Cause Urocystis magica (syn. = U. cepulae) and U. colchici, fungi that survive many years saprophytically in infested soil. They attack only members of the onion family including onion, leek, and Welsh onion and are more a problem on muck soils. Susceptibility decreases as tissue ages. Affected bulbs are predisposed to other infections in storage.
Symptoms The disease appears in the plant stem as it emerges. Infected areas have a raised, blister-like appearance near ground level. Blisters rupture to expose black powdery spore masses. Many seedlings die during emergence; those that live produce distorted bulbs with darkish streaks and numerous smutty lesions.
Cultural control Small onion sets and young onion plants may be set out in infested soil without apparent danger of infection.
Chemical control
- Seed treatment.
- Thiram 50WP Dyed (Group M3) at 3 oz/100 lb seed (not labeled for Oregon) or 42-S Thiram (Group M3) at 6 fl oz/100 lb seed plus a dye. See label for reentry restrictions.
- Pro-Gro (Group 7 + M3) at 2.5 lb/100 lb seed by professional applicators only. See label for reentry restrictions.
- Control smut by an in-furrow drench at seeding.
- Dithane DF Rainshield (Group M3) at 3 lb/A in 75 to 125 gal/A water. Dry bulb onions only. 24-hr reentry.