Q. Can you tell me what this is on one of our peach trees?
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A. I am looking at your picture now of the peach leaf. It was hard to see what the problem might be with only one picture and that picture was low resolution.
What I saw was one leaf inside the canopy in some shade with some black spots developing on the leaf margins. I couldn't be certain but it looked like there was a yellow halo around the black spots on the margins.
I don't know if this is typical of all the leaves or just the leaves in the shade. For me there were two possibilities; irrigation or a disease called shot hole fungus or Coryneum Blight.
If it was over the entire canopy and it involves leaves in full sunlight then I would tend to think it was irrigation related. It usually occurs if the tree is not getting enough water at the time of an irrigation or if you waited too long between irrigations. If the tree has gotten considerably larger in the last two years than I would add any emitter or two to the irrigation of the tree. I would also mulch the surface of the soil to conserve water and reduce water stress.
If this is Coryneum blight then you would spray the tree with a copper-based fungicide such as Bordeaux mixture immediately when the leaves fall from the tree this early winter. You would follow up with the spray in the spring as the leaves are coming out and new growth is emerging but after blooming has finished.