Covered smut of barley (caused by Ustilago hordei) is/are

✅ Explanation: Covered smut of barley, caused by the fungus Ustilago hordei, is primarily an externally seed-borne disease. The fungal spores contaminate the seed surface during harvest or handling. When the infected seed is sown, the spores germinate and infect the seedling, leading to the characteristic smutted heads where kernels are replaced by black spore masses.

📌 Other Options Explanations:
Internally seed borne Loose smut of wheat, Loose smut of barley