Ask Mr. Smarty Plants

When we searched on our Native Plant Database, we found 7 species of ash with "mountain ash" as a common name. Two of them - Sorbus americana (American mountain ash), and Sorbus decora (Northern mountain ash) -  are native to Michigan, so we chose American Mountain Ash to check out; usually, plants that closely related will share benefits and problems.

When we followed the plant link to our webpage on this tree, we found this statement:

"Conditions Comments: Mountain ash is usually short-lived due to fire blight, mildew, and borers as well as a number of other diseases and insects. In cultivation this species does best in the northern part of its range and at high altitudes. Young stems and thin bark need protection from winter browsers."

Since this is a plant that wouldn't grow in Texas, we don't have any first-hand knowledge of what the problem might be, so we are going to look for articles, perhaps with pictures, online to help.

University of Wisconsin Extension: Mountain Ash disorder: Fire blight.

Previous Mr. Smarty Plants question on fungus in Mountain Ash

From Yardener Solving Problems with Mountain Ash

From Dave's Garden Mountain Ash - A Multifaceted Tree

From Virginia Tech Mountain Ash: Finally, one with a picture like you describe, and this comment: "Bark: When young, smooth with numerous lenticels, grayish brown, developing cracks, splits and scale patches with age."

So, maybe you don't have anything to worry about after all.