In this phase of the disease the fungus forms dark brown to black hard masses on the surface of the tuber.
These are called sclerotia and are resting bodies of the fungus.
Although black scurf is the most noticeable sign of Rhizoctonia, stem canker, is the most damaging of the disease as it occurs underground and often goes unnoticed .
Early in the growing season, the fungus attacks germinating sprouts underground before they emerge from the soil.
The sprout may be killed outright if lesions form near the growing tip.
Damage at this stage results in delayed emergence and is expressed as poor and uneven stands with weakened plants