Hi Jill,

Thank you so much for reaching out to the UVM Extension Master Gardener Helpline.

I'd like to preface my response with a caution because I really cannot diagnose anything from these photos. If you can get a close-up picture of the black sections, that would be really helpful.

Having said that, there is a fungus that can plague birch trees called Inonotus Obliquus (also called canker rot or chaga) which is characterized by the following description:

• Rough, cracked black, knob or cone like fungal growths protrudes from trunk, often near an old wound or branch stub
• Internally fungal growth is spongy, colored yellow-brown to
  rust-brown
• Infected trees often break at the point of the fungal growth
• Mottled white and reddish brown wood decay can be seen within infected trunks
• You should try to, as much as possible, remove the cankered sections. Try to avoid any stress on the tree. If there is drought, water it well. Mulch well under the tree and avoid damaging the bark (like from a weed wacker or mower).
• There is no cure for cankers other than removal and burning/destroying below the disease and improving the vigor of the tree. Trees have had a tough time for the past several years with defoliation by spongy moth, the wet weather 2 years ago and then again this spring followed by the flash drought. All of these stresses may have made the tree more susceptible to disease. White birch are not long lived trees so it may be reaching the end of the road, but if we have good weather this growing season with adequate rainfall and you prune out the cankers, the tree may continue to grow and look good. 

Here are some useful links that show an example of the damage caused:

https://apps.extension.umn.edu/garden/diagnose/plant/deciduous/birch/branchesgrowth.html

https://www.maine.gov/dacf/mfs/forest_health/documents/Chaga_Cinder%20Conk_MFS_FactSheet.pdf

I hope this is helpful and best of luck with those trees!

Suzanne