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Beginner

Scientific Name
Rhizoctonia solani

Damage from Rhizoctonia is called damping off or wire-stem depending on the growth stage affected. Bottom, root and head rot from Rhizoctonia can also occur, mainly on cabbage.

Identification

• Seedlings affected by damping-off fail to emerge or if they do, they quickly decline, topple over and die
  
• In older seedlings, there is purpling on the lower leaves and the lower stem becomes constricted and dark-brown near the soil surface
  
• Other symptoms may include seed decay, rotting roots and cankers on lower petioles

Often Confused With
Wireworm damage
White rot

Period of Activity
Infection can occur at any time during the growing season, but the seedling stage is most susceptible. Incidence and severity of disease at all growth stages are influenced by weather, soil conditions and inoculum level. Seedling infection is favoured by cool weather. As well, incidences of wirestem are most severe in spring and fall when soils are wet and cool. Root rots generally occur during periods of warm, wet weather and may affect the plant at any stage of development.

Scouting Notes
It is important to note that Rhizoctonia is not the only cause of damping off.  Therefore, it’s important to make sure that a diagnosis is made by someone skilled at identifying plant diseases.

While walking fields, look for wilted plants and/or gaps in the plant stand where the seedlings have failed to emerge. Dig around the seed trench to look for the cause of the damage. Record the percentage of the field impacted by damping-off or root rot damage.

Thresholds
None established.

Advanced

Scientific Name
Rhizoctonia solani

Diseases caused by Rhizoctonia spp. can affect most plants at various stages of growth. Depending on the growth stage, the disease is given different names. Rhizoctonia can be found in all soil types and in all regions of the province. In Ontario the strain that attacks crucifers prefers cool, wet soil.

Identification
Damage from Rhizoctonia is called damping off or wire-stem depending on the growth stage affected. Bottom, root and head rot from Rhizoctonia can also occur, mainly on cabbage.

Damping off occurs in seedbeds and in direct-seeded plantings. Seedlings simply fail to emerge or if they do, they quickly decline, topple over and die.

When older seedlings are attacked by Rhizoctonia, the lower stem becomes constricted and dark-brown near the soil surface, a symptom called wire-stem. In general, the first symptom of wire-stem is purpling of the lower leaves. Symptoms may appear in sections of rows or in areas of the field. A close examination of the seedling exposes the withered stem just above the soil line. The outer tissues slough off and all that is left is the dark, wiry, woody stem. Affected plants remain standing but are severely stunted.

Bottom and root rot occurs as a result of contact between plant tissue and moist, infested soil. The midribs of lower leaves or roots below the soil surface will develop light or dark grey lesions with purplish borders extending along the axis of the rib or stem, respectively. Leaves of affected plants may become yellow and drop off. In severe Rhizoctonia cases, the fungus may attack the head of the plant up to several layers deep. Dark, web-like mycelium and small, brown sclerotia may be found between leaves. Late in the season such plants may develop rot in storage.

Often Confused With
Wireworm damage
White rot

Biology
Rhizoctonia may be carried in or on seeds, serving as an inoculum in seedbeds or trays. Otherwise, spread of this disease can be carried out by wind, water or machinery. The fungus can also survive in the soil in the form of sclerotia, a hardened mass of fungal threads. The sclerotia are resistant to cold, heat, drought and most chemicals. When favourable conditions occur, fungal threads grow out of the sclerotia and penetrate roots or other plant tissue with which they come in contact, excreting pectolytic enzymes. Eventually, the disease may colonize vascular tissue, causing the collapse of the infected plant.

Period of Activity
Infection can occur at any time during the growing season, but the seedling stage is most susceptible. Incidence and severity of disease at all growth stages are influenced by weather, soil conditions and inoculum level. Seedling infection is favoured by cool weather. As well, incidences of wirestem are most severe in spring and fall when soils are wet and cool. Root rots generally occur during periods of warm, wet weather and may affect the plant at any stage of development.

Scouting Notes
It is important to note that Rhizoctonia is not the only cause of damping off.  Therefore, it’s important to make sure that a diagnosis is made by someone skilled at identifying plant diseases.

While walking fields, look for wilted plants and/or gaps in the plant stand where the seedlings have failed to emerge. Dig around the seed trench to look for the cause of the damage. Record the percentage of the field impacted by damping-off or root rot damage.

Thresholds
None established.

Management Notes

• Make sure flats and soil for growing seedlings has been sterilized.
  
• Use fungicide-treated seed and plant only when soil and weather conditions are favourable for vigorous crop development.
  
• Avoid overcrowding the seedlings.  This helps to improve ventilation and drying-off of the crop and soil. Damping-off fungi thrive under moist conditions.
  
• Plant seed at the proper depth in well-prepared, moist, warm soil. Retarded seed germination and emergence make seedlings more susceptible to damping-off fungi. Seeding outdoors should be no deeper than 4.0 cm (1 ½ in.) at a soil temperature of at least 10°C (50°F).
  
• In greenhouses and cold frames, water seedlings only as necessary, with pre-warmed water, ensuring adequate drainage. Waterlogged soils promote damping-off.
  
• Maintain optimum growing conditions. Plants which are "leggy" from insufficient light are weak and prone to attack by pathogenic fungi. Avoid excessive fertility which promotes succulent growth or salt accumulation, both of which favour disease. Banding fertilizer close to the seed may delay emergence.
  
• During cultivation do not throw soil onto the plant heads.
  
• Do not store cabbage heads infected with bottom rot.
  
• Rotate out of crucifers for 3 years following any crucifer crop when bottom rot is evident.