Hi, that's the beginning of a nutrient deficiency or high salt intake. What you are actually seeing is some sunburn where the leaves are suffering. Here's what I suggest:
1) Keep off those conventional fertilizers...they are salt based and could cause this effect.
2) You don't say "in a Pot" or "in the Ground". Plants in Pots will always become nutrient deficient.
That's the Good and the Bad of container growing, You control the medium) Remember to
fertilize regularly. (That's a lot more frequently in a pot, Micro nutrients too)
3) In spring plants are "off and running" so it's not abnormal for some plants to become deficient.
(Plants are metabolizing super rapidly and even when a nutrient is available, some get missed)
4) If your tree is in a pot, be very aware of it's spring water needs.
So, if I were you, here's what I'd do. First be sure there is no salt build up. No chem fert's and also be sure to leach the root zone on your plant. (ie run lots of water to your plant every couple of months) If you already see a problem in spring, give those trees some quickly absorbed nutrients. (Yes, you need the top 3 NPK, but what I do is give my trees Fish emulsion and Kelp (Seaweed) , thereby getting the NPK but also all the organically available Micro nutrients as well. It is quickly absorbed and you will see a response quickly too. I don't see a pathogen on your leaf, just burn. That's a Good Thing! Good Luck, Lori
