What is the primary survival structure of stem rust in wheat?

The primary survival structure of stem rust in wheat is teliospores. Teliospores are specialized structures produced by the fungus that allows it to survive in adverse environmental conditions, particularly during the winter months. These spores remain dormant until conditions are suitable for germination, at which point they can develop into basidiospores, which are responsible for initiating new infections in wheat plants.

While urediniospores play an important role in the lifecycle of the stem rust pathogen by facilitating the asexual reproduction cycle during the growing season, they do not serve as the primary survival structure. Mycelium, the fungal tissue, can exist within infected plant material but is not specifically a survival structure in the same way that teliospores are. Basidiospores are also involved in the life cycle but primarily function in spreading the disease during periods of favorable conditions.

Thus, teliospores are crucial for the long-term survival of stem rust in wheat, particularly through challenging climatic conditions, making them the correct answer.