I deep water my Nectarine tree once a week. I have not had any problems the last two years. I now have a white powdery substance on most all the new fruit and lots of them are oozing with sap. What can I do to get rid of this
I don't have fruit trees myself, but at a guess it sounds like powdery mildew. Does it look like this:
https://www.ipm.ucdavis.edu/PMG/r540100511.html
If that is the case, it is a fungal condition. I would get rid of all already diseased fruits and then spray the tree with either a milk solution or a baking soda solution. (Put milk treatment into the search the forum feature if you aren't familiar with it.)