a relatively narrow host range.  Other forms of Beta vulgaris, such as table beet and Swiss chard, are susceptible.  Six additional Beta spp. are susceptible, but only Beta macrocarpa might be important in the epidemiology of the disease in the United States.

The conidia of powdery mildew fungi are unique in their response to humidity.  Most fungal spores require 100 percent relative humidity or free water to germinate.  Powdery mildew conidia are capable of germination at any humidity.  This enables the pathogen to spread during the entire season, and not just during periods of rainy weather.  Disease spread does increase at higher humidity.

 The fungal structures are very sensitive to low temperatures and will not overwinter in the sugarbeet production areas of the Great Plains and Rocky Mountain areas.  The fungus probably overwinters in California on winter-grown sugarbeet crops and on weed species such as Beta macrocarpa.  The beet seed fields of Oregon could also be a source of inoculum and may provide inoculum for beets grown in Modoc and Siskiyou Counties.  It is thought that wind currents during the summer months move the conidia long distances.  These spores infect sugarbeets and produce secondary inoculum, and the disease spreads throughout the crop.  Some of these secondary conidia will also be blown long distances, and the cycle repeats.  In this way the disease progressively moves from the southwestern part of the country throughout all of the sugarbeet production areas of the United States.

The age of the sugarbeet crop is an important factor in susceptibility to disease.  The disease is rarely seen in the field until eight to 12 weeks after emergence.  The disease occurs first on the older leaves.  If allowed to go unchecked, the disease progresses and within a month will cover all the leaves in a field.

Yield loss due to powdery mildew will occur if the disease is allowed to go unchecked.  The earlier the disease occurs during the season, the greater the loss.  Loss will occur due to decreased root yield as well as decreased sucrose concentrations.  Gross sugar yields may be decreased by as much as 40 percent under severe infection.  In addition, powdery mildew will cause a reduction in purity.  Infected plants have higher concentrations of sodium and amino-nitrogen in the roots.  The decreased purity will reduce the amount of extractable sugar.

Little is known about variability of the pathogen.  Major gene resistance has not been deployed, so selection of races has not occurred.  In the early 1980's, strains resistant to benzimidazole fungicides quickly developed.  Some reports of triazole fungicide resistance have remained unsubstantiated.

 2) CURRENT PEST MANAGEMENT PRACTICES

Chemical Control:  A very effective control procedure is the application of sulfur.  This material is used throughout California to control powdery mildew on sugarbeets.  Not only are sulfur applications very effective for control, they are also inexpensive.  Sulfur is not a hazardous pesticide.  Sulfur dust applied at 20 to 40 pounds per acre will provide excellent control of the disease.  Wettable sulfur applied at three to ten pounds per acre in at least ten gallons of water per acre also will protect the crop from serious loss.  It is very important that the sulfur application be made with an adequate volume of water, or its effectiveness will be reduced.  Often, if the sulfur application is to be made by an aerial applicator, less water is applied.  If disease onset occurs early in the season, more than one application of sulfur may be required.  Usually the second sulfur application is made three to four weeks after the first.

Sulfur controls the disease by dramatically lowering the rate of disease spread.  The sulfur will decrease the production of secondary conidia and will protect non-infected leaf surfaces from becoming infected.  The non-infected tissues will remain healthy.

 Foliar sprays of triadimefon are also used for powdery mildew control.  Triadimefon is a triazole fungicide and is classed as a steroid demethylation inhibitor.  The fungicide provides systematic control of powdery mildew.  It is particularly useful under lower ambient air temperatures when sulfur is not effective. Triadimefon is on the U.S. EPA FQPA list as a unquantitative carcinogen.

Host Plant Resistance:  Most sugarbeet cultivars grown in California have some resistance to powdery mildew.  This resistance is quantitative or minor gene resistance and is not in itself adequate for control of the disease under the extreme infection pressure inherent in California.  Major gene or qualitative resistance does exist but has not been deployed.  In most powdery mildew host systems where this type of resistance has been used, the population of the pathogen quickly shifts to a virulent form that will overcome the resistance genes.

Cultural Control:  Cultural practices have not played a big part in powdery mildew control.  There is some speculation that overhead irrigation may increase severity of the disease.  This is due to the irrigation water washing the sulfur from the leaves of the plant.

Biological Control:  Currently, there are no biological control options being implemented for powdery mildew control in sugarbeets.

 3) REDUCED-RISK OPTIONS

A new generation of fungicides may become available that could increase the options for powdery mildew control.  The new class of fungicides called B-methoxyacrylates have been shown to be very effective against the powdery mildew fungi as well as other fungal pathogens.  A new experimental fungicide, CGA-279202, from Novartis has been reported by the company to be very effective for controlling powdery mildew on sugarbeets.

 4) CHALLENGES

If the onset of powdery mildew occurs a month or more before the crop is to be harvested, control is usually warranted.  Because of the rapid rate with which the disease increases, control measures must be initiated at the first observation of symptoms.  A delay will cause control measures to be less effective.

Before control measures are commenced, consideration must be given to the time of harvest.  If the field is to be harvested within three to four weeks after disease onset, control is probably not necessary.  If a field is being harvested on a daily quota, perhaps the disease in only a portion of the field should be controlled.  These are some of the variables that must be considered.

Several challenges are presented to threaten the current system of managing powdery mildew in sugarbeets.  Sulfur has been under review.  Existing regulations have limited its use near inhabited areas.  Restrictions exist on the use of some sulfur formulations by aerial applicator.  Triadimefon is under review by the EPA.

  5) INNOVATIVE FEATURES IN REDUCED-RISK PROGRAM

The new family of fungicides may prove to be very effective in control of powdery mildew on sugarbeets.  The first of these new products, azoxystrobin (Heritage®), was registered on February 7, 1997, and is a new fungicide for use on golf courses and commercial turf.  It is the first of a new class of pesticide compounds called B-methoxyacrylates which are derived from the naturally occurring strobilurins.  Strobilurins are organic compounds produced by some naturally occurring fungi.  They function by inhibiting the electron transport of other potentially competitive fungi.  It has low application rates and longer intervals between applications than most alternatives.  The broad control spectrum and new mode of action should make it a likely candidate for use in resistance management.  It is labeled for use in integrated pest management programs.  According to EPA risk assessment, this product has no acute risk levels of concern for birds, mammals, and bees.

 6) BARRIERS TO ADOPTION OF REDUCED-RISK METHODS

There are two likely barriers to adoption of new methods.  The most likely is regulatory.  Registration of new products for sugarbeets have been slow to materialize.  Manufacturers are reluctant to pursue registration of chemicals on minor crops such as sugarbeets because of the small amount of potential sales.  The second barrier is economic.  The cost of the new fungicide is unknown and may be substantially more than the current method.
 

REFERENCES

Hills, F. J., Chiarappa, L, and Geng, S. 1980. Powdery mildew of sugar beet: Disease and crop loss assessment. Phytopathology 70:680-682.

Hills, F. J., Hall. D. H., and Kontaxis, D. G. 1975 Effect of powdery mildew on sugarbeet production. Plant Dis. Rep 59:513 - 515

Hills, F. J., Hall. D. H., Skoyen, 1.0. And Leach, L. D. 1974. Powdery mildew, a new disease in California sugar beet fields. Calif Sugar Beet (1974): 54-55

Palus, A. 0., Harvey, 0. A., Nelson, J. And Meek, V.1975. Fungicides and timing for control of sugarbeet powdery mildew.
Plant Dis. Rep. 59:516-517.

Ruppel, E.G., Hills, F. J., and Mumford, D. L. 1975. Epidemiological observations on the sugarbeet powdery mildew epiphytotic in western U.S.A. in 1997. Plant Dis. Rep. 59:283-286.

Skoyen, I. 0., Lewellen, R. T., and McFarlane, J. S. 1975. Effect of powdery mildew on sugarbeet production in the Salinas Valley of California. Plant Dis. Rep. 59:506-510