Rhizoctonia Root Rot

Rhizoctonia Root Rot

Rhizoctonia is a serious problem in sugarbeet causing root and crown rot. While some fungicides are available to combat the disease, advances in plant breeding continue to provide the best chance of success against Rhizoctonia.

The fungus responsible for Rhizoctonia, Rhizoctonia solani, is common in sugarbeet in all countries where the crop is grown and climate, soil type and crop rotations make conditions favorable for its development.

Progress in the development of better genetic tolerance to Rhizoctonia has been slow compared to other sugarbeet diseases like Rhizomania due to the complexity of its multi-gene structures and the difficulty in managing these structures with genetic markers. In addition, the source of the Rhizoctonia-resistant gene structure goes back to wild relatives of the sugarbeet. This means introducing resistant genetics has an adverse effect on many of the desireable characteristics of cultivated sugarbeets. To put it simply, Rhizoctonia-resistant varieties have yet to produce the same consistent yield and quality as conventional seed varieties.

But there’s hope. Great strides continue to be made, thanks in part to research done in the United States. Seedex, Inc. is very proud of its key contributions to the development of Rhizomania-Rhizoctonia tolerant hybrids. These commercial varieties provide industry-leading solutions for most environments plagued with Rhizoctonia.

Marc Meulemans
Plant Breeder
Seedex, Inc.

Introduction, Domestic Rhizomania, Geographical Distribution, and Epidemiology

Introduction
Rhizomania (RZM) is one of the most widespread sugarbeet diseases. Greek for “crazy root”, Rhizomania is characterized by a proliferation of lateral rootlets along the main tap root. The disease is caused by the Beet Necrotic Yellow Vein Virus (BNYVV) and is transmitted to the sugarbeet by an intermediary root parasite called Polymyxa betae, a protist living in the soil. The disease typically attacks the plant in June during the crop canopy phase. The affected sugarbeet will lose sugar content and increase tare in the field when harvested.

Domestic Rhizomania
Rhizomania was first reported in the United States in the state of California in 1983. However, the widespread accounts of its devastation that same year suggest that the disease had come to the United States much earlier. Since the first report, Rhizomania has been identified in most domestic sugarbeet production areas.

Over the last few decades, plant breeders have worked tirelessly to improve the availability and quality of varieties resistant to Rhizomania.

Geographical Distribution
Rhizomania is found in all regions of the world where sugarbeets are grown. Domestically, most counties are affected by the disease. Generally speaking, it has become fairly difficult to find estimates of the distribution of Rhizomania or the percentage of the area contaminated in these counties for two reasons. First, the disease is considered to have been present in nearly all sugarbeet producing regions and second, the use of resistant varieties has become essentially a standard procedure.

Many specialists believe climate change will cause a more rapid propagation of Rhizomania. In recent years, unseasonably wet conditions and a reluctance by farmers to discontinue the use of susceptible varieties has resulted in a seven fold increase in inoculum build-up of Rhizomania compared to regions where more disease-resistant varieties were used.

Epidemiology Cycle
Transmission of the virus occurs through the root parasite, Polymyxa betae, a protist that dwells in the soil. It survives in the form of sporosores – masses of highly resistant, dormant spores. In harsh conditions, the Polymyxa betae/BNYVV complex can remain dormant and maintain its infectious potential in the soil for decades.

When the temperature of the soil becomes favorable, 59 degrees to 77 degrees Fahrenheit, and has a high moisture content, the dormant spores germinate and produce primary zoospores. Attracted by secretions from the rootlets of the host plant, they swim through water in the soil, propelled by their flagellum. When the zoospores reach the outer surface of the rootlets, they hook onto it and discharge their cellular contents inside the plant’s cells. This union results in the formation of plurinucleated cytoplasmic mass more commonly referred to as plasmodium. This is important because when the zoospores release their own cellular contents into the root, they also release the virus.

After the incubation period in the plant cells, the Polymyxa betae plasmodium begins to evolve in one of two ways depending on the climactic conditions.

If conditions are unfavorable, cold and dry, the parasite will form survival units (the cystospore).
If conditions are favorable, warm and wet, the parasite will multiply and colonize new host cells.
Regardless of condition, each new spore will contain the Rhizomania virus, Beet Necropic Yellow Vein Virus (BNYVV).

Dispersal & Growth Factors
Polymyxa betae spores can be dispersed by water (rain, run-off, irrigation, etc) or by soil (wind, farming equipment, sugarbeet transportation, etc). The transplant of soil from machinery is the most likely cause of dispersal because it relocates both active and dormant spores.

The environmental factors that will contribute to the development of the disease are:

  • the presence of a host plant
  • a warm, wet spring (high temperatures, abundant rainfall)
  • a neutral to alkaline soil pH. The increase in fertilizer prices has resulted in an increase in the use of composted green waste, which often has very high pH levels. High application rates on low fertility soils may exacerbate the infection.

Hosts
The Polymyxa betae/BNYVV complex generally feeds on plants belonging to the chenopoiacea family (sugarbeet, chenopodium, spinach) and amaranth family.

Symptoms
While outbreaks of Rhizoctonia occur early in the season during canopy closure, symptoms are not generally visible until late summer or early fall. At times, a farmer will not be aware of the infection until harvest.

Rhizoctonia spreads down crop rows
In Foliage:
Rhizoctonia root rot infestation always appears as limited patches that spread along the rows. These small infected patches grow gradually. In extreme cases, entire rows or the whole field can be affected. What starts as a sudden wilting of the foliage gradually evolves into a chlorosis or complete necrosis of the leaves. The dead foliage remains attached to the crown of the sugarbeet, forming a brown rosette in the middle. New leaves may appear, however this is a sign the plant is near death.

In Roots:
Rhizoctonia root and crown rot in sugarbeet.
Damage in the roots and crown is defined by a dark, black or brown dry rot. It can be observed on the surface and/or below the crown depending on the severity of the outbreak. In some cases, the entire sugarbeet will disappear entirely.

Diagnosis
Diagnosing root rot is fairly easy due to the stark discoloration. However, Rhizoctonia may be confused with other root rots such as Pythium or Aphanomyces. Even lightning damage may resemble a Rhizoctonia outbreak. If uncertain of the particular affliction, growers should consult their Seedex dealer, an agriculturalist or other specialist.

Economic Importance
Rhizoctonia is present across all areas of the United States where sugarbeets are grown. The severity of an outbreak may vary considerably depending on a number of environmental conditions.

In highly infected patches, the economic impact of an outbreak can be devastating. The consequences of a severe infection are:

  • Major losses in yield (losses range from 25% to 100%)
  • Reduction in sugar content
  • Increased soil tare. Soil sticks to the fungus’ mycelium
  • Poor industrial quality due to increased levels of sodium, potassium and nitrogen.
  • Storage problems of sugarbeets in piles

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.

Summary

Rhizoctonia root rot is caused by the soil fungus, Rhizoctonia solani. The disease causes a dark black or brown rot that envelops the root and crown of sugarbeets.

Outbreaks occur in favorable conditions with abundant rainfall and high spring or summer temperatures. Poor soil structure consisting of tightly compacted soil increases the spread of the fungus. Infection typically occurs in the crown when contaminated soil is deposited onto the crown during cultivation or by wind or rain.

The strain of Rhizoctonia solani that causes Rhizoctonia root rot is fairly common and can attack a wide range of hosts such as corn or beans. Planting these crops in rotation with sugarbeets greatly increases the risk of infection.

Symptoms appear late in the growing season, spreading in patches or down the rows in the field. The damage is characterized by permanent wilting of the foliage and dark rot spots on the root or crown.

In severe cases, Rhizoctonia root rot causes significant economic devastation through major losses in yield, reduction in sugar content, an increase in soil tare and poor industrial quality.

Complete immunity from a Rhizoctonia outbreak is not possible, however several things can be done to minimize the risk of infection.

Adopt the following agronomic measures:

  • 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.
  • In the United States, some fungicides are available to help combat the fungus. Growers should consult their agriculturalists and chemical representatives for more information on managing the disease after planting.

Varieties with resistance to both Rhizoctonia and Rhizomania continue to provide growers a higher level of tolerance and the opportunity to grow this valuable crop.

Seedex, Inc. is a leader in dual Rhizoctonia-Rhizomania tolerant varieties. These consistent Seedex varieties blend a high degree of resistance, excellent yield performance and the strong emergence that Seedex is known for.

Everyman’s Guide to Rhizoc (PDF)

Rhizoctonia Map 2016 ACS

Rhizoctonia Map ACSRhizoctonia Management ACS

Rhizoctonia Map – ACS 2016 (PDF)