Thank you for contacting the UVM Extension Master Gardener Helpline about how to help your peach tree! The image you included with your post makes it hard to confirm the Peach Leaf Curl (PLC) diagnosis, so I'm basing my advice on the assumption that is what it is.

There is no need to go in and remove the diseased leaves by hand. There is no secondary spread of this disease from leaves infected in the spring to new leaves produced later in the growing season. Once infected leaves drop, no further symptoms will appear during that growing season. Diseased twigs become swollen and stunted, and may have a slight golden cast. They usually produce curled leaves at their tips...The fruit on affected trees should be thinned to compensate for the loss of leaves. Over-cropping the tree will weaken it and make it more susceptible to winter injury.

You mentioned that you applied copper sulfate spray this Spring. Spring application must be made before bud swell. If spring applications are made, temperatures must be monitored closely. Unusually warm weather during late winter months can encourage bud swell thereby making fungicide sprays ineffective. Once the fungus enters the leaf in the spring, the disease cannot be controlled until Fall. If the weather was not dry enough around the time that you treated the tree and/or if you treated the tree after the buds appeared, it is likely too late for this spring for further intervention other than making sure the tree is healthy; focus on treatment again in the fall.

Peach leaf curl alone doesn't usually kill trees, but it can weaken severely infected trees. This can cause the trees to be more susceptible to other problems (diseases, insects, stress). Your tree will need some babying this year to protect it. So what can you do now to support your tree and hopefully set it up for success next year?

• Thin the fruit to reduce demand on the tree. Thinning fruit to about every 8” with about 10 leaves in the immediate clump/ fruit would be a generous allotment of energy for development of each peach.
• Irrigate to reduce drought stress during this growing season if needed. More info on irrigation options.
• Fertilize the tree with nitrogen by June 15. Be careful not to overstimulate the tree.

For 2024, keep in mind the best time to treat your tree to prevent the return of PLC next Spring is to treat it this fall, just after leaf-drop. Feel free to reach back out in mid-summer to request recommendations on treatment options for the fall.

Further reading:
"Peach Disease - Peach Leaf Curl" PennState Extension - https://extension.psu.edu/peach-disease-peach-leaf-curl
"Peach Leaf Curl" Ohio State University Extension - https://ohioline.osu.edu/factsheet/plpath-fru-26
"Good Growing: Peach Leaf Curl" Illinois Extension - https://extension.illinois.edu/blogs/good-growing/2020-03-11-peach-leaf-curl
"Time To Straighten Out Peach Leaf Curl" - https://bygl.osu.edu/node/1895
"Drip Irrigation for Tree Fruit Orchards in Pennsylvania" - https://extension.psu.edu/drip-irrigation-for-tree-fruit-orchards-in-pennsylvania