I realise this is an old thread, but it helped me discover that it wasn't rust from the washing machine or iron that was ruining my shirts, but sap from the wisteria running along our garden path, from brushing it within a day or two of its being pruned. The stains only appeared after the clothes were next put through the wash, making it very hard to track down the cause. So thank you!
Fortunately, after trying various approaches, I've also discovered how to remove the stains entirely from cotton clothing, even from plain white shirts, which is what I'd like to share here now, in case anyone else stumbles on this thread in future: put a couple of scoops of oxygen bleach in a cup (Tesco Oxy Powder worked for me), lower the affected area of the clothing into the cup, and pour on boiling water from the kettle. Leave for 2 minutes. Rinse, and the stain will have vanished.
Of course, delicate or dyed fabrics may need gentler treatment, but I was willing to try anything!
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what about sap from a white wisteria that turns red on new concrete?
Yes, I have the same problem. The sap stains the concrete. Anyone have luck removing it?