Spots on Leaves

I was given a Brug seedling this year. It's done well but has now developed dark spots on the leaves. What is the problem?

Close-up of leaf --

Have you looked at the undersides of the leaves for insects or mites? If you can't find any and you don't fertilize often it light be potassium deficiency since the veins look yellow and the rest of the leaf surface looks pale and blotchy. Those dried brown spots are one of the symptoms of potassium deficiency. How often do you fertilize? What do you use when you fertilize? Fertilize with a complete liquid fertilizer or a fertilizer that has to be dissolved in water to use. You might check the pH level to see if the pH is too high or too low which make nutrients unavailable. PH should be between 6.5 and 7 for optimum nutrient uptake.

I will investigate further on those things. When fertilizing, I use Miraclo Gro that's applied with a water hose. I'll check on the soil tomorrow and let you know what I find.

Thank you.