One of the best-selling small trees in Northern North America is the ‘Schubert’ chokecherry (Prunus virginiana ‘Schubert’)… and I have to wonder why.
Not that it’s not an attractive tree: with green foliage in the spring that quickly becomes dark purple, it really stands out from the crowd. In addition, the plant as usually sold has a strong, straight trunk and an attractive rounded crown.
Its clusters of small white flowers in spring are pretty, but not spectacular (they suffer from comparison with the much more colorful crabapples and ornamental cherries), but the berries, green at first, then shiny dark purple, almost black, are not without charm, plus they attract birds. And ‘Schubert’ is incredibly hardy too: absolutely thriving as far north as in zone 2.
What is disappointing is its health. It seems to me that every ‘Schubert’ chokecherry I see is infected with black knot (Dibotryon morbosum, syn Apiosporina morbosa), a nasty fungal disease.
What is Black Knot?
Black knot is most visible as black galls that form along branches. They are puffy, hard, cylindrical growths, charcoal black in color and of variable length. They not very visible in summer: you have to look very closely to see them, because the foliage hides them from view. It’s in fall and winter, when the tree is leafless, that they are very clearly visible.
The affected branches grow normally at first, but eventually the gall, one-sided at first, girdles the branch and cuts off its flow of sap. Thus all growth beyond the gall dies.
At first galls mostly appear on the tree’s outer branches, but the disease then seems to evolve inwardly, reaching increasingly larger branches over time. Eventually, the trunk itself is reached and the tree eventually dies, but that may take 7 to 12 years.
Black knot produces spores that are mostly carried from tree to tree by wind, although once the tree is infected, it is also readily spread by rain and infected pruning tools.
Not Much You Can Do
If you turn to the Internet for help, you’ll get some pretty useless advice. Something like “prune off infested branches, cutting about 4 inches (10 cm) below the base of the gall and sterilizing your pruning shears between each cut with rubbing alcohol so as not to spread it.” Sounds great in theory, but have any of the experts actually tried it? Like a cancer that metastasizes, new galls appear on other branches even after a thorough pruning job has been done. I know of no one who has ever managed to cure a ‘Schubert’ chokecherry of black knot by pruning.
You may also be told you can spray the tree with some sort of fungicide, like lime sulphur or Captan, usually in early spring. Have fun with that one too: yet another exercise in frustration!
From what I can see, once the tree shows the first galls (and there is rarely just one, even the first year), it is doomed. It’s just a question of time before it either dies outright or looks so ugly after repeated prunings that you simply remove it.
Here’s another good one: to prevent black knot, you’re told to eliminate any wild cherries or plums and any infested tree growing within 600 feet (180 m) of any ‘Schubert” chokecherry, because they can be hosts of the disease. Good luck with that, because it means getting the cooperation of the entire neighborhood: 600 feet is basically 2 city blocks and I’m not sure that all homeowners of the sector will feel they have to get involved. I mean, when you tell one tree owner he has to remove his tree to protect yours, how likely do you think it is that he’ll agree?
Are ‘Schubert’ Chokecherries Already Infested When You Buy Them?
Now that you know more about black knot, my question is: is it possible that the ‘Schubert’ chokecherries we buy were already infected with the disease before purchase? That the disease was spread in the nursery? I became even more suspicious when I learned that genetic studies have shown that the strain of black knot that affects ‘Schubert’ is genetically distinct from those affecting wild plums and that even the wild chokecherries in most regions usually suffer from different strains of the disease. So, the infection is not coming from “wild plums and cherries” as is usually claimed, but rather from other ‘Schubert’ chokecherries!
I have no proof of what I advance here. It is possible that the disease spreads from ‘Schubert’ chokecherry to ‘Schubert’ chokecherry strictly by spores carried by the wind (you have to admit that this tree is very widely planted in many areas, so few ‘Schubert’ chokecherries are truly growing in isolation) and that every nurseryman producing ‘Schubert’ chokecherries grows them with utmost care, making absolutely sure all specimens sold in nurseries are completely free of the disease and that all infected plants are burned as soon as the first symptom is noted. But I remain skeptical. Before buying a ‘Schubert’ chokecherry, or recommend it to any other gardener, I’d like a confirmation that it is not already contaminated.
What to Do?
Given the current situation, where almost all ‘Schubert’ chokecherries seem to suffer from black knot, I suggest not planting this tree until more is known about the source of the disease. If you already own one and you prune it annually to remove the galls that appear, I suggest not waiting too long before planting another tree (and certainly not a ‘Schubert’!) as a replacement. Thus, when you do need to remove yours, there will already be a substitute in place that is actively growing and there won’t be a gaping hole in your landscape.
There is no lack of small trees similar size to the ‘Schubert’ chokecherry that you can can then use as substitutes: crabapples, Japanese lilacs, Amur maples, hornbeams, hawthorns, smaller magnolias, etc. There are even a few plums and cherries that are considered resistant to black knot, including Amur chokecherry (Prunus mackii), but personally I’d be a little afraid of tempting fate by planting even a supposedly resistant Prunus species near an infested ‘Schubert’ chokecherry.
I’m curious to know if this blog will generate any kind of response from commercial nurseries growing ‘Schubert’ chokecherry. If I get one, I’ll keep you posted.