Which of the following is a resistant survival stage for downy mildew of lettuce?

The correct answer, oospores, represents a resistant survival stage for downy mildew of lettuce. Oospores are thick-walled structures produced by the sexual reproduction of certain fungi, including those responsible for downy mildew. They are capable of surviving adverse environmental conditions, such as extreme temperatures or droughts, which allows the pathogen to persist in the soil and infect host plants when conditions become favorable again.

These structures can remain dormant for extended periods, helping the pathogen endure unfavorable conditions that would otherwise eliminate it. When the right host and environmental conditions return, oospores can germinate, giving rise to new infections of downy mildew.

In contrast, sporangia are specialized structures for asexual reproduction, and while they play a crucial role in the immediate spread of the disease, they do not serve as a long-term survival mechanism. Zoospores are a motile stage that is important for the short-range spread of the pathogen but are also not resistant to environmental stresses. Mycelium, while part of the fungal life cycle, does not represent a survival stage as robust as oospores in terms of enduring difficult conditions.