Onion smut: Streaks on bulb
Pathogen: Urocystis colchici and U. cepulae
Symptoms
Dark areas can be seen first on cotyledons soon after their emergence from soil. On older plants raised, blister-like lesions can occur near the base of the scales, and large lesions cause leaves to curve downward. Streaks may develop within the leaves, leaf sheaths and bulbs. Mature lesions contain a black, powdery mass of spores.
Disease Development
The fungus can over-winter as resting spores in the soil for several years. Spread of the fungus occurs through infected onion sets, transplants and when spores are transported by wind, equipment and water. Onion seedlings are susceptible to infection from just after germination until they reach the first true leaf stage. As each new leaf emerges it goes through a growth phase where it is susceptible to infection. After that growth phase, infection does not occur. Optimum temperatures for spore germination and growth are 13-22°C (56-72°F) while both are decreased above 25°C (77°F).
Disease Management
Chemical seed treatments can protect seedlings through the susceptible stage. In addition, any cultural practice that is favorable for rapid growth can shorten the susceptible stage of the onions. Healthy onion sets and transplants that are planted into infested soil may escape infection. A crop rotation out of onions for three or more years also reduces disease.
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