Mancozeb Help With Junipers

My junipers have recently reverted to putting out juvenile foliage. The base of these shoots are turning light green. Looks like some brown spots there as well. Possible cankers? Is this a fungal infection. I have Mancozeb being delivered tomorrow. Is it possible to stop this with just this fungicide? Do I soak the soil or protect my soil? Does Mancozeb kill microrhizae? Is this only effective as a preventative?