1) SUMMARY

Cercospora leaf spot is a foliar disease caused by the fungus Cercospora beticola.  Disease development is favored by temperatures in the range of 68 to 95º F and must be accompanied by high humidity or moisture on the leaves.  In most years it occurs in a very limited geographic area, the east side of the San Joaquin Valley, where still nights result in heavy dews starting in August.  In the fall harvest area of Glenn County, Cercospora leaf spot can also be a problem.  Sprinkler irrigation can increase disease levels or result in disease development in locations where Cercospora leaf spot usually is not a problem.  Symptoms will usually become evident in August and continue through the fall.  In most cases, the areas affected are harvested in September through November.  Very little research has been conducted on this disease in the last two decades.

Cercospora causes severe reduction in sugar content, reducing grower profits.  Infected leaves die, causing the plant to produce more leaves at the expense of sugar production and growth.  The plants develop a large crown, which disrupts efficient harvest operations.

 2) CURRENT PEST MANAGEMENT PRACTICES

Current management of this disease is to spray at the first sign of symptoms and continue as necessary.  If spraying for insect problems, such as armyworms or Empoasca leafhoppers, a fungicide will be included.  The list of active ingredients registered for control include copper sulfate, cupric hydroxide, copper oxychloride, mancozeb, maneb, manzate, and thiophanate-methyl.  With the except of thiophanate-methyl, recommendations are to repeat every seven to ten or seven to 14 days.  Thiophanate-methyl recommends treatment every 14 to 21 days.  Also, the USDA in Colorado has confirmed low levels of resistance to benzimidazoles in the Tulare County area.

In addition, growers can minimize disease levels by reducing crop canopy and irrigation.  Crop quality and root health are the main reasons that growers do not apply excess fertilizer or irrigation.  Further reductions in these two inputs would probably hurt production.

Standard recommendations for this disease are to incorporate crop residues and avoid planting back in the same field.  Growers already incorporate residues and control volunteers for controlling virus problems.  Also, they do not plant back into a field with sugarbeets for three to five years to control/avoid other disease and pest problems.

 3) REDUCED-RISK OPTIONS

 The most likely management options that could reduce pesticide use for this disease are reduction of crop canopy by reducing nitrogen and water (likely to have an impact on yield) and use of tolerant/resistant varieties.  California has some major requirements for disease resistance, such as curly top and beet yellows virus, in the varieties used here that make it unwise to use varieties that have Cercospora tolerance but no tolerance to other more serious and geographically widespread diseases.  Because the geographic area of California sugarbeet production that is affected is relatively small, the development of Cercospora tolerance is not a top priority in breeding programs for California.

 4) CHALLENGES

Pest management challenges include the need for tolerant varieties and the need for effective fungicides that do not require frequent re-application until tolerant varieties are available.  As discussed in the paragraph above, the development of tolerant varieties is not likely in the near future.  Where this disease is a problem in other states, another fungicide (Duter®) that is more effective than the ones available in California can be used.  Duter® is a tin compound and not likely to be registered in California, but other states are looking at replacements for Duter® due to fungal resistance and review by EPA.  If an effective alternative fungicide is identified, it would be desirable but probably a regulatory challenge to have it registered in California.

 5) INNOVATIVE FEATURES IN REDUCED-RISK PROGRAM

At this time, there is no innovative or alternative method of managing this pest.

 6) BARRIERS TO ADOPTION OF REDUCED-RISK METHODS

  No information available.
 

REFERENCES

Sugarbeet Pest Management: Leaf Diseases. 1982. Paulus, A. O. et al. Division of Agricultural Sciences, University of California Special Publication 3278.