Loose Smut

Pathogen

  • Ustilago nuda f.sp. tritici (U. tritici) - Wheat
  • Ustilago nuda f.sp.hordei - Barley
  • Ustilago avenae - Oats

Hosts

The disease affects wheat, barley and oats.

There are distinct forms of the pathogen which are crop specific.

Symptoms

Loose smut is easily recognised at ear emergence as the each grain is usually completely replaced by a mass of black fungal spores.  Partly affected ears are sometimes seen. The spores are released as soon as the ear emerges, leaving only the bare remains of the ear rachis. Because the blackened ears are so very obvious in the crop at ear emergence the disease appears to be very severe, even at very low incidence levels.

Life Cycle

Loose_smut-life-cycle.gif

Spores are released from infected ears and are carried by the wind to the open flowers of surrounding healthy plants. There they germinate and the fungus grows into the developing grain site. Weather conditions during flowering affect the length of time that the florets remain open and hence the time that the plant is susceptible to infection. Thus, the likely level of infection varies considerably from season to season. The fungus lies dormant within the embryo of the seed until the seeds are sown and germinate. When the infected seed germinates the fungus grows within the developing shoot, eventually reaching the ear primordia . The fungus develops within the young ear, eventually replacing spikelets with masses of fungal spores which are released once again as the ear emerges.

Importance

The UK Seed Certification Scheme is undoubtedly successful in ensuring that loose smut remains at very low levels in UK seed stocks and is of low importance. Seed crops grown under the scheme are inspected for loose smut and because the disease is so easily seen at low levels it can be detected by visual examination.

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