SUGARBEET POWDERY MILDEW:
BIOLOGY, ECONOMIC IMPORTANCE
John J. Gallian
Powdery mildew is a perennial disease problem on sugarbeets in many production areas of the western U.S. Powdery mildew is an insidious disease because the loss is not as obvious as it may be with other diseases or conditions. A grower can have significant infection and loss, yet still harvest a marketable crop. There are many instances in which the crop is not being treated when it should. Yield loss from powdery mildew can be as high as 35% if not controlled.
EPIDEMIOLOGY
The disease is caused by the fungus Erysiphe polygoni (syn. = Erysiphe betae), which is an obligate parasite, requiring a living host to develop and reproduce. The first symptoms of the disease are small, disperse, white colonies first appearing on the older leaves. Often the initial colonies first appear on the underside of the leaves. As the disease progresses, younger leaves become infected, and the plant takes on a dusty white appearance. The disease can cover entire leaves, and severely infected leaves may become yellowed, then purplish-brown.
Specific moisture and
temperature conditions are important in the development of powdery mildew.
Relative humidity of 30-40% is optimum for conidia (spore) production,
the asexual reproductive stage of the fungus.
High relative humidity of 60-100% inhibits conidia production and
viability. Conidia can germinate,
however, at very low relative humidity and germination increases as relative
humidity increases up to 100%. Free
moisture inhibits spore germination and colony development.
The optimum temperature for conidia germination, infection, and colony
development is 68°-86°F
(20°-30°
C). Conidia formation and viability
increases with increasing variation in daily temperature, up to a maximum of 27°F
(15°C)
variation. For these reasons
powdery mildew is well adapted to environmental conditions of semi-arid regions
with warm, dry climates and large diurnal fluctuations of temperature and
relative humidity. Because the
microclimate under sprinkler irrigation will be more moist, disease development
is usually slower than under surface irrigation. The sexual stage of the fungus
has rarely been observed in the U.S.
CONTROL
Because powdery mildew has the
ability to increase rapidly, the concept of reaching a disease threshold before
treating is of little practical value. Early
detection is essential for good control, and it is far more effective to treat
at first appearance of the disease, or preferably just prior to its appearance,
than to wait until the disease is widespread.
When the disease is treated is far
more important than which fungicide is used.
If powdery mildew has been found in a nearby field, the recommendation is
to treat knowing that it is coming. A
delay of 2 weeks in implementing control measures can result in serious loss
because inoculum levels can become too high for adequate control.
In order to see young colonies just beginning to develop, the angle of
the light and the visual angle should be as low as possible in relation to the
leaf surface. Young colonies will
likely be missed looking directly at the leaf at a perpendicular angle.
The date of powdery mildew appearance is rather predictable in most sugarbeet growing areas in southern Idaho, and experienced growers and fieldmen are usually prepared and know when to look for it. In southwestern Idaho, disease first appears about July 1, but in some years it appears the last week of June. The earlier the disease appears, the more severe the disease will be. Traditionally the disease occurs about two weeks later in the western portion of Twin Falls County. Travelling to the east, the first appearance is later.
In cases where disease begins to appear late in the season, the general rule based on field experiments is that treatment should be made if the disease first appears 5½ weeks or more before harvest. Later treatment will be of no economic benefit. Where multiple treatments are required, applications should be repeated as the disease reappears, but no closer to harvest than 5½ weeks.
DISEASE MONITORING METHOD
Growers, fieldmen and consultants can use a simple, consistent method for monitoring disease progress and determining when to treat. First, randomly select the most recently fully expanded mature leaf from a representative number of plants in the field and estimate the percentage of each leaf surface covered by the powdery mildew fungus. Calculate an average of the sampled leaves to determine the percent mature leaf area diseased (% MLAD). Any appearance of powdery mildew should trigger the first treatment, and an increase % MLAD in subsequent readings signals a repeat treatment. Research shows that there is negligible economic loss when the average season long % MLAD is maintained below 10%.
FUNGICIDES AND CONTROL
ECONOMICS
Sulfur is the only fungicide currently available that is recommended for control. Azoxystrobin (QuadrisÒ) is registered for powdery mildew on sugarbeets, but control has not been consistent, and it is not recommended at this time. Good coverage is essential for control. Fungicide should be applied in as much water as is necessary for good coverage. The calculations here are based on liquid sulfur applications, but experience has shown that sulfur dust is more effective than liquid sulfur applications because dust provides superior coverage.
A few simple calculations clearly illustrate the economic benefit of timeliness in controlling powdery mildew. Based on field data, assuming the first appearance of disease between July 1 and 15 and a harvest date of mid-October, it may be necessary to make three applications to prevent an estimated 15% loss. Using 25 tons/acre yield and $40.00/ton return as an estimated basis, a grower can expect a $150.00 loss without treatment (3.75 tons x $40.00). Cost of three applications of sulfur is about $40.50 (3 x $13.50), which gives a net return of $109.50/acre.
When powdery mildew first appears 5½ weeks before harvest, which is about August 31 in many parts of south-central and southeastern Idaho, one application should be made to prevent an estimated 5% loss, based on field measurements. Using the same price basis as in the previous example, a grower can expect at least a $50.00 loss without treatment (1.25 tons x $40.00). Cost of one sulfur application is about $13.50, which gives a net return of $36.50.
DISEASE RESISTANCE
There is at least one sugarbeet variety that has limited resistance to powdery mildew and is available in some growing areas. Unfortunately, resistance is often incorrectly equated with immunity, and growers may incur unnecessary losses when the resistant variety is not treated when it should be. With a resistant variety, disease development will usually be slower and not reach the same level of severity as with susceptible varieties. The number of treatments required for control can usually be reduced, and in situations where only one treatment would normally be required, it may be eliminated in some years. Specific data for treating a powdery mildew resistant variety has not been developed.
FURTHER READING
Danderevski, C. A. 1978. Powdery mildews of beet crops. In: The Powdery Mildews. D. M. Spencer, ed. Academic Press, London.
Duffus, J. E. and E. G. Ruppel. 1993. Diseases. In: The Sugar Beet Crop. Science Into Practice. D. A. Cooke and R. K. Scott, eds. Chapman and Hall, London.
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Presented at the Snake River Sugarbeet Conference, Twin Falls, Idaho, January 11-12, 2001.