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There are two common bacterial cankers on peaches, cytospora canker: https://www.gardeningknowhow.com/plant-problems/disease/weeping-amber-in-trees.htm
and bacterial canker: http://www.ipm.ucdavis.edu/PMG/r602101011.html
You can make use of your county extension service to identify which you have, and how to deal with it. This site will help you locate one: https://www.gardeningknowhow.com/garden-how-to/info/what-is-extension-service.htm

Unfortunately I can't tell you what the problem is from a description "yellow leaves." You need to examine the leaves for things like spotting, leaf deformities, size changes, green veins, etc. While leaves simply turning yellow is a sign of nitrogen deficiency, adding nitrogen to the soil might not show a change in the leaves in only 2 months. And it's never a good idea to use a treatment without a specific pest or pathogen as target. Here's an article on common peach tree diseases that might give you an overview of what to look for: https://www.gardeningknowhow.com/edible/fruits/peach/common-peach-diseases.htm
The best idea would be to take leaf,branch, and soil samples to the nearest extension service, where they can identify the problem and give you a solution. This link will help you locate one: https://www.gardeningknowhow.com/garden-how-to/info/what-is-extension-service.htm

This article should provide help with the brown rot: https://www.gardeningknowhow.com/edible/fruits/peach/brown-rot-fungus.htm
For an anti-fungal soak, I prefer vinegar - white or apple - a couple of tablespoons in a bowl of water, soak the peaches for 5 minutes. I soak all my fruits and vegetables like this before I eat them or cook them.

Sometimes when you plant new trees or plants, they go into "shock" it stops growing and loses leaves. I think you should look up ways on how to care for it now that its in shock.

Here is information on planting peach seeds over the winter:
https://www.colostate.edu/Depts/CoopExt/4DMG/VegFruit/Fruits/pits.htm

But I am not sure that you are looking for that. Are you looking to plant them over the winter or do you want to store them elsewhere? Let us know and we can be of more help.

From your description, it could be peach twig borer: http://www.ipm.ucdavis.edu/PMG/r602300611.html
You need to get an analysis from a professional. You could have samples examined by a nursery, university botany dept, or the master gardeners. They have loads of information and assistance available for home gardeners. This link will help you locate the nearest group:

http://mastergardeners.org.uk/get-involved/find-your-nearest-master-gardener/