how to get rid of these stains on my boots?

A few days ago I picked up stains on the toes of these boots after walking through the rain for a while. They had initially left behind a white-ish residue so I assumed they were salt stains and on some advice from a friend tried cleaning them with a mixture of water and vinegar. At first it looked like the stains were gone, so I polished them up with a bit of saphir wax and left them. Checked again a few hours later and noticed these stains persisted. What can I do to get rid of them?
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