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Extension > Garden > Diagnose a problem > What's wrong with my plant? > Vegetable > Potato > Spots on tuber

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Potato > Tuber > Spots on tuber

1 of 8
  • Common Scab 1
  • Common Scab 2

Common Scab
Streptomyces spp.

  • Round rough, brown spots on tuber surface
  • Spots may be slightly raised and rough or slightly sunken depending on cultivar
  • Disease only affects tubers
  • Disease is favored by alkaline and dry soils
  • More information on Common Scab...
2 of 8
  • Early Blight 1
  • Early Blight 2

Early Blight
Alternaria solani

  • Potato tubers develop dark, sunken spots surrounded by a purplish brown raised border
  • Rough, corky dry tissue can be seen below tuber spots when cut open
  • Leaf spots are round, brown with target like rings inside
  • When severe many leaf spots grow together resulting in large brown patches and leaves turn yellow
  • Stem lesions are dark, slightly sunken areas
  • More prevalent in warm and humid weather
  • More information on Early Blight...
3 of 8
  • Late Blight 1
  • Late Blight 2
  • Late Blight 3

Late Blight
Phytophthora infestans

  • Irregular sunken, dull brown to purple lesions around the eyes on potato tubers
  • Inside of potato tuber has a reddish-brown dry rot
  • Large irregular olive to brown leaf spots that are not restricted by leaf veins
  • In wet weather, leaves, stems and petioles rapidly turn brown and shrivel
  • Powdery white fungal growth can be seen on infected tissue in wet weather
  • More prevalent in years of cool and wet weather
  • More information on Late Blight...
4 of 8
  • Black Scurf/Rhizoctonia Canker 1
  • Black Scurf/Rhizoctonia Canker 2
  • Black Scurf/Rhizoctonia Canker 3

Black Scurf/Rhizoctonia Canker
Rhizoctonia solani

  • Small hard black spots stick to the skin of the tuber like dirt that won't wash off
  • Delayed or no emergence, have few stems, and poor growth
  • Underground stems and roots have brown, sunken dry spots
  • Tuber production is low, tubers may be small and deformed
  • In severe cases, small green tubers form on the stem above ground
  • Stem has a white to gray flaky growth just above the soil line.
  • Most problematic in cool wet soils
  • More information on Black Scurf/Rhizoctonia Canker...
5 of 8
  • Fusarium Dry Rot

Fusarium Dry Rot
Fusarium sp.

  • Tuber surface has sunken dark spots, eventually becoming wrinkled in concentric rings
  • In storage, tuber develops a dry decayed cavity often with white fluffy fungal growth or pink to yellow spores inside
  • Plant may not emerge, or may emerge small and weak
  • More information on Fusarium Dry Rot...
6 of 8
  • Black Dot 1
  • Black Dot 2
  • Black Dot 3

Black Dot
Colletotrichum coccodes

  • Irregular patches of gray discoloration on surface of the potato tuber
  • Sunken dark brown to black lesion on stem that turns white in the center with age
  • Many pinprick sized black dots can be seen on infected stems and tubers (handlens is helpful)
  • Leaves may have random brown spots, or may yellow and wilt due to infection of stem and roots
  • More information on Black Dot...
7 of 8
  • Corky Ring Spot 1
  • Corky Ring Spot 2
  • Corky Ring Spot 3

Corky Ring Spot
Tobacco Rattle Virus

  • Potato tubers can be cracked and malformed if infections comes in early
  • Brown rings and arcs form on the skin and in the flesh of tubers
  • Leaves may have no symptoms or have yellow rings, spots and arcs
  • Stems may be mottled green and yellow
  • More information on Corky Ring Spot...
8 of 8
  • Potato Virus Y 1
  • Potato Virus Y 2

Potato Virus Y (PVY)

  • In severe cases, light brown rings form on tuber skin
  • Leaf symptoms vary greatly depending on strain of virus present
  • Light and dark green mosaic pattern on leaves, occasionally leaves crinkle
  • More information on Potato Virus Y (PVY)...

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