True loose smut (Ustilago nuda)

True loose smut: biology

The true loose smut fungus is seed-borne, and as such it over-winters in the seed.

Seed-borne fungal mycelium establishes itself in the embryo of the seed at flowering. As the seed matures, the mycelium becomes dormant. When infected seed germinates the following spring, the mycelium begins to grow and penetrates the growing point.

Barley & wheat plants are infected during flowering by wind blown spores from nearby smutted heads. Rain and insects may also disseminate spores at this time.

True loose smut: damage description

The black spores of true loose smut on barley

Kernels covered in black spores

Loss of kernels on true loose smut infected heads

The most obvious symptom occurs at heading, when kernels and glumes are converted to masses of black spores. The spores blow away, leaving a bare stem. Heads of infected plants usually emerge from the boot slightly earlier than normal. Prior to heading, plants may have dark green, erect leaves, sometimes with yellow streaks. Results in complete loss of kernels on infected heads. Spores are dispersed before harvest, therefore harvested seed quality is not affected.

True loose smut: management

1. Grow loose smut resistant cultivars. The winter wheat cultivars available for production in western Canada are susceptible to loose smut. Therefore, control of loose smut in this region is presently dependent upon the use of smut-free seed and fungicide seed treatments.
2. Plant smut-free seed. Select seed from fields that were free of loose smut. When own-farm seed stocks are replaced, purchase seed that is certified free of loose smut. Certification is usually based on field inspections that are made prior to harvest to determine the presence of loose smut infected heads. Suspect seed lots can be tested for loose smut by plant disease laboratories.
3. Routinely treat the seed of cultivars that have poor or very poor loose smut resistance with a registered systemic fungicide. Systemic fungicides penetrate the seed coat to provide disease control within the seed and seedling.

Management info source: USask.ca