Check under-side of leaves, looking for Potato Leaf-Hoppers--- they can be hard to see--- same color as the leaf, and can move very fast over the edge of the leaf to hide on the other side, usually running sideways. Adults fly away when disturbed.
These green leafhoppers inject a toxin that causes leaf-curling and induced nutrient deficiencies. It doesn't take many of them to bring on the visible symptoms. Eventually the growing tip falls off.
If insecticide is used before growth has stopped, the next leaves will be normal.