It might have something to do with all the rain. Mine did ther same thing. I got over 15" of rain in less than 3 weeks. All of my nectraines are spillting from all the rain.

Lacking in vigor because there is no new growth and few leaves in your picture. Peaches/nectarine should look more like this: Something like malathion will usually kill the scale. In winter treat with dormant oil to smother the over wintering forms. You can't wash them all off. And usually there are the big adults and smaller ones that need killing as well. Many people use a copper product like Kocide 3000 https://www.keystonepestsolutions.com/kocide-3000-fungicide-4-pounds-387 for PLC. It must be sprayed before bud swell in spring. Spraying later does nothing. If bad spray fall, winter, and early spring.

Trees that are newly planted and not yet established do not perform according the calender like other older, more established specimens would. They do not yet have the root system established that would allow them to be more self-sufficient. That's why you need to provide extra water for at least the first two growing seasons. It is just getting ready to enter dormancy. October is pretty typical for that to happen just about anywhere in the US except for the deep South or SoCal. And dropping leaves is SOP for that process. Don't panic - the tree is fine. Just make sure it gets enough water (but never too much) until the fall rains come or in any extended dry periods. If it loses its leaves earlier than your other trees, NBD. It should leaf out again at the proper time in spring.

Agree that the problem is probably due to thrips. They are the most likely cause of russeting on stone fruits. "This tree is planted In the grass and gets watered by the sprinklers twice a day. Would this supply it with enough water? " I assume this is a pop-up irrigation system that comes on for just a few minutes at a time? If so, OK for lawns (although far from ideal) but an extremely poor choice for anything else. No tree needs water daily, even those in very arid climates!! A long and slow deep soaking - as needed; not on any kind of schedule - is what is called for here. Too frequent but inadequate volumes of water do nothing to encourage healthy tree growth, especially of the root system. And an unthrifty tree without a well-developed and established root system is just going to be prone to all manner of disease and insect issues. You will need to treat for the thrips but they and any other insect and disease problems will be far less of an issue if you adopt proper watering practices!