Last week the question was: What is this on an oat leaf?
This is an up close view of a rust pustule. There are lots of species of rust fungi that infect a number of different crops, some of which include southern rust of corn, stem and leaf rust of wheat, and crown rust of oats. The pustules seen above are full of spores called urediospores. The pustules split open and the release the urediospores which can be carried by the wind for long distances. Rust fungi have very complex life cycles, which you can read more about here.
Here is this week’s question: What happened to this collard plant?