I think its normal. Mine have some yellowing today while looking at them. I'm guessing the heat has something to do with it (yesterday was 97F and humid). Otherwise the tree is growing like crazy. Wouldn't worry unless you suspect something else is at play.

This is not unusual, and many people have posted their worries about yellowing peach leaves on this forum over the years. I wouldn't worry if it's the older smaller leaves that are turning. The newer, larger leaves should be remain green. If those are turning yellow, then I'd say you have a problem.

You wrote:
" I have been spraying regularly with sevin and sulfur, like always, and on one occasion added diazinon to the mix."

Why you're doing that. All those sprays could be adding to your problems.

jean001 asks why I'm doing that. "All those sprays could be adding to your problem," she says.

Now that you ask, Jean, I'll tell you. Without those sprays, and one more that I didn't mention, Ferbam, my tree would be suffering from peach leaf curl, worms, shot hole disease and brown rot. Instead of getting delicious peaches the size of large tennis balls every year, I'd be getting nothing. Although my posting may have made me sound like a newbie I've been growing peaches for 30 years, and I experienced all of the above before I learned how to cope with them. The yellowing leaves (a slight rust color, actually) is new to me and I thought I'd throw it out there for an opinion or two.

Two ideas that you may have already considered, spraying when it is too hot can damage leaves or using the wrong formula when mixing the chemicals. I sprayed my nectarine trees earlier this year and the leaves turned yellow and I lost about half of the leaves. I was using a new combination of chemicals and I may not have gotten the formula just right. I changed some things around in my spray formulation, watered the trees deeply and they got over the problem. Who knows? A new chemical in the mix or just hot, dry weather or a combination of both. I have damaged a tree or two just about every year, either from spraying the wrong combination of chemicals or spraying when it was too hot. (or maybe some other physiological change not related, but coincidental to my spraying)

So, it could have been something with the mixture or it may have been too hot when you sprayed. I have not lost any trees from my possible errors, but have thought for sure I killed an occasional tree.