Phal Leaf Dying

[scy]

Last month, there was lots of new root growth and a new leaf growing on my phal sogo grape. However, the bottom leaf has gradually turned lighter and now is pretty pruny and not looking so great. It hasn't dried out yet, but it will probably fall off soon. I know that it is common for bottom leaves to drop but this doesn't have many leaves to begin with. Once it drops, I'll have 2 leaves and a new one forming. Should I be concerned? Should I leave that leaf and wait for it to drop on its own? Thanks for the advice!

Susan