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My experience with brown spot (a fungal infection) is many fewer spots, and much larger spots. The pattern suggests an environmental cause. Perhaps the foliar urea spray, or perhaps "weather fleck", which is ozone damage to the leaf. Air quality over Lithuania today looks good. But air quality over Easter Europe is presently rather bad. Since weather fleck develops over an extended time period, just simple shifts in wind patterns could have brought the poor air quality over Lithuania. I would do nothing about the leaf now, and just watch. Brown spot will cause an increase in the diameter of the spots, with concentric circles, while a weather fleck pattern will not likely change with time. If the cause is the foliar urea spray, I would also expect no change with time.
Bob