Apple Tree Brown Leaves

I planted 3 apple pips in October of last year, and since then 3 small plants have grown. Everything was fine until a few weeks ago, where I noticed the leaves of one of my plants had a brown stripe halfway up the leaf and was crispy beneath that. We took the plants to out local Hillars Garden Centre after many of the leaves had turned brown on one of my plants and had spread to my second plant. The lady at the garden centre told us that it was scorch, and I should water the tree at the base and make sure the leaves do not get wet. I accepted that and took her advice, however, I noticed the brown had spread all over the plants save the top few leaves, so I put it them in my plastic un-heated greenhouse as there had been a few leaves buzzing around the room which the lady assured were from compost eggs. I checked this morning on the plants and the brown is starting to go slightly golden and shiny. It has also spread to my third plant which I was hoping would be safe. Does anyone know what this could be??
Many Thanks, Charlie.