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Extension > Garden > Diagnose a problem > What's wrong with my plant? > Vegetable > Summer Squash and Zucchini > Spots on leaves

Summer Squash and Zucchini > Leaves > Spots on leaves

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  • 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...
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  • Image: Anthracnose 1
  • Image: Anthracnose 2
  • Image: Anthracnose 3

Anthracnose
Colletotrichum obriculare

  • Irregular brown leaf spots
  • Circular sunken dark spots on fruit up to ½” across
  • Salmon colored spores form in the center of fruit spots when wet
  • Commonly appears mid to late season
  • Also common on melons and cucumber
  • More information on Anthracnose...
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  • 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...
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  • 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...
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  • Image: Scab 1
  • Image: Scab 2
  • Image: Scab 3

Scab
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...
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  • 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...
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  • 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...
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  • 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...
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  • 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...

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