Symptom Cause  
Sunken spots on leaves; later, a purplish mold develops over spots
Downy Mildew
Plant grows slowly; turn yellow; wilt, die
Onion Maggot
White streaks, blotches on leaves; tips distorted, plants wilt, wither, brown, die
Onion Thrips
Roots turn pinkish or red, eventually rot; plant is stunted with wilted tops
Pink Rot
Block spots on leaves/between the sections of the bulbs; plants have twisted leaves
Smut (fungus)