Disease Control . Fungicide sprays for other diseases should be discontinued once bulbs have reached full size. At harvest, tops should be cut close to the bulb. Onion Smut Introduction Onion smut is a disease common to temperate growing regions, especially where onions are grown from seed. Most onions and related crops are susceptible to smut.
Lesions appear as dark brown streaks running up and down the leaves.
The streaks initially appear as long blisters on the leaf surface. As the lesions mature, they turn brown and contain a mass of dark powdery spores that give the tops a sooty appearance. Diseased leaves may bend or twist abnormally and usually are shed prematurely. Smut infected plants normally are stunted and produce bulbs highly prone to soft rot. Disease Cycle .