Which of the following is a resistant survival stage for apple scab?

The resistant survival stage for apple scab is found in fallen apple leaves. This is because apple scab, caused by the fungus Venturia inaequalis, produces ascospores that survive winter in the litter layer composed of fallen leaves. These leaves harbor the overwintering structures of the fungus, allowing it to persist in the environment until conditions become favorable for infection in the spring.

Fallen apple leaves not only provide a substrate for the pathogen to survive during the dormant season but also serve as a source for the potential spread of the disease once new apple foliage emerges. In contrast, conidia are the asexual spores of the fungus that facilitate rapid infection during the growing season, and appressoria are specialized structures used by the fungus to penetrate host plant tissues during infection, but neither of these structures represents a survival stage during adverse conditions. Ascospores also play a role in initiating infection but are not the primary resistant structure that endures winter conditions in the same way as fallen leaves do.