lwabirds

I have a few groups of basjoos in my yard. The pups begin to turn brown/black when the break the surface of the ground and then die. I've sprayed for mites numerous times. I did noticed mites in last spring. Could it be over fertilizing? Mites? I've never had this happen in the 5 yeas that I've had them in the ground. Any help would be appreciated. The big "parent" plants seem stunted and not putting out new leaves.
I fertilized once every 1-2 weeks. i've been using a bag of miracle grow in a hose end sprayer.

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