Black knot on Serviceberry

Question: I just purchased and planted a Serviceberry this week. It had a black knot that looked similar to the black fungus on plum trees. What is it?

responder: Answered by the Home and Garden Information Center (HGIC), University of Maryland Extension. For timely gardening information and tips, subscribe to our Maryland Grows blog and quarterly newsletter. Follow us on Facebook, Instagram, and YouTube. Replied February 24, 2023, 12:22 PM EST
response: Although the knobby growths on plum trees from Black Knot can superficially resemble those in your photos, your Serviceberry probably has rust. This is a type of rust called Cedar-Quince Rust, which is noted for causing these twig infections and swellings instead of the more commonly-noticed orange spore-producing lesions on foliage or unripe berries. (That's a different type of rust, like Cedar-Apple Rust.) Although Serviceberry and cherry/plum trees are in the same plant family, in our experience Serviceberry does not contract Black Knot, which we see regularly on plums (flowering or fruiting).Still, the approach to treatment is similar with both fungal issues: prune out the infected twig sections (just trim down to the next-lowest junction, not necessarily any further) and dispose of the trimmings (don't compost). While use of a fungicide might be reasonably successful, such efforts are preventative only (not curative) and may still fail to work some years when infection pressure is high due to weather conditions or a nearby alternate host laden with spores. Multiple treatments would need to be applied each year, starting around early spring. Fungicides might risk harm to other organisms, so we tend to discourage their use; plus, they must be applied vigilantly (according to label directions) since a missed re-application can give spores an opportunity to infect.Rust fungi are unusual in that they require two unrelated host plant species to complete their life cycle. The infectious stages of spore production switch between one and the other -- spores on the Serviceberry are going to infect the other host, not more Serviceberry tissue. The alternate host plant for this group of rust fungi are junipers, like our native Eastern Redcedar (Juniperus virginiana), an abundant evergreen in sunny natural areas. The wind-blown spores produced on fungal growths on infected junipers can travel over a mile to susceptible Serviceberry hosts, so elimination of this reservoir of fungus is not practical. Eastern Redcedar is an important native plant for wildlife value, and many non-native junipers which may also serve as hosts are commonly used in landscaping. (Rusts don't typically disfigure the junipers if their infections are mild, so they tend to go unnoticed.)The severity of any plant fungal infection can be highly weather-dependent, with wet springs favoring higher rates of infection. (Wet leaf surfaces are usually easier for spores to infect.) Due to the variability of the weather, an outbreak one year won't necessarily repeat itself every year thereafter, and fortunately these particular infections don't generally result in serious dieback. That said, Cedar-Quince Rust does tend to cause more damage overall (on both hosts) than Cedar-Apple Rust since it can kill young twigs as the swellings cut off the supply of moisture and nutrients to the growth beyond it.There are some rust-susceptible trees that have been bred to be resistant, like multiple Crabapple varieties, but to our knowledge there is currently no Serviceberry cultivar which has proven to be reliably resistant to infection.If you're interested in learning more about this and similar types of rust, the linked pages below provide images and more details about the life cycle of these interesting fungi. Although they feature a variety of rust types and host plants, the basic information applies here as well:Cedar-Quince Rust - Univ. of MissouriRusty Hawthorns - Ohio StateJunipers Garnished with...Goo - Ohio State (similar article to the above) Emily

responder: The Question Asker Replied February 24, 2023, 2:12 PM EST
response: Hi Emily, thank you so much for your thorough response. If that was the only location on the serviceberry that I could find evidence of rust, would that mean that it is not elsewhere, or has it just not developed elsewhere on the tree?Also, I have a row of red cedars, which I love, should I not keep the serviceberry if it is a risk? Heather On Fri, Feb 24, 2023 at 12:22 PM Ask Extension <<personal data hidden>> wrote:

responder: Answered by the Home and Garden Information Center (HGIC), University of Maryland Extension. For timely gardening information and tips, subscribe to our Maryland Grows blog and quarterly newsletter. Follow us on Facebook, Instagram, and YouTube. Replied February 24, 2023, 2:33 PM EST
response: Hello Heather,  It might be a fairly localized infection for now, but time will tell since symptoms of infection (for various plant diseases) don't always manifest quickly. If you see more twig swellings appear over time, especially if they develop an orange-y coating, trim them off and discard them. Although growing vulnerable hosts next to redcedar may bring an increased risk of infection, honestly, there are so many other wild redcedars and other junipers in gardens that the pathogen could still blow-in from elsewhere when the weather cooperates and a tree is infected in the general area. You can definitely look over your redcedars to check for galls, though, and trim any in reach off as soon as they're detected since April (possibly March) tends to be the time when we start to see those juniper galls disperse spores during wet mild weather. Once the spores are released and windborne, there's little you can do unless you're using a fungicide on the serviceberry. (Even then, it's borderline too late to prevent infection for that year in that example.)  In our opinion, serviceberry offers a variety of aesthetic and wildlife benefits that it's probably worth the risk to grow regardless of this vulnerability. We don't regularly observe rust killing serviceberries, at least, though sometimes infections can be an eyesore depending on their severity. To be fair, sometimes the cultivars most commonly sold (like 'Autumn Brilliance') seem to be the most susceptible to rust, whereas straight-species trees may have a better chance at having resistance because they're more genetically unique from individual to individual, but this isn't something that can be predicted or determined by the plant's outward appearance.  Miri