Introduction, Damping Off, and Epidemiology

Introduction

Rhizoctonia root rot disease is caused by a soil fungus called Rhizoctonia solani. Infection typically occurs when contaminated soil is deposited onto the crown during cultivation or by wind or water. Infected sugarbeets will develop a dark brown rot on the root and crown. Outbreaks generally begin in June when rows begin to canopy. However, symptoms of the disease are not apparent until the end of summer. Often times, a farmer will only become aware of the diseased crop during harvest.

While Rhizoctonia has been diagnosed on sugarbeets since the mid-1970s, its presence was rare. Recently however, outbreaks have been more common, particularly in areas with heavy rotations of corn or beans. The strain specific to these crops, AG 2-2 IIIB, is widely distributed around the world and seen in all types of soil.

Rhizoctonia root rot is the most common root rot in the United States. According to one study in 2003, the disease was present in approximately one third of all sugarbeet growing areas.

Damping Off

The fungus responsible for Rhizoctonia, Rhizoctonia solani, not only causes Rhizoctonia root rot, but is also a key part of a group of fungi responsible for damping off. Rhizoctonia solani carries two primary strains of infection. One strain, AG 2-2 IIIB, is responsible for Rhizoctonia root rot. The other, AG 4, aids the other fungi in damping off sugarbeets. Both strains may coexist in the same field meaning both damping off and root rot could occur successively.

The other fungi involved in the complex that causes damping off are Phoma (Phoma betae), Pythium (Pythium ultimum) and Aphanomyces (Aphanomyces cochloides, which may also cause root rot). These fungi, in addition to Rhizoctonia solani, attack young plants at the same time, making it difficult to identify the pathogenic agent.

Damping off causes the constriction of the beet crown and the blackening of roots in young plants (also called “black foot”). Sometimes, the part located under the cotyledons may also turn black. Unhealthy root structures make the plant extremely weak or kill it altogether.

This fungi complex prefers warm, wet weather. Late sowing increases the risk of spreading the fungi complex. Black foot is widespread and devastating if not attended to quickly. Seed coatings have not been successful in minimizing the spread of the disease. Folian fungicides have some promise to minimize infection if applied at the optimum time.

Epidemiology

A constricted crown caused by damping off.

Life Cycle

Without a host plant, Rhizoctonia solani dwells in the form of small, brown or black structures called “sclerotia” where it can survive for many years. Rhizoctonia solani can survive in soil for many years in the form of mycelium.

When the soil reaches a favorable temperature (roughly 60º F), the host plant, in this case sugarbeets, begins to excrete chemicals. These secretions arouse the dormant sclerotia and the fungus begins to produce a mass of long filaments (hyphae). The hyphae extend through the soil until they make contact with the host plant.

Once attached to the host plant, the mycelium proliferates on the root and produces T-shaped structures called “infection cushions.” Here the mycelium releases enzymes capable of dissolving cell walls. The mycelium then colonizes the intra- and inter-cell spaces of the root tissue.

The fungus parasite begins to develop by feasting on the plant’s nutrients. Eventually, the fungus invades all the cells, killing them and reproducing survival structures (sclerotia) inside. The host plant dies as its xylem vessels are attacked, leaving these sclerotia behind in the soil.

Dispersal & Growth Factors

Rhizoctonia solani survival structures (sclerotia) can be dispersed by the wind, water (rainfall, drainage, irrigation, etc) and soil movement (erosion, machinery, uprooting, etc). Many phytopathologists believe that the dispersal of Rhizoctonia solani may not be as central to the possibility of infection as once suspected. These scientists site the fact that Rhizoctonia solani is already found in most soil where sugarbeets are grown. Its prevalence means a combination of environmental factors may cause outbreak, such as:

  • presence of a host plant
  • abundant rainfall or irrigation
  • increased temperatures in spring and summer

Soil compaction reduces drainage, creating a favorable environment for Rhizoctonia.

Hosts

Various strains of Rhizoctonia solani exist all over the world. AG 2-2 IIIB, the strain responsible for Rhizoctonia root rot, can be found in a broad array of host crops. Corn and beans are particularly susceptible to Rhizoctonia. This means crop rotation plays an important role in determining the possibility of a Rhizoctonia outbreak in the field.

Note: There is a strain of Rhizoctonia solani that attacks potatoes.

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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