To know what is wrong with your trees will require an arborist. It could be a root problem or environmental. If they do have a fungal disease then you won’t want to plant near the same location and you will want the replacement trees to be resistant to what killed your trees if they indeed do not survive. Here is a link to how to find an arborist; https://extension.umn.edu/planting-and-growing-guides/how-hire-tree-care-professional

Nother expert thinks it might be winter injury. It may leaf out again and the second flush of leaves is usually smaller than normal. 

Thanks.   I checked with U of M disease lab and they are unable to test for fungus until tree dies and I can provide root sample.  They gave me the ISA web site and I found some certified arborists in the area.  I may hire an arborist but will probably operate on the assumption that it is winter injury for now.    I suppose I should add some fertilizer this summer to assist those trees?   I have generally just relied on the "deer pellets"  to fertilize in the past as they seem to be frequent visitors particularly when apples are falling.  Any recommendations for specific fertilizer treatment or simply general fruit tree regime?   Thanks so much.  This is a fantastic resource for those of us who "play" at gardening.

Fertilizing is not recommended because it stimulates growth and this is stressful for a tree. The tree is already stressed and trying to recover from some disease or winter injury. Providing adequate water, mulching the root zone and keeping grass that competes for water out of root zone will help.