Is this rust?

Growing three Auto Critical Mass plants outdoors. Last weekend, we had a lot of rain and the temps dropped pretty low over night. A few days after I came back, I was tending to my plants and one of them looked to have rust growing on a few of the leaves?

Does anyone know for sure what the cause for the discoloration is? Nutrient deficiency? Fungus?

Not an expert but kinda does look like some kinda fungus would treat with a fungicide

Here is a pic of a plant with rust I found in rollitup threads I goggled (rust fungus weed plants) and it was the first thing that came up