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Ministry of Agriculture, Food and Rural Affairs

Rhizoctonia or Black Scurf

Rhizoctonia cankers on sprout Rhizoctonia black scurf on tuber Black sclerotia on late-harvested tubers Sclerotia on seed tuber Sprouts pinched off by Rhizoctonia Rhizoctonia cankers on young stems Longitudinal cracks on older stems Fungal mycelium on the lower stem Rhizoctonia symptoms on upper leaves Aerial tubers due to severe infection Aerial tubers due to severe infection Tight cluster of tubers near soil surface Small, malformed tubers due to Rhizoctonia infection
Click to enlarge.

Beginner

Scientific Name
Rhizoctonia solani

Identification
Rhizoctonia symptoms are found on above- and below-ground portions of the plant. Reddish-brown to brown lesions develop on sprouts, stolons and young stems. Rhizoctonia lesions that pinch off sprouts before plant emergence result in severe crop damage. The secondary sprouts that develop from the pinched off sprouts are less vigorous and emerge much later, causing irregular, uneven stands. Early infection of stolons often results in pruning before tuber formation or interrupted development of newly formed tubers. As the stem lesions mature they become cankers. Cankers on young stems cause the emergence of weak plants.

Mid- and late-season infections result in long, deep, sunken cankers on the stems, the formation of aerial tubers and deformed daughter tubers. Aerial tubers are produced on the leaf axils and/or at ground level. Small, malformed tubers are produced in a tight cluster close to the surface of the soil.

A white to grey mat of fungal mycelium develops at the base of the stem. The mycelium is easily rubbed off. The affected stem area remains green.

The upper leaves roll upwards and turn reddish. These symptoms could be confused with potato leaf roll.

At the end of the season, black sclerotia form on the skin of daughter tubers. Sclerotia are hard, black structures of irregular shape and variable sizes tightly attached to the tuber skin. They are commonly called the dirt that won’t wash off.

Rhizoctonia may cause tuber cracks.

Often Confused With
Late blight: white mycelium at the base of the stem.
Leaf roll: upper; leaves roll upwards

Period of Activity
Throughout the season. Rhizoctonia is more prevalent during cool, wet springs, when plant emergence is slow.

Scouting Notes
Fields should be monitored at least twice a week. Always check low spots in fields and areas of poor drainage.

When walking fields, stop at many sites as possible and check plants at random.

Thresholds
Rhizoctonia is a seed and soil-borne fungus. Planting sclerotia-free seed is an effective management strategy that reduces both the amount of inoculum and disease incidence.

Advanced

Scientific Name
Rhizoctonia solani

Identification
Rhizoctonia symptoms are found on above- and below-ground portions of the plant. Reddish-brown to brown lesions develop on sprouts, stolons and young stems. Rhizoctonia lesions that pinch off sprouts before plant emergence result in severe crop damage. The secondary sprouts that develop from the pinched off sprouts are less vigorous and emerge much later, causing irregular, uneven stands. Early infection of stolons often results in pruning before tuber formation or interrupted development of newly formed tubers. As the stem lesions mature they become cankers. Cankers on young stems cause the emergence of weak plants.

Mid- and late-season infections result in long, deep, sunken cankers on the stems, the formation of aerial tubers and deformed daughter tubers. Aerial tubers are produced on the leaf axils and/or at ground level. Small, malformed tubers are produced in a tight cluster close to the surface of the soil.

A white to grey mat of fungal mycelium develops at the base of the stem. The mycelium is easily rubbed off. The affected stem area remains green.

The upper leaves roll upwards and turn reddish. These symptoms could be confused with potato leaf roll.

At the end of the season, black sclerotia form on the skin of daughter tubers. Sclerotia are hard, black structures of irregular shape and variable sizes tightly attached to the tuber skin. They are commonly called the dirt that won’t wash off.

Rhizoctonia may cause tuber cracks.

Often Confused With
Late blight: white mycelium at the base of the stem.
Leaf roll: upper; leaves roll upwards

Biology
Rhizoctonia is a common seed- and soilborne fungus that survives as black resting structures called sclerotia. In the soil, the fungus can also survive as mycelium on organic debris. Planting seed potatoes infested with sclerotia increases the amount of Rhizoctonia inoculum in the soil. Rhizoctonia is more prevalent during cool, wet springs, when plant emergence is slower than the growth of the fungus. Infections start when sclerotia in the soil or on seed tubers germinate, producing mycelium that infects roots, sprouts and stolons. Rhizoctonia cankers that develop during mid-season cause the formation of small, deformed tubers in a tight cluster near the soil line. Also, aerial tubers are formed at leaf axils. The black resting structures called sclerotia begin to form late in the season, mainly after vine death.

Period of Activity
Throughout the season. Rhizoctonia is more prevalent during cool, wet springs, when plant emergence is slow.

Scouting Notes
Fields should be monitored at least twice a week. Always check low spots in fields and areas of poor drainage.

When walking fields, stop at many sites as possible and check plants at random.

Thresholds
Rhizoctonia is a seed and soil-borne fungus. Planting sclerotia-free seed is an effective management strategy that reduces both the amount of inoculum and disease incidence.

Management Notes
Management of Rhizoctonia requires an integrated program of cultural practices which include the following strategies:

  • Plant only certified seed free of sclerotia. Seed growers should plant only sclerotia-free seed.
  • Planting seed tubers shallow in warm soil (above 8°C) speeds up emergence reducing the risk of stem canker.
  • Plant fields with coarse-textured soils first because they are less likely to become waterlogged and will warm up faster.
  • Harvest the crop as soon as the tuber skin is set. Delaying harvest in the fall increases the amount of sclerotia on the tubers.
  • Do not dump infested tubers on future potato fields as they can become sources of inoculum.
  • Chisel plowing has been shown to reduce the incidence and severity of the disease.
  • Increasing the soil organic matter tends to reduce Rhizoctonia incidence