Disease Control and Dual Resistant Varieties

Disease Control

Symptom of Rhizoctonia crown rot form; permanant foliar wilting, forming a brown rosette.

Some fungicides have been approved by the USDA to control Rhizoctonia root rot. Growers should consult their Agriculturists and chemical representatives to discuss which approved fungicide is best. In addition to spraying approved fungicides or planting Rhizoctonia-Rhizomania dual tolerance varieties, growers are encouraged to engage in the following agronomic practices:

  • Extend Rotation. A three to five year buffer between sugarbeet crops in the same field is ideal.
  • Avoid susceptible crops. This means supplementing corn, beans and sugarbeet crops with non-host crops.
  • Exercise careful weed control.
  • Use caution cultivating as it can result in the spread of contaminated soil to the beet tops.
  • Maintain good soil structure. Use appropriate fertilizers, sow a cover crop in the winter and avoid the use of heavy machinery in wet or unfavorable conditions.
  • Minimize soil compaction. Use wide tires.

Dual Tolerance Varieties

Rhizoctonia infection in the row.

Breeding for Resistance

The United States provides the primary source for Rhizoctonia resistance. It differs from Rhizomania resistance in that, it is controlled by a large number of genes rather than a single gene. The process of multi-gene management is also known as quantitative resistance.

Production of Rhizoctonia and Rhizomania-resistant varieties requires several steps. First, pedigree plants, (those with high sugar content, good root yield, extractability, etc) are bred with wild species resistant to the disease. Next, the populations are “back-crossed” several times with the elite parent. Each generation is grown in a greenhouse with high levels of Rhizoctonia and Rhizomania pressure. The plants with the best results are harvested and prepared for the next step.

Advances in genetic marking continue to enhance the success of the hybrids. The primary challenge for breeders is to develop varieties that are not only highly resistant to Rhizoctonia and Rhizomania, but maintain the characteristics of quality plants (high sugar content, large yield, etc).

Advantages and Disadvantages

A variety with Rhizoctonia-Rhizomania dual tolerance is an effective solution where Rhizoctonia root rot has been known to exist. Compared to varieties that only provide Rhizomania resistance, the dual tolerance performs much better in terms of yield, sugar content, soil tare and extractability where Rhizoctonia pressure is high.

For optimum results, dual tolerance varieties must be combined with proactive agronomic practices.

Varieties with Rhizoctonia-Rhizomania resistance have the following limitations.

  • They are susceptible to bolting.
  • There is no immunity to Rhizoctonia. Resistance means a higher level of protection or tolerance
  • Rhizoctonia-Rhizomania resistance does not protect against the other fungi that cause damping off.

Nevertheless, dual tolerance varieties currently on the market continue to be a viable source of effective seed.

Contents

Rhizoctonia Root Rot
1. Introduction, Damping Off, and Epidemiology
2. Symptoms and Economic Importance
3. Disease Control and Dual Resistant Varieties
4. Summary

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