Cucumber
Symptom Cause  
Water-soaked spots on leaves
Angular Leaf Spots
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Small, dark spots on leaves; eventually leaves turn black; fruits may blacken/drop
Anthracnose (on plants)
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Leaves wrinkled or curled; discolored, stunted, tend to fall off
Aphids
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Plants wilt quickly; white, sticky material might be seen when a stem is cut
Bacterial Wilt
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Bites taken out of cucumber
Birds
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Holes chewed in leaves, leaf stalk, stems; plant wilts, dies.
Cucumber Beetles
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Plants disappear all or in part
Deer
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White powder on leaf undersides; foliage wilts
Downy Mildew
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Leaves riddled with shot holes; weak plant
Flea Beetles
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Yellowing of the leaf margins (chlorosis) is an early symptom on the plant, and light- to dark-brown spots (necrosis) can appear, plus water-soaked regions on the stem.
Gummy Stem Blight (black rot)
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Flowers and new fruits damaged
Melonworms
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Yellow-green mottling of leaves
Mosaic Viruses
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Leaves pale; turning yellow
Nitrogen Deficiency
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Flowers and new fruits damaged
Pickleworms
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White spots on leaves; eventually entire leaf is covered with powder; fruits ripen prematurely and have poor flavor/texture
Powdery Mildew
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Cucumbers disappear
Raccoons
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Sunken green spots on cucumbers
Scab
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Large ragged holes in middle of leaves
Slugs
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Leaves stippled, yellowing or bronze; dry out and drop may be webbing visible; sometimes galls form on leaves
Spider Mites
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Wilted; dried-up leaves
Squash Bugs
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Sudden wilting of plants, leaves turn black at edges, sawdust material at stem base
Squash Vine Borers
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Bites taken out of cucumbers
Squirrels
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Cucumbers taste bitter
Various Plant Stresses
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Yellowing of foliage and gradual defoliation; plants may become stunted
Verticillium Wilt
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Weakened plant, leaves yellowed, appears like flying dandruff when foliage disturbed
Whiteflies
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Plants disappear all or in part
Woodchucks
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