Answer: Nematodes are often the cause of wilting impatiens. However, the rapid onset after planting of your plant's symptoms tends to eliminate nematodes from the list.
Frequent watering in the summer heat can create an ideal environment for disease problems. Take a close up look at the stems near the soil line. You may see a brown, water-soaked area indicating a disease infection. To combat this problem, remove infected plants and water less often giving plants a good soaking when you water.
A final possibility is that a grub or other soil insect is feeding on the roots. Carefully dig underneath an affected plant to see if the culprit is present.
Good luck with your impatiens!
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