Hi Maureen.  Thanks for contacting the UVM Extension Master Gardener Helpline.

Unfortunately, it's impossible to diagnose your tree from the photos you provided.  The lichens you're seeing can be indicators of stress, age, or just very good air quality.


You may want to fill out the Diagnostic Worksheet.  This will provide information on how old the tree is, when were signs first noticed, any green growth at all or is entire tree dead, fertilizing, pruning, etc.  Photos of the entire tree would also be helpful.  With that additional information, we might be able to give you a better answer to your question of what killed your tree.

In the meantime, here are some articles that may provide useful information about pests and diseases that affect crabapple trees and good growing practices.
https://extension.psu.edu/crabapple-diseases
https://portal.ct.gov/-/media/caes/documents/publications/fact_sheets/plant_pathology_and_ecology/commondiseasesofcrabapple110512rpdf.pdf
https://www.mofga.org/resources/orcharding/grow-crabapple-trees/
Hope this information helps.