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Extension > Garden > Diagnose a problem > What's wrong with my plant? > Vegetable > Potato > Flecks, spots, streaks, or blotches on stem

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Potato > Stem > Flecks, spots, streaks, or blotches on stem

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  • Early Blight 1
  • Early Blight 2

Early Blight
Alternaria solani

  • Stem lesions are dark, slightly sunken areas
  • 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
  • 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
  • More prevalent in warm and humid weather
  • More information on Early Blight...
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  • Late Blight 1
  • Late Blight 2

Late Blight
Phytophthora infestans

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

White Mold
Sclerotinia sclerotiorum

  • Elongated tan dry lesions form on stems
  • Leaves wilt but remain green
  • Fluffy white fungal growth and small hard black fungal sclerotia can be found on or in infected stems
  • More information on White Mold...
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  • Black Dot 1
  • Black Dot 2
  • Black Dot 3

Black Dot
Colletotrichum coccodes

  • Sunken dark brown to black lesion on stem that turns white in the center with age
  • Irregular patches of gray discoloration on surface of the potato tuber
  • 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...
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  • Blackleg & Soft Rot 1

Blackleg & Soft Rot
Pectobacterium carotovora

  • Brown to black slimy sunken lesions on stems, most common starting at the soil line
  • Seed potato may rot in the soil, plants do not emerge
  • In severe cases, plant is stunted, with yellow and wilted leaves
  • Tuber flesh is white, soft and rotted. In severe cases the entire tuber rots away, leaving on the skin
  • Favored by cool wet weather
  • More information on Blackleg & Soft Rot...
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  • Black Scurf/Rhizoctonia Canker 1
  • Black Scurf/Rhizoctonia Canker 2
  • Black Scurf/Rhizoctonia Canker 3

Black Scurf/Rhizoctonia Canker
Rhizoctonia solani

  • Stem has a white to gray flaky growth just above the soil line.
  • Underground stems and roots have brown, sunken dry spots
  • Late or no emergence, have few stems, and poor growth
  • Small hard black spots stick to the skin of the tuber like dirt that won't wash off
  • Tuber production is low, tubers may be small and deformed
  • In severe cases, small green tubers form on the stem above ground
  • Most problematic in cool wet soils
  • More information on Black Scurf/Rhizoctonia Canker...
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  • Corky Ring Spot 1
  • Corky Ring Spot 2
  • Corky Ring Spot 3

Corky Ring Spot
Tobacco Rattle Virus

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

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