Issue with Mountain Ash Tree

Asked July 12, 2018, 10:16 AM EDT

Hello: I work at the Cottonwood County Extension office in Windom, MN. I had a gentleman come into my office with questions regarding his Mountain Ash tree. The leaves are falling off and have brown and yellow spots on them. I have taken two photos of the sample - a front and back of the leaf. Can you please tell me what his issue is and how he should proceed to fix it. Thank you. Teresa Schwartz - deter086@umn.edu or 507-831-4022

Cottonwood County Minnesota

5 Responses

A kind of leaf spot disease (there are many) appears to have affected the mountain ash. The following bulletin discusses leaf spot diseases and how to manage them. Although the information is not specific to mountain ash, it can be applied in this case.

Thank you for your response Bob. A follow up question to this, if I am reading the bulletin correct, at this point in the summer there is really no spray that can be used to fix the problem for this year. Should my homeowner be applying something next Spring? Second question, does this come back from year to year if left untreated? Thank you.

Controls (fungicides), if needed, are applied in early spring usually just before and just after the leaves begin to unfold. Nothing can be done now to reduce or reverse the extent of disease damage this growing season. However, raking and removing fallen leaves and fertilizing and watering the tree when needed may reduce the amount of disease next year which may or may not be present depending upon the extent of conditions favorable to its development. Control may not be needed unless defoliation occurs frequently.

We can't identify the disease by viewing the photos, but information in this bulletin may be of general interest:

Jumping in on this because I am curious about this tree. I have never seen a mountain ash with leaves like this; they are typically compound leaves with many small leaflets. The leaves in this sample appear to have only 2 small leaflets and then a large, terminal leaflet shaped like an oak leaf. I do not believe this is a mountain ash, and if not, the diagnostics and recommendations might be very different.

Granted that the leaves are desiccated and difficult to identify for that reason, those shown were probably taken from an oak leaf mountain ash.