Loose smut of barley

Loose smut of barley is caused by the fungus Ustilago nuda. It is a seed-borne, systemic disease that significantly impacts barley production worldwide. The pathogen remains dormant as mycelium within the embryo of the barley seed, making it invisible to the naked eye during inspection of the seed lot before sowing.

The disease primarily affects barley (Hordeum vulgare). The fungus is highly host-specific, which means it cycles within the barley crop, infecting the floral parts during the flowering stage. Unlike covered smuts, loose smut replaces all parts of the spikelet with a mass of dark, powdery spores that are easily dispersed by wind and rain.

The characteristic symptom is the transformation of the ear into a black mass of spores that appears as soon as the plant heads out. The spores are eventually blown away, leaving only the bare central axis (rachis) of the spike. Because the infection process happens during flowering, farmers often do not realize the crop is infected until the following season when the symptomatic ears appear.

The development of Ustilago nuda is heavily influenced by environmental conditions during the flowering phase of the crop. High humidity and moderate temperatures (around 18–22 °C) are ideal for the spores to land on the stigma, germinate, and penetrate the developing seed embryo. Once inside, the fungus goes dormant and waits for the seed to germinate to continue its life cycle.

The economic impact of loose smut is direct yield loss, as infected plants fail to produce any grain in the affected ears. Effective management focuses on preventing the spread of the pathogen through infected seed. Systemic seed treatments are the primary method of control, as they can penetrate the seed coat and eradicate the mycelium residing within the embryo.

• Use of certified, disease-free seed lots.
• Selection of resistant barley cultivars.
• Seed treatment with systemic fungicides (e.g., carboxin or triazoles).
• Monitoring of seed crops to ensure low infection rates.