Question: Can the Mountain Ash get the ash borer also? Our mature Mountain Ash has several large branches with dried leaves. G.P.
Answer: Mountain ash is not a type of ash. Mountainash (Sorbus) is a member of the rose family (Rosaceae) and not related to ashes (Fraxinus), which are in the olive family (Oleaceae). Although not often used, the correct way to write the common name for Sorbus is one word – mountainash – which might help eliminate the confusion with ash. The good news is your mountainash isn’t susceptible to the emerald ash borer insect that is decimating ash trees in the Midwest. However, mountainash is prone to a number of disease and insect problems and tends to be short-lived.
European mountainash or Rowan tree and its cultivars are most commonly used in the landscape. They are small (20 to 30 feet tall) trees with attractive white flowers in spring followed by showy clusters of red-orange berries, and in autumn brightly colored leaves. Our native American mountainash, which is also short-lived, grows in cold swamps or bogs so not a good choice for residential landscapes.
Mountainash are susceptible to several canker causing diseases. Many of these diseases are caused by fungi growing under the tree bark. Typical symptoms are sunken or discolored bark on stems or trunk. Some cankers will cause bark cracks. Cankers may be circular or irregularly shaped. In some cankers tiny dots, which produce the fungal spores, may be visible. Canker fungi kill the cambium and vascular tissue which is directly under the bark. This disruption of the vascular system leads to dieback of the leaves and stem beyond the canker.
Like all members of the Rose Family, mountainash can get the bacterial disease fireblight. Typical symptoms of fireblight are black or dark brown leaves that remain on the branch. Dead branch tips often curl into a shepherd’s crook. Infected flowers will blacken or brown. Fireblight also causes stem or branch cankers, which ooze in warm, wet weather of spring.
For fungal cankers and fireblight, remove infected branches by pruning several inches below the cankered or dead area. Disinfect your pruning tools between cuts by spraying with a household disinfectant containing at least 70 percent alcohol.
Mountainash are also at risk for oystershell scale insects, borers, and fungal leaf diseases including apple scab and cedar/apple rust. As the trees age they lose their attractiveness due to dead or removed branches. In my experience most mountainash succumb to a complex of problems before they are 25 to 30 years old. For the time being, enjoy your mountainash for its multi-season beauty.
Barb Larson is horticulture educator for the Kenosha County University of Wisconsin Extension. She holds a master’s of science in horticulture from the University of Wisconsin-Madison. If you have a plant or gardening question, email her at firstname.lastname@example.org or call 262-857-1942.