What are they planted in, the ground, or a raised bed or something else? I can't work out what I'm seeing along the base of the trees...

I'm wondering about the spraying, when it was done and why
Also what was sprayed, I've not foliar fed or insecticide sprayed conifers like these.....

I thought you were talking about birds for a moment! How lovely that would be!

Looks like a fence behind the trees the bottom of which stops a foot above the ground. Below the bottom border of the fence is a cloth or plastic continuance that touches the ground. The gold rests bases have a layer of red mulch with a plastic barrier defining the muched area and the lawn. Perhaps too much foliar spray and or insecticide was used. Read the directions carefully on both items used and see if you did something wrong in their application.

Yes, they are planted on the ground, 10-12 cm away from the neighbor wall, there is a green metal fence on the wall which is about 1m high. The main function of these trees is screening, to provide some private space. The soil is clayish.

The spraying was made based on gardener advice and intructions were followed to our best efforts, but I have doubts about the proficiency of the gardener. Some trees looked ill, it was an attempt to cure those trees but few weeks after the spray (probably 3 weeks), all the trees turned from green to brown/gray color and the former ill ones did not improve at all. I also do not know if spraying should be blamed, does 3 weeks sound like the right amount of time for the wrong spraying to take effect?

Yesterday I gave some organic vermicompost to the trees. From what I read, vermicompost has no negative effects to the garden. I am hoping it helps the trees to fight with the situation.

Apart from that, I have no idea what I should be doing? Does this look like a disease? Do I need to replace the trees or wait and see? I have seen people talking about fungus in similar trees and recommending each other the removal of the infected trees or even the soil. I hope that is the case for me.

What would you do in my shoes?

And another thing, it is quite windy in here. The wind speed averages about 40 km/h in the afternoon and evening with may be gusts of 60 km/h, not as strong in night and morning hours.

I know conifers don't regenerate from brown wood so it worries me.....
Usual advice is to wait and see......maybe talk to the gardener who gave the advice, the spraying is not something I'd heard of...
Done in the heat of the day so it burnt the foliage is another possibility

Foliar feeds for conifers are common in America, but rarely used in the UK - it might help to know precisely which product was used to spray with, or it might not - if he used a foliar feed when the roots were really dry, that could account for what you're now seeing. Or mixed it in the wrong ratio or, as Pam said, sprayed in bright sunlight or applied it at the wrong time of year. Spraying with insecticide isn't a good idea unless you've identified a particular pest.

They don't particularly look like they've got root rot (phytophthera), that blackish grey colour is not typical of that disease. This particular conifer can suffer windburn, but that presents as brownish tips to the needles. I'd say the blackish grey colour is related to whatever they were sprayed with - all you can do is keep them watered as necessary and see what happens.

It looks from the photos that the backs of the trees nearest the fence have not been affected - could you have missed these with your spray? But if the trees were looking ill before you sprayed its a mystery. It doesn't look like windburn.

If the worst happens and they have to be removed you will have the opportunity to plant a new hedge a proper distance from the boundary - 12cm is much too close.