Answer: It sounds to me as though you are doing everything right for your plants. The white patches are sunscald. Lily of the Nile grows best in full sunshine, but often plants in nurseries are set under shade cloth to reduce the intensity of sunlight so plants retain their vibrant colors and won't need to be watered as frequently. (Something difficult to do in garden center environments.) Because of this, the foliage is accustomed to filtered sunlight and can sunburn when exposed to hot afternoon sunshine. At this point I'd suggest continuing to give your new plants the wonderful care you are giving them and wait for them to produce healthy new foliage. As the foliage develops in sunny conditions it will be hardier and more accustomed to full sunshine. Just think of your plants as going through an adjustment period. Once they adjust, they should not wilt in the sun.
Best wishes with your new plants.
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