What do the pustules found on rust-affected grass contain?

The pustules found on rust-affected grass are primarily filled with spores, which are the reproductive structures of the rust fungi. These spores are crucial for the lifecycle of the fungi as they are dispersed by wind, water, or contact with other organisms, allowing the fungus to infect new host plants. The formation of pustules occurs as the fungus develops within the plant tissue, leading to visible lesions on the grass where spores accumulate. When environmental conditions are favorable, these spores can be released, potentially leading to further infection of other plants, illustrating the importance of understanding plant pathology for effective pest management and control strategies.

Fungicides are used to manage such infections but are not contained within the pustules. The other options, such as bacteria and insect eggs, are not relevant to the rust fungi's lifecycle or infection process on grass.