Cercospora leaf spot

Cercospora leaf spot (CLS) is a fungal disease that commonly affects sugar beets grown in Idaho, Oregon, Washington and other sugar beet growing regions. CLS symptoms appear on sugar beet foliage, causing leaves to develop circular spots ranging from 0.02 inch (0.5 mm) to 0.2 inch (6 mm) in size (Figure 1). Initially, these lesions are found on older leaves and petioles, but will progress to young leaves. Individual lesions have a brownish to reddish purple border with a tan to light brown center, turning greyish with visible spore baring structures later in the infection cycle. As the disease progresses, these small lesions combine to form large patches of necrotic tissue, and can cause leaves to yellow, wither and prematurely die. CLS symptoms will be more severe in areas of a field with higher humidity (e.g., low areas or near wind breaks).

CLS can be distinguished from other common foliar diseases (such as Alternaria leaf spot, Phoma leaf spot, Ramularia and bacterial leaf spots) through examination of the lesions (Figures 2-4). When CLS is the cause, lesions will be small and round in shape, with black spots in their center. These black spots are spore bearing pseudostromata, which may be covered in cobweb-like masses of spores when examined under a hand lens during periods of high humidity.

Biology

CLS is caused by Cercospora beticola, a necrotrophic, imperfect fungus. The fungus infects living plant tissue but can also persist in dead plant tissue (plant debris) for up to two years. During the growing season, C. beticola can be spread by wind, splashing water or the movement of insects, animals, workers or equipment.

Disease development and spread is greatly facilitated by warmth (77°F to 95°F days, nights above 60°F), and high humidity or leaf wetness. However, spore production and spread are still possible outside of these optimal conditions. An infection cycle can be completed in 7-21 days, depending on favorability of conditions and susceptibility of the host plant.

Damage

CLS causes damage by killing leaf tissue (Figure 5), thus reducing root growth and sugar content. Though CLS does not infect the sugar beet root directly (e.g., by forming lesions on the root), it can still cause losses in tonnage and sucrose of 40% or more when outbreaks are not controlled. CLS also causes post-harvest losses by reducing sugars and increasing impurities, which lead to losses during storage and sugar processing. Economic losses from CLS occur when 3% or more of the foliage (about 80 spots per leaf) is affected by the time of harvest.

Monitoring

Early detection of CLS is key for effective management and to prevent economic losses. Scouting should begin at canopy closure and extend through harvest. The development of CLS is highly dependent upon the presence of inoculum, so monitoring efforts can be targeted to areas that have suffered from CLS in the past or near potential reservoirs of the fungus such as weedy areas or some other crops that may be infected (e.g., spinach, safflower or table beet). Monitoring is also particularly important when conditions favor the development and spread of the disease. Periods of six hours or more where leaves remain wet or humidity is high (>90%), while temperatures remain warm (60-90°F (15-32°C)) favor establishment and spread of CLS.

Economic losses from CLS occur when leaf spots cover 3% or more of the foliage by harvest (about 80 spots per leaf). Treatment with fungicide is recommended when 1% of foliage is affected (one to five spots per leaf).

Management

Primary management tactics

Monitoring should commence when conditions favor CLS outbreaks in areas that have suffered from CLS in the past. Implementation of cultural practices that reduce inoculum such as deep tillage to bury refuse and a minimum of three years crop rotation reduce likelihood of CLS outbreaks.

Cultural

• Plant new beets at least 300 feet from areas planted in sugar beets the previous season.
• Avoid replanting sugar beets in the same ground for at least three years.
• Plow post-harvest to completely bury beet refuse

Under severe infection pressure:

• Reduce planting density to allow leaves to dry more quickly
• Manage irrigation to allow plants to dry by nightfall and avoid irrigating at night
• Consider planting CLS tolerant varieties which can reduce disease severity and spread

Chemical

• In areas with known CLS pressure, apply fungicides prophylactically when environmental conditions are conducive for infection, and rotate between fungicides with different modes of action to reduce risk of developing fungicide resistance.
• Tankmix protectants such as EBDC, copper, oxidate
• Apply fungicides with sufficient water to ensure good coverage. Good crop coverage is necessary for effective control of CLS.
• Discuss existing fungicide resistance with your local crop consultant or agronomist, as surveys have shown increasing fungicide resistance in Idaho
• Recommendations for pesticides to use in the management Cercospora leaf spot can be found on the PNW Pest Management Handbooks website

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.