Hi everyone, due to my job I sometimes have to leave for extended periods of time,leaving my neighbour to water my bonsais in training. Lately I had to leave for almost 3 weeks and when I returned one of the ash trees showed heavy symptoms of what I think is some deficiency. I've been fertilising the soil with a NKCFe bonsai fertiliser,so the only thing I could think of with the symptoms is Boron.
Symptoms include: new growth has small,almost white leaves,dried margin,but no precise color distinction between veins and the rest. Internodes,on the other hand,are long (might be due to an awful spring with lack of sun).
Could any expert confirm what this could be? See attached picture for a reference.

I would have thought lack of light is the strongest possibility. I hope you only took it in to take that picture, and don't keep it in.

I would have thought so too,but it was outside the whole time and,as much as it rained,it seemed very unlikely that that could be the cause. Due to the heavy rains a local gardening center owner told me it looks like Iron deficiency,but the veins are not very visible,even though I have to admit that even wild plants around this area are showing iron deficiency symptoms. I started using a different fertiliser with micro elements,I'll see if there's any improvement in the next few weeks.
Thank you!

Strange it's happening to all the plants in the area. Could be someone sprayed a chemical, possibly defoliant like glysophate (Round-Up).

Quick & easy way to get iron is to put nails in a container of water & sit for a week or so. Swirl it around every now & again. Screen any debris out & add to watering can.