Skip to Main navigation Skip to Left navigation Skip to Main content Skip to Footer

University of Minnesota Extension
We're looking for your feedback on the Extension website. Please take our survey.

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

Print Icon Email Icon Share Icon

Cucumber > Leaves > Spots on leaves

1 of 9
  • Image: Powdery Mildew 1
  • Image: Powdery Mildew 2
  • Image: Powdery Mildew 3

Powdery Mildew
Podosphaera xanthii

  • Initially pale yellow spots develop on the oldest leaves
  • White to gray powdery felt in spots, blotches or complete covering of leaves
  • Lesions develop on underside of leaves, or on shaded leaves first
  • Older leaves infected first
  • Severe infection can cause leaves and vines to wither and die
  • Fruit may be blotchy or have sunscald
  • More information on Powdery Mildew...
2 of 9
  • Image: Anthracnose 1
  • Image: Anthracnose 2
  • Image: Anthracnose 3

Colletotrichum obriculare

  • Somewhat circular dry brown leaf spots form near veins
  • Center of leaf spot often falls out, leaves appear ‘shot hole’
  • Circular sunken dark spots on fruit, salmon colored spores form in the center when wet
  • Commonly appears mid to late season
  • Also commonly infects melons and pumpkin
  • More information on Anthracnose...
3 of 9
  • Image: Alternaria Leaf Blight 1
  • Image: Alternaria Leaf Blight 3
  • Image: Alternaria Leaf Blight 5

Alternaria Leaf Blight
Alternaria spp.

  • Leaf spots start as small flecks, 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, wither and die
  • Most common on muskmelon (cantaloupe), but can also infect watermelon, squash, and cucumber
  • More information on Alternaria Leaf Blight...
4 of 9
  • Image: Angular Leaf Spot 1
  • Image: Angular Leaf Spot 2
  • Image:Angular Leaf Spot 3

Angular Leaf Spot
Pseudomonas syringae pv. Lachrymans

  • Leaves develop small, angular, brown or straw-colored spots with a yellow halo
  • Leaf spots dry and drop out, leaving irregularly shaped holes in the leaves
  • Water soaked to tan small circular spots on fruit
  • Sticky drops of whitish liquid for on the underside of the leaf when wet, dry to a crust when dry
  • More information on Angular Leaf Spot...
5 of 9
  • Image: Scab 1
  • Image: Scab 2
  • Image: Scab 3

Cladosporium cucumerinum

  • Irregular gray leaf spot with a yellow halo
  • Center of leaf spot falls out, leaves look shot holed or ragged
  • Cucumber, summer squash, zucchini, & pumpkin have sunken fruit spots are covered with greenish black velvety fungal growth
  • Winter squash have raised corky areas around fruit infections, internal rot varies depending on resistance of variety
  • Watermelon is very resistant
  • More information on Scab...
6 of 9
  • Image: Phytophthora Blight 1
  • Image: Phytophthora Blight 2
  • Image: Phytophthora Blight 3

Phytophthora Blight
Phytophthora capsici

  • Large irregular brown spots on leaves
  • Stem and leaf petiole lesions are light to dark brown, water soaked and irregular
  • The entire plant may collapse if root and crown rot occurs
  • Fruit develop soft, water-soaked lesions which expand to large sections of the fruit
  • Infected fruit are soft, easily punctured and often collapse
  • Infected fruit are covered with white fungal growth
  • More information on Phytophthora Blight...
7 of 9
  • Image: Gummy Stem Blight and Black Rot 2
  • Image: Gummy Stem Blight and Black Rot 3

Gummy Stem Blight and Black Rot
Didymella bryoniae

  • Leaf symptoms vary from browning at leaf edges and between veins to circular tan to brown spots
  • Yellow halos may occur around leaf spots and older spots are often dry and cracked
  • Tan lesion on stems, often with black spots and dark gummy exudate
  • Fruit rot initially looks water soaked and eventually turns completely black
  • Tiny black dots can be seen in fruit infections
  • Fruit infection often starts on the side of the fruit touching the soil
  • Fruit often collapse due to other rotting organisms
  • More information on Gummy Stem Blight and Black Rot...
8 of 9
  • Image: Fourlined Plant Bug 1
  • Image: Fourlined Plant Bug 2
  • Image: Fourlined Plant Bug 3

Fourlined Plant Bug
Poecilocapus lineatus

  • Feeding causes brownish or blackish sunken spots on leaves
  • Nymphs are red or red and black; Adults greenish yellow with 4 black stripes, 1/4 - 1/3 inch long
  • Occurs May to early July
  • More information on Fourlined Plant Bug...
9 of 9
  • Image: Downy Mildew 1
  • Image: Downy Mildew 2
  • Image: Downy Mildew 3

Downy Mildew
Pseudoperonospora cubensis

  • Pale green to yellow spots on upper surface of leaves, later turn brown
  • Angular spots bounded by leaf veins
  • Dark purplish grey fuzz on underside of leaf when high humidity
  • In wet or very humid conditions disease progresses rapidly, leaves turn brown and look like they were killed by frost
  • On watermelons, an exaggerated upward leaf curling is common
  • Cucumber and muskmelon most susceptible
  • More information on Downy Mildew...

Don't see what you're looking for?

  • © Regents of the University of Minnesota. All rights reserved.
  • The University of Minnesota is an equal opportunity educator and employer. Privacy