Finding problems in the field.
From this to this, in three days.
What happened?
Is there a pattern? Is it spotty or all over? You say it's spotty, but do you see pattern in the spots? When you see salt damage or leaf blight (often the causes are the same, one a salt accumulation, the other a fungus hitting salt/water stressed trees), you go straight to the irrigation. Timing? Amount? Distribution? If other trees are doing fine, then the amounts and times are right. So it's the distribution? Emitter plugging? Emitter alignment? Animal damage?
Or is it root damage? Are there roots? Are there roots in the wetted zone? What color are the roots? It's getting cold, but there should still be some white/light colored roots. You got a resistant rootstock, but it can still get root rot. Maybe a shot of phosphite? Or Orondis?
I don't like fungicides in general, because they are such a short fix, but sometimes they can help through a tough time. I don't think it warrants Orondis at this time, though. Not yet, until you see what is causing the problem. Check the irrigation system and the roots first.
And this all happened on a weekend when there was wind and heat. Attention to the irrigation must be paid