Potato Diseases

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Potato Diseases

• Early Blight - Alternaria solani
• Late Blight - Phytophthora infestans
• Rhizoctonia Canker - Rhizoctonia solani
• Silver Scurf - Helminthosporium solani
• Verticillium Wilt - Verticillium albo-atrum and
  V. dahliae

• Common Scab - Streptomyces scabies

• Bacterial Black Leg and Soft Rot - Erwinia
  carotovora pvar. carotovora and pvar. atroseptica
• Bacterial Ring Rot - Clavibacter michiganensis
  subsp. sepedonicus

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Early Blight
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Alternaria solani
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Early Blight
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Tuber symptoms
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Hygrothermograph
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Potato Diseases - Early BlightKey Points

• Pathogen Alternaria solani
• Symptoms
• Leaf lesions frequently have "bulls-eye" or
  "target spot" appearance
• Advanced lesions usually have angular margins
  because they are limited by leaf veins
• As infection progresses, entire leaf becomes
  chlorotic, then dies
• The fungus persists in the soil, infected debris,
  and tubers
• The disease is more severe when potato plants are
  under stress, have been injured or have poor
  nutrition

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Potato Diseases - Early Blight Control Strategies

• Protectant fungicides
• apply according to blight forecasting systems for
  maximum efficacy
• Allow tubers to mature in ground before digging
• wait at least 7 days after vine kill before
  harvest.
• Avoid injury of tubers during harvest

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Late Blight
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Late Blight
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Late Blight
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Late Blight
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Late Blight
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Late Blight
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Late Blight
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Late Blight
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Late Blight
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Potato Diseases - Late Blight Key Points

• Pathogen Phytophthora infestans
• Symptoms
• leaf lesions initially small, water-soaked
• expand rapidly to large brown to purplish-black
  lesions killing entire leaflets and spreading
  through petiole to stem, eventually killing
  entire plant
• under moist conditions white velvety area of
  sporangia and sporangiophores visible around edge
  of lesions (mainly on underside of leaf)
• Disease progresses very quickly under optimum
  conditions.
• Fungus overwinters in infected tubers (cull piles
  or infected seed tubers)
• Also infects tomato and other solanaceous crops
  and weeds such as hairy nightshade

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Potato Diseases - Late BlightControl Strategies

• Eliminate cull piles
• Control weed hosts
• Eliminate volunteer potatoes
• Plant disease-free seed
• Treat crop with currently registered fungicides
• Combine with disease forecasting
• Soil coverage by means of adequate hilling can
  reduce infection of tubers
• Kill vines two weeks before harvest so sporangia
  on infected plant tissue die
• Check and remove infected tubers at harvest
• Monitor stored potatoes for decay

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Rhizoctonia Canker
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Rhizoctonia Canker
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Potato Diseases - Rhizoctonia CankerKey Points

• Pathogen Rhizoctonia solani
• Symptoms
• plants most severely damaged in spring just after
  planting
• stem lesions are reddish-brown, may girdle stem
  leading to
• death of shoot (before or after emergence)
• stunting of plant
• aerial tubers
• leafroll symptoms
• discoloration of foliage, chlorosis
• tuber symptoms black or dark brown sclerotia on
  surface
• Can overwinter as sclerotia on tubers or in the
  soil, or as mycelium on plant debris.
• Low soil temperatures and high soil moisture
  favor infection

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Potato Diseases - Rhizoctonia CankerControl
Strategies

• Crop rotation
• Use disease free seed pieces
• Plant on well drained soil
• Deep planting increases risk of stem infection

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Silver Scurf
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Silver Scurf
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Potato Diseases - Silver ScurfKey Points

• Pathogen Helminthosporium solani
• Symptoms
• light brown, circular spots with indistinct
  borders may enlarge to cover considerable area of
  tuber
• affected areas have distinct silvery sheen
• tubers more likely to shrivel in storage
• Disease found world wide wherever potatoes are
  grown.
• Transmission is primarily from infected seed
  pieces
• The disease continues in storage as long as
  conditions are favorable

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Potato Diseases - Silver ScurfControl Strategies

• Use disease free seed
• Treat with with currently registered fungicides
• Harvest tubers as soon as they are mature
• Avoid free moisture on tubers

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Verticillium Wilt
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Verticillium Wilt
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Verticillium Wilt
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Potato Diseases - Verticillium WiltKey Points

• Pathogens Verticillium albo-atrum and V.
  dahliae
• Symptoms
• early senescence of plants - leaves become
  yellow, die
• single stems or one side of stem often affected
  first
• brown vascular discoloration
• Soil fungi that infect many species and can
  survive in the soil in the form of microsclerotia
• Wilting first occurs on hot dry days
• Inoculum can be distributed by movement of
  contaminated soil which adheres to seed tubers or
  equipment
• Incidence of Verticillium Wilt is increased with
  the presence of parasitic nematodes

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Potato Diseases - Verticillium WiltControl
Strategies

• Rotate with non-host crops such as cereals,
  grasses and legumes
• Control weed hosts
• Treat soil with a currently available fumigant if
  pathogen population is above economic threshold
• Avoid over irrigation during period of emergence
  to tuberization

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Common Scab
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Common Scab
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Potato Diseases - Common ScabKey Points

• Pathogen Streptomyces scabies
• Most common on soils with pH 5.5-7.5
• Warm, dry soils and early season stress favor
  infection and lesion development
• Soil borne and tuber borne
• May be introduced on seed potatoes
• Limited host range - carrot, beet, parsnips,
  radishes, rutabagas, spinach, sugar beets,
  turnips
• Tubers susceptible as soon as they form
• Infection through lenticels
• Once periderm forms, tubers no longer susceptible
• Survives digestive tract of animals, distributed
  in manure

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Potato Diseases - Common ScabControl Strategies

• Manure applied to soil may favor scab infection
• Consider 3-5 year rotations - alfalfa, corn,
  peas, rye, snap beans, soybeans
• Adjust soil pH to 5.2-5.8
• Avoid moisture stress especially at tuber
  initiation and early tuber growth
• Some cultivars with high tolerance e.g. Frontier
  Russet, RedDale, Russet Burbank, Superior
• Treatment of seed with mancozeb fungicide dust
  reduces scab
• In-furrow application of Mocap granular
  insecticide reduces scab

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Bacterial Black Leg Soft Rot
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Black Leg
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Bacterial Soft Rot
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Potato Diseases - Bacterial Black Leg and Soft
Rot - Key Points

• Pathogens Erwinia carotovora pvar. carotovora
  and pvar. atroseptica
• Symptoms
• Soft rot wet, slimy cream to tan colored decay,
  with distinct margins. May become brown to
  black, foul smelling with ropy consistency with
  invasion of secondary organisms
• Black leg black, wet looking decay of stem,
  moving from seedpiece up stem. Plants commonly
  stunted, wilted
• Bacteria overwinter on or in tubers in storage
• Primary inoculum is in the tuber seed piece
• Bacteria enter lenticels, wounds or growth cracks
• Bacteria can move in soil water
• Tubers can become infected in storage if inoculum
  runs onto clean potatoes and enters through
  lenticels or wounds

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Potato Diseases - Bacterial Black Legand Soft
Rot - Control Strategies

• Plant in well drained soil
• Do not over irrigate
• Cut tuber seed pieces 3-5 days before planting,
  keep at 55-60 F and 90 RH to allow cut surfaces
  to suberize before planting.
• Disinfest seed piece cutting tools
• Harvest tubers only when mature and when soil
  temperatures are less than 68 F
• Do not wash tubers before storage
• Keep storage temperatures from 34 to 40 F in a
  well ventilated area

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Bacterial Ring Rot
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Bacterial Ring Rot
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Potato Diseases - Bacterial Ring RotKey Points

• The pathogen - a bacterium, Clavibacter
  michiganensis subsp. sepedonicus (formerly
  Corynebacterium sepedonicum)
• Symptoms
• wilting of leaves and stems after midseason
• often affects only one or two stems of a plant
• lower leaves, slightly rolled at the margins and
  pale green usually first to wilt
• milky white exudate can be squeezed from vascular
  system of tubers
• Symptom expression is most common following warm
  dry weather
• Overwinters in tubers either in storage or left
  behind in the soil
• Contaminated knives or picking equipment can
  disseminate the inoculum
• Infection occurs through wounds of any plant part

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Potato Diseases - Bacterial Ring RotControl
Strategies

• Use certified, disease free seed and follow
  careful sanitation procedures
• Dispose of all tubers if the disease is found in
  the crop
• Disinfest all equipment used for growing and
  harvesting the crop
• Use clean new bags for each new years crop