Pathogen. The black dot fungus persists as tiny sclerotia on the surface of tubers or in plant debris in the soil.

Disease development. The black dot fungus needs a wound to infect potato plants. If the fungus enters underground stems, it moves upward. If infections start in injured leaves or in stems, the disease progresses downward into the stems and into the roots.
The disease is associated with sandy soils, low nitrogen, high temperatures and poor soil drainage.

Symptoms. The top leaves in the canopy turn yellow and wilt in mid- to late summer. The symptoms progress down the stem to the middle and lower leaves. Wilt caused by black dot may be confused with Verticillium wilt. Black dot reduces yield by killing stems before the tubers size.

|Initial symptoms of black dot on stems could appear
as dark brown lesions that can be confused with late
blight Lesions often develop at the base of the stems
Older lesions have a bleached appearance
Stems completely covered with sclerotia
turn black
Yellowing and wilting of top leaves
Tiny sclerotia are produced on the stem surface
As stems become dry the internal tissue flakes away easily The interior of the vascular ring turns purple
Sclerotia formed inside an old stem
Close up of sclerotia produced inside a stem
Sclerotia are produced within the discoloured
areas of the tubers
Infected roots may appear purplish
Infected stolons are purplish and may remain
attached to the tubers
Black dot produces brown lesions on the skin The lesions are usually sunken
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T.G. Lynn
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