Onion Diseases

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Onion Diseases

• Botrytis Blight - Botrytis squamosa
• Botrytis Neck Rot - Botrytis allii
• Purple Blotch - Alternaria porri
• Fusarium Basal Rot - Fusarium spp.
• Downy Mildew - Peronospora destructor

• Translucent Scale

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Onion Diseases - Botrytis Blight
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Onion Diseases - Botrytis Blight
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Onion Diseases - Botrytis Blight
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Onion Diseases - Botrytis BlightKey Points

• Pathogen Botrytis squamosa
• Symptoms and signs
• white necrotic lesions on leaves, surrounded by a
  whitish halo
• ozone injury symptoms may mimic Botrytis symptoms
  but no halos are present around lesions
• fungus produces weather-resistant sclerotia
• Overwintering
• onion cull piles and infected debris in soil
• sclerotia survive in soil for long periods of time

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Onion Diseases - Botrytis BlightControl
Strategies

• Fungicide sprays
• Elimination of cull piles
• Long rotations with non-host crops
• Wide row spacing
• Irrigation early in the day

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Onion Diseases - Botrytis Neck Rot
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Onion Diseases - Botrytis Neck Rot
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Onion Diseases - Botrytis Neck RotKey Points

• Pathogen Botrytis allii
• Symptoms and Signs
• Generally do not appear until bulbs are in
  storage
• Collapse of neck tissue
• Internal watery decay
• Sclerotia of the fungus frequently found on the
  neck of onion
• Overwintering
• Sclerotia survive in soil for long periods

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Onion Diseases - Botrytis Neck RotControl
Strategies

• Cut roots under bulbs before harvest
• Avoid damaging necks or bulbs at harvest
• cutting necks 1 above bulbs is preferable to
  ripping off tops
• Rapid and proper curing is important
• Proper storage conditions
• just above freezing
• prevent moisture condensation on bulbs

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Onion Diseases - Purple Blotch
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Onion Diseases - Purple Blotch
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Onion Diseases - Purple BlotchKey Points

• Pathogen Alternaria porri
• Symptoms and signs
• watersoaked areas followed by white necrotic
  spots on foliage
• girdling lesions on the stem
• black concentric rings on purplish lesions
• fungus requires rain or persistent dew for
  reproduction and penetration of plant tissue
• Overwintering
• fungus can survive in crop debris

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Onion Diseases - Purple Blotch Control Strategies

• Rotation with non-host crops
• Maintain good air movement around foliage
• Commercial growers can use currently registered
  fungicides

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Onion Diseases Fusarium Basal Rot
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Onion Diseases Fusarium Basal Rot Key Points

• Pathogen Fusarium oxysporum f. sp. cepae
• Symptoms
• Leaf tip yellowing and necrosis
• Affected plants may wilt
• Decay of bulbs progressing up from basal plate
• White mycelium on and surrounding basal plate
• Pathogen is soil borne
• Commonly introduced into home gardens on onion
  sets
• Infection increased by insect injury to roots,
  stem plate or bulb

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Onion Diseases Fusarium Basal Rot Control
Strategies

• Crop rotation minimum of 4 years between onion
  or garlic crops
• Plant disease free onion sets into home garden
• Careful sorting of onion sets before planting
  helps to eliminate diseased bulbs
• Some disease resistant cultivars available

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Onion Diseases Translucent Scale
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Onion Diseases Translucent Scale
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Onion Diseases Translucent Scale Key Points

• Cause Frequent high temperatures (gt90? F) and
  high relative humidity during the last 6-8 weeks
  of growth
• Symptoms
• Scales are translucent and resemble freezing
  injury
• Scales become grayish and water soaked
• In cross section, affected scales are brownish
• Symptoms are most common on outer scales
• Affected scales shrink, giving bulb slightly
  irregular surface
• Symptoms continue to progress in storage
• Delays of 2-4 weeks between curing and cold
  storage can increase incidence

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Onion Diseases Translucent Scale Control
Strategies

• Little can be done to change growing environment
• Affected onions should be cured and temperatures
  in storage rapidly decreased to just above the
  freezing mark to minimize losses
• If observed in early storage, prolonged storage
  will result in additional losses

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Onion Diseases Downy Mildew
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Onion Diseases Downy Mildew
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Onion Diseases Downy Mildew Key Points

• Pathogen Peronospora destructor
• Symptoms
• Elongated leaf patches that are slightly paler
  green or tan
• When wet with dew, grayish violet and furry
  growth visible on lesion surface
• Affected leaves become pale green and soon
  collapse
• Favored by cool temperatures and wet weather
• Under favorable conditions, this disease rapidly
  destroys susceptible tissues
• Fungus produces oospores that can be soilborne
  for several years

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Onion Diseases Downy Mildew Control Strategies

• Rotation of at least 3-4 years between onion
  crops
• Prompt tillage after harvest to hasten breakdown
  of onion tissues
• Protectant fungicide sprays are effective
• Careful irrigation to avoid prolonged leaf wetness