Good Afternoon,

It looks like dutch elm disease( DED).

However, the only way to diagnose DED is through an arborist visit, or sending branch samples of a specific size to a lab.

Diagnosing other diseases or pests would require a certified arborist visit, too.

There are ways to protect the elm tree sinc it looks like it isn't severely affected yet.

I recommend you hire a certified arborist, a professional who has taken training in care, diseases, pests and passed certification tests. He/she will come on site and give a complete diagnosis and a plant care plan for your trees, including an assessment of the dead tree. . Find certified arborists by zip code here—-

www.treesaregood.org

These references describe how dutch elm disease is managed:

https://www.canr.msu.edu/resources/dutch_elm_disease

https://extension.umn.edu/plant-diseases/dutch-elm-disease

If you plan to replace the tree there are American elm hybrids that have been bred to resist dutch elm disease, and would be well worth planting.

https://extension.umn.edu/trees-and-shrubs/dutch-elm-disease-resistant-elm-trees

I hope this helps.  Thanks for using our service.