Rhizoctonia Canker / Black Scurf

Causal Organism: Rhizoctonia solani

Crops Affected: potatoes

Disease Cycle:

• Disease persists as sclerotia (hardened hyphal masses) on tubers or in the soil; may also persist as mycelium on plant debris in the soil
• Sclerotia germinate and produces mycelium that infects potato sprouts, roots, stolons and tubers throughout the season
• Sclerotia form on tubers as tubers mature and plant tops die
• Spores are not an important part of the disease cycle
• Sclerotia do not develop in storage

Symptoms:

• Most severe when soil conditions are cold and wet or when there are short intervals between potato crops in rotations
• Less severe (even with high levels of inoculum) under high temperature conditions during early plant growth

Canker:

• Potatoes may fail to emerge
• Roots, stolons, newly emerged sprouts and new stems may have a reddish-brown discolouration and dark, sunken stem cankers
  • Cankers and lesions can kill sprouts and girdle stems
• Aerial tubers may be visible on infected stems

Black Scurf:

• Dark brown to black, irregularly shaped sclerotia form on tuber surfaces (superficially)
  • Note, these do not penetrate the tuber
• Some malformation of the tubers can occur

Other symptoms associated with presence of the disease:

• Greyish-white mycelium may occasionally be visible on stems at the soil surface
• Some foliar symptoms may be visible, including leaf rolling, wilting and purpling and premature vine death
  • Some symptoms of other disease can be similar to these symptoms

Management:

• Plant clean seed tubers
• Seed treatments can control seed-borne inoculum, but will not protect developing tubers
• Maintain a rotation of at least 3 years between potato crops; rotate with cereals or grasses
• Encourage rapid plant growth and emergence in the spring
  • Avoid planting into cool soil
  • Warm seed
  • Seed shallow and hill after emergence
  • Delay irrigation until after sprouts have emerged
• Early harvesting and top killing can reduce scurf