Loose smut

Pathogen

Ustilago nuda (barley)

Ustilago tritici (wheat)

Ustilago avenae (oats)

Hosts

The disease affects wheat, barley and oats. There are distinct crop-specific forms of the pathogen.

Symptoms

Usually, a mass of black fungal spores replaces grain, affecting all or part of the ears. Spore release occurs as the ear emerges. This leaves only the bare remains of the ear rachis. Because the blackened ears are obvious, the disease appears to be very severe, even at very low incidence levels.

Life cycle

Wind spreads spores, released from infected ears, to open flowers. Weather affects how long florets remain open and, hence, the time that the plant is susceptible to infection. Consequently, the infection levels vary considerably from season to season. Once spores germinate, the fungus grows in the developing grain site. The fungus lies dormant within the embryo of the seed. 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.

Importance

Most important in UK barley, the disease is rare in wheat. The UK Seed Certification Scheme helps ensure that loose smut remains at very low levels in UK seed stocks. Fungicidal seed treatments also control it well. Infected crops suffer yield loss. Although an infection of 0.1% (1 in 1000 ears affected) looks severe, it only results in a yield loss of 0.1%. However, smutted ears can rapidly spread infection to surrounding crops. The grain of these crops becomes infected which, when re-planted, do not show symptoms until the following spring.

High-risk factors

  • Sowing untreated infected seed
  • Using untested farm-saved seed
  • Varieties with a longer flowering time are more susceptible to infection. Cool, moist conditions can prolong flowering
  • Windy conditions can increase spread

Management

Use clean seed and apply a fungicidal seed treatment to infected seed. These are the most effective methods to control loose smut. Additionally, control volunteers. Foliar fungicide treatments do not control loose smut. Visual examination of loose smut forms part of the UK Seed Certification Scheme. Percentage infection can also be established through seed tests – embryo extraction (takes 48 hours). The maximum acceptable infection is 0.5%. However, seed to be sold at a premium has a lower infection threshold of 0.2% (Higher Voluntary Standard).

Visually inspect farm-saved seed crops prior to harvest. Do not farm-save seed from heavily infected areas (including crops and volunteers). As different loose smut pathogens affect wheat and oats, a high level in barley does not confer risk of higher levels in wheat and oats.

Varietal resistance to loose smut is not tested by the AHDB Recommended List, but varieties planned as seed crops that have an open flowering habit and longer flowering time, can be more at risk.

Summary

  • Inspect seed crops for signs of infection
  • Buy certified seed or test home-saved seed for the presence of the pathogen
  • Use a fungicidal seed treatment on infected seed
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