Nectarine tree - 8 years old. It develops fruit, but the fruit never really...
Nectarines do not grow that well in MD. They are very frost sensitive and also susceptible to insect and disease problems which usually require a spray schedule for control. If your tree is not too large you can cover with an old tarp or blanket to protect from the frost tonight. You will have to remove in the morning so heat does not build up.
There may be several reasons why your fruit does not enlarge and ripen. Fruits not thinned properly, lack of pruning, drought or cold temperatures, low soil fertility, possible insects, (you will have to cut open the fruit and look for this), etc. See our publication for more information. http://extension.umd.edu/sites/default/files/_images/programs/hgic/Publications/HG77_IPM_Series_Ston...
We have some general information on fruit trees on our website http://extension.umd.edu/growit/fruit-profiles/getting-started-tree-fruits
but you will have to check other extension websites such as Penn State for detailed information http://extension.psu.edu/plants/gardening/fphg/stone