Diagnois cause of yellowing of new leaves on apple tree

I have a few young apple trees on B9 rootstock that are showing symptoms that I am not certain what the cause is. New leaves on the terminal tips of the leader and branches are yellow in appearance and show weak growth. Even to the point of some die back. Some older leaves are mottled with yellow. Trees were growing vigorously until this started roughly 3 weeks ago.

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Kearney County Nebraska trees and shrubs apples tree health fruit trees apple trees about 3 years ago