SOIL sick plants

[joanieinmaine]

My sister's medicinal plants have this circumferential girdling by a white gray substance about a foot above ground that is killing the plants. You will see I scraped it and found some black stuff inside. What is it?
Any treatment? thanks

 

[D.A.A.S.69]

Looks like where buck deer rub their horns.

 

[Jackf21792]

Could be mold, you can spray them with certain products to mitigate it.