Powdery mildew

Powdery mildew is a fungal disease in many sugar beet producing areas of the world and is a recurring disease problem in Idaho and parts of Oregon. Powdery mildew is particularly well adapted to the warm, dry climate of Idaho’s sugar beet growing regions, where the disease can spread rapidly.

Symptoms begin with the development of small, white mycelial mats (branching fungal growths). These mats are about half the diameter of a penny and widely scattered (Figure 1). As the disease progresses and entire leaves become covered in mycelium, lending the leaves the appearance of being covered in powder and giving the disease its common name (Figure 2). Under severe infection, leaves turn yellow to purple-brown and may develop tiny black spots within the white mycelial mats, usually on the upper surfaces of older leaves (Figure 3).

Biology

Powdery mildew in sugar beets is caused by the fungus Erysiphe betae. This fungus affects only sugar beets, field or fodder beets, Swish chard and wild Beta species. E. betae is introduced annually via wind-transported spores from southern California. Powdery mildew spores (also called conidia) may arrive in the southwestern part of the state from the end of June through mid-July. It then takes a further two to three weeks for the fungus to spread to southcentral Idaho (the Magic Valley), and then on to the easternmost growing regions in the state.

For infection to occur, first a spore must land on a susceptible sugar beet variety. For that spore to germinate and infect the plant specific environmental conditions must ensue. Optimal spore germination occurs at 100% relative humidity, without free water on the leaf surface and at 77 degrees Fahrenheit (25°C). Daily temperature fluctuations of up to 27 degrees Fahrenheit (25°C) also favor spore production and viability. However, spore germination and infection can occur at relative humidity as low as 30-40%, and at any temperature between 59- and 86-degrees Fahrenheit (15-30°C).

Once the spore germinates, it produces infectious hyphae (threadlike structures) that penetrate the host plant’s cells. As the hyphae continue to grow, they form the expanding “powdery” white spots characteristic of the disease (mycelial mats). From mycelial mats new spores are produced and spread. Though high relative humidity (60-100%) favors spore germination, it inhibits production of the spores and reduces their viability. Later in the season another form of reproductive structure, chasmothecia (Figure 3), may form to facilitate sexual reproduction. Chasmothecia are thought to be important for winter survival and development of fungicide-resistant strains, but these structures are rarely observed.

Damage

Powdery mildew can cause losses of up to 35%. Losses are most severe when infection occurs early in the season and conditions favor rapid spread. Powdery mildew outbreaks can be particularly severe in arid and semi-arid climates, with large day-to night temperature fluctuations, such as in the sugar beet growing regions of Idaho. Infection tends to be more severe in surface irrigated fields (where humidity is often higher) than in sprinkler irrigated fields (where spores are washed off leaves and free moisture inhibits spore germination). Economic losses from powdery mildew are more severe when plants are also drought stressed. Delay of control by as little as two weeks may allow inoculum levels to rise above adequately controllable levels, increasing the severity of losses. Unfortunately, though losses may be severe, it can be more difficult to detect powdery mildew infection than other diseases and conditions.

Monitoring

Monitoring for powdery mildew should begin in the middle or end of June and continue until a month before harvest. To increase chances of early detection, focus scouting efforts on areas most likely to be infected such as surface-irrigated fields, fields that were water stressed, and fields that suffer from poor fertility. Plants exposed to physiological stressors, located along field borders or in areas with standing water, are most likely to show early infection. Start by examining the lowermost leaves in the canopy, as this is where the disease will initially appear. It can be difficult to detect young colonies, as they are hard to see when the leaf surface is viewed flat. To better detect young colonies, roll the leaf over, with the suspected spot facing upward, and view it in profile against the sky or another light-colored background (Figure 4) .

The presence of any powdery mildew should trigger the first treatment. To determine if treatments are needed subsequently, it is necessary to calculate the average percentage of mature leaf area diseased (% MLAD) within the field. To do so, randomly select the most recent fully expanded leaf from various plants in the field. Ensure that this sample is representative by taking a sufficient number of leaves, and from various parts of the field. Estimate the % MLAD for each leaf, and then calculate the average across all leaves sampled. Maintain average % MLAD below 10% to avoid economic losses.

Management

Primary management tactics

Diligent scouting facilitates correct control timing, which is key for effective disease management. Treat at first disease onset in a field, when powdery mildew is found in a nearby field or just prior to its historical appearance.

Cultural

• Manage irrigation to avoid drought stress which can interact with powdery mildew infection to produce more severe infection and greater losses
• Powdery mildew tolerant varieties do not provide complete immunity to the disease but may reduce symptom severity and/or reduce the number of fungicide applications necessary to achieve adequate control

Chemical

• Apply control measures as soon as powdery mildew is detected. Treatment within six weeks of harvest is of no economic benefit.
• Reapply as necessary every 2-3 weeks to keep diseased leaf area below 10%
• Apply fungicides with enough water to ensure good crop coverage
• Consider tankmixing sulfur for fungicide resistance management purposes
• Avoid fungicides belonging to strobilurin or Group 11 since powdery mildew is known to show resistance to this mode of action
• Recommendations for pesticides to use in the management of powdery mildew on sugar beets can be found by contacting local crop consultants, Extension representatives or by visiting the PNW Pest Management Handbooks webpage.

Further reading

Pesticide warning

Always read and follow the instructions printed on the pesticide label. The pesticide recommendations in this University of Idaho webpage do not substitute for instructions on the label. Pesticide laws and labels change frequently and may have changed since this publication was written. Some pesticides may have been withdrawn or had certain uses prohibited. Use pesticides with care. Do not use a pesticide unless the specific plant, animal or other application site is specifically listed on the label. Store pesticides in their original containers and keep them out of the reach of children, pets and livestock.

Trade names — To simplify information, trade names have been used. No endorsement of named products is intended nor is criticism implied of similar products not mentioned.

Groundwater — To protect groundwater, when there is a choice of pesticides, the applicator should use the product least likely to leach.