Serviceberry and Juglone

(Question)

IMG_1183A previous Q&A states that Serviceberries are sensitive to juglone even though they are a native species. There is a lot of conflicting info on the web. My 20 yr. old Serviceberry is about 25 ft from my neighbour’s black walnut & is not doing well; many leaves appear to have rust on them, some leaves have a white fuzzy material on the underside & it has lost 65% of its leaves already. There seems to be a number of problems but if the juglone is going to kill it anyway, I may as well just cut it down. What do you think? Are pagoda dogwoods juglone resistant?

(Answer)

Sorry to hear that your serviceberry is not doing well.  From your description of the symptoms, it sounds like the tree has just had a bad year.  Serviceberries generally do not have serious insect/disease problems, and should not be affected by black walnut trees.

From my research, the serviceberry (Amelanchier canadensis) as well as at least some dogwoods, Cornus alternifolia (Pagoda Dogwood) and Cornus florida (Flowering dogwood), are tolerant to juglone.  For example,  Penn State’s Landscaping and Gardening Around Walnuts and Other Juglone Producing Plants.  I also located several additional references on point.

As well, your serviceberry’s symptoms sound quite different than what one would expect with juglone toxicity. You might want to look into other causes of the plant’s problems – e.g., the Missouri Botanical Garden’s Amelanchier canadensis notes that rust and powdery mildew are among the conditions that may affect the serviceberry. Here’s a link to a helpful Q&A in our archives, Something on my serviceberry leaf  This response discusses some insect pests that can affect these plants – including those that can cause a white coating on the underside of leaves…(sound familiar?).  Near the end of the post, there is a terrific link concerning insects that attack serviceberries – Pest Damage on Serviceberry – which sets out what pests attack these plants – by season!

Although I do not believe the black walnut tree is responsible for your serviceberry’s symptoms, it is useful to review how the toxin from that tree can affect susceptible plants. The black walnut (Juglans nigra) produces juglone, a chemical that inhibits growth of sensitive species.  Symptoms of juglone damage include stunted growth and partial or total foliage wilting/yellowing. These symptoms can occur quickly, with the plant progressing from healthy to deathly in 1-2 days. Unfortunately, once wilting starts, there’s no way to reduce the effect.  Black walnut roots can extend 50-80 feet past the outer canopy of the mature tree, although it is those sensitive plants located directly under the tree’s canopy that are most at risk, due to direct contact with the roots, or toxin accumulation from leaves/nut hulls. Black walnut trees start to cause symptoms in sensitive plants when they are 7-8 years old.  More detailed information is set out in Juglone and Black Walnut: A Toronto Master Gardeners Guide.

To try and minimize any damage the black walnut may be causing to susceptible plants, it is important that debris from the tree be removed (e.g., your kind neighbours might agree to keep the area clean). As well, check the plant’s drainage – excellent drainage will increase the chance of the plant surviving. Where soil drains well and is well-aerated, a plant would be vulnerable only when its roots are in direct contact with the walnut roots; where water pools, the juglone (although it is poorly water-soluble so does not move far in soil) gets into the water and can affect the roots.   As well, it is important that the soil around the susceptible plant have a healthy population of microbes (e.g., from decaying organic matter, compost), which speeds up the decomposition of juglone.  I’ve mentioned all these strategies, because at least two of them – good drainage, healthy soil – are important in keeping most plants – including your serviceberry – healthy, no matter the cause of the symptoms.

Other strategies to consider include removing the affected leaves and branches from your serviceberry, pruning to ensure good air circulation and not spraying the leaves with water. See also Ask a Master Gardener Q&A Fungus on serviceberry (which has some good links).  I’d suggest that you ask someone at your local gardening centre to help diagnose the problem, and (if the above cultural practices are not sufficient) perhaps suggest products that could help the plant recover.

Additional references you might be interested in include:

Ontario Ministry of Agriculture, Food & Rural Affairs (OMAFRA). Walnut Toxicity

University of Wisconsin Extension. Black walnut toxicity 

University of Wisconsin Extension. Walnut and butternut toxicity