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Ministry of Agriculture, Food and Rural Affairs

Black dot

Older lesions have a bleached appearance Initial symptoms of black dot on stems Black dot sclerotia on the stem surface Internal symptoms in stem Black dot sclerotia inside an old stem Advanced symptoms on stem Infected roots may appear purplish Infected stolons are purplish Lesions on the tuber skin Sclerotia in the discoloured tuber tissue
Click to enlarge.

Beginner

Scientific Name
Colletotrichum coccodes

Identification
The top leaves in the canopy turn yellow and wilt in mid- to late summer, as do plants infected with Verticillium.
Tiny dot-like sclerotia may develop on stems both above and below ground level. Sclerotia are easily seen after vine kill. Sclerotia can also form on tubers, stolons and roots.

Brownish to gray discolored areas develop on tubers. Tiny sclerotia form on those infected areas.
Infected stolons, roots and stem pith may turn reddish-purple.

Often Confused With
Verticillium wilt on leaves
Silver scurf on tubers

Period of Activity
Infection of below ground plant parts may start early in the season especially when plants are under stress. The infection continues throughout the season if crop stress persists.

Scouting Notes

  • Scouting should start before the rows close.
  • Fields should be monitored at least twice a week.
  • When walking fields, stop at many sites as possible and check plants at random.
  • Always check areas of poor drainage.

Thresholds
It is a soil borne fungus. Scouting is essential to determine the initial level of disease incidence so that growers can make management decisions.

Advanced

Scientific Name
Colletotrichum coccodes

Identification
The top leaves in the canopy turn yellow and wilt in mid- to late summer, as do plants infected with Verticillium.
Tiny dot-like sclerotia may develop on stems both above and below ground level. Sclerotia are easily seen after vine kill. Sclerotia can also form on tubers, stolons and roots.

Brownish to gray discolored areas develop on tubers. Tiny sclerotia form on those infected areas.
Infected stolons, roots and stem pith may turn reddish-purple.

Often Confused With
Verticillium wilt on leaves
Silver scurf on tubers

Biology
The black dot fungus has a very wide host range, thus, it is found in most soils where potatoes are grown.

The black dot fungus is both tuber and soil-borne. It persists as tiny propagules called microsclerotia on the surface of tubers or in plant debris in the soil. 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.

Period of Activity
Infection of below ground plant parts may start early in the season especially when plants are under stress. The infection continues throughout the season if crop stress persists.

Scouting Notes
  • Scouting should start before the rows close.
  • Fields should be monitored at least twice a week.
  • When walking fields, stop at many sites as possible and check plants at random.
  • Always check areas of poor drainage.
Thresholds
It is a soil borne fungus. Scouting is essential to determine the initial level of disease incidence so that growers can make management decisions.

Management Notes

  • Use healthy seed
  • Select well-drained fields.
  • Avoid crop stress:
    • Provide adequate levels of nutrients
    • Manage other pests
    • Match irrigation with crop needs. Do not over-irrigate.
    • Harvest tubers when the skin is set and avoid mechanical injury.