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Alternaria leaf blight

Quick facts

  • This disease is most common in melon, but can also affect cucumber, pumpkin and squash.
  • Alternaria leaf blight first infects mature leaves near the crown of the plant. Fruit can suffer from sunscald due to leaf loss.
  • Wet, rainy weather favors diseases. Damage can be very severe in warm, wet conditions.
  • The fungus survives from season to season in plant debris.
  • Do not work in plants when wet.
  • Rotate vegetables so that a location goes three or more years without growing any member of the squash family.

The fungus Alternaria cucumerina causes Alternaria leaf blight. This disease is most common melon, but can also affect cucumber, pumpkin and squash. Alternaria leaf blight does not commonly infect fruit. It can reduce yield and quality through reduced plant vigor and sunscald of exposed fruit.

Identifying alternaria leaf blight symptoms

  • Alternaria leaf blight first infects mature leaves near the crown of the plant.
  • Leaf spots start as small brown spots, often with a yellow halo, and grow into irregular brown spots (up to 3/4").
  • Leaf spots sometimes develop a target-like pattern of rings.
  • Severely infected leaves turn brown, curl upward, wither and die.
  • Fruit can suffer from sunscald due to leaf loss. They are not common victims.

What causes alternaria leaf blight

  • Wind currents can carry Alternaria cucumerina a long distance.
  • Alternaria cucumerina can also spread within the field by splashing water.
  • Wet, rainy weather favors diseases. Damage can be very severe in warm, wet conditions.
  • The fungus survives from season to season in plant debris.

Preventing and managing the disease

  • Rotate vegetables so that a location goes three or more years without growing any member of the squash family.
  • Use drip irrigation instead of overhead sprinklers if possible.
  • Do not work in plants when wet.
  • Remove and destroy infected plants at the end of the season in small gardens.
  • In large fields, till under crop residue at the end of the season.
  • Several fungicides are available for use against Alternaria leaf blight. Preventative sprays are effective but are only necessary in fields with a history of Alternaria leaf blight.
  • Commercial growers should refer to the Midwest Vegetable Production Guide for specific fungicide recommendations.

CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.

Michelle Grabowski

Reviewed in 2018

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