Rust ?

I was shooting the borage and noticed yellow patches on
the underside of leaves. Are these rust or substance produced
by the plant ?

 

Rust of course, comfrey rust.

 

Natural control of this ubiquitous (if not a pest) plant.

 

David, just to be clear, the plant in your image is comfrey, Symphytum, not borage, Borago.

 

Thanks for pointing out my error.
Always mix up the two officinale !