Onion
Symptom Cause  
Flowers, leaves, roots chewed
Blister Beetles
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Sunken spots on leaves; later, a purplish mold develops over spots
Downy Mildew
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Yellowing of foliage and gradual defoliation; plants may become stunted
Fusarium Wilt
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Plant stunted, leaves yellowed, lesions on roots
Nematodes
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White streaks on leaves; tips distorted; plant may wilt, whiter, turn brown; bulbs small
Onion Thrips
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Roots turn pinkish or red, eventually rot; plant is stunted with wilted tops
Pink Rot
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Onion bulbs damaged or gone
Pocket Gophers
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Young plants wilt in heat, may yellow/die
Root Maggots
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Orange or brown pustules on leaves
Rust (on plants)
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Block spots on leaves/between the sections of the bulbs; plants may have twisted leaves
Smut (fungus)
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Plant wilts, underground stems and roots chewed.
Wireworms
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