What do the pustules on rust-affected grass contain?

The pustules that form on rust-affected grass contain spores. Rust diseases are caused by fungal pathogens that produce these spores within the pustules, facilitating the spread of the disease. When conditions are favorable, the spores can be released into the air or onto the surrounding foliage to infect other plants, continuing the cycle of infection. This is a crucial aspect of understanding plant pathology, as recognizing that spores are the reproductive structure of the fungus helps in managing and controlling the spread of rust diseases in grass and other plants. Other options, such as water droplets, petals, and roots, do not relate to the characteristics or functions of rust pustules. Water droplets may be present on the surface of plants but are not a component of the pustules themselves; petals pertain to flower structures and have no involvement in rust diseases, and roots are part of the plant's anchorage and nutrient intake, irrelevant in the context of the disease symptoms observed in rust infections.