Ask a Master Gardener: Weird spikes on berries are caused by fungus
A fungus called rust can produce weird formations on fruits such as crabapples and serviceberries.
Q: What's wrong with my serviceberries? What should I do?
A: By the looks of the photos you sent in, your serviceberries have a case of rust. Your serviceberries, also called juneberries or saskatoons, should be beautifully bluish-purple and plump right now. Yours, sadly, are not. Most are not ripened and are covered in tiny tubes or spikes. These tubes contain fungus spores. When the tubes open, powdery, bright orange spores are released.
From one of your photos, I can see that your serviceberry is planted next to a juniper bush. This is the perfect recipe for this type of fungi to grow. The fungus creating the rust, called Quince Rust, needs two different, specific host plants to grow — and you have both of them right there: a plant from the Cupressaceae family, which includes red cedar and juniper, and the other from the Rosaceae family, which includes crabapple, mountain ash and, you guessed it, serviceberry.
There is another type of rust that can develop from a different fungus, but also caused by the proximity of these two plant families. It is called Juniper broom rust, and it affects the leaves of the plants of the Rosaceae family. This disease can be spotted by yellow and slightly raised spots that may have a red border. Yellow droplets will form in the center of the leaf spots and eventually turn into raised, black dots. You may also see finger-like tubes on the leafstalks, stems and leaf veins.
You’d best check your juniper for rust, too, as they are susceptible to the diseases from the fungus formed from these plants. You can learn much more at: extension.umn.edu/plant-diseases/cedar-apple-rust#symptoms-on-eastern-red-cedar-and-other-junipers-1173261 .
The good news is these rust diseases will probably not kill your serviceberry. The No. 1 rule is not to plant members of the eastern red cedar and juniper plants within a few hundred yards of susceptible Rosaceae plants.
If you have already planted them, as you have, there are some other things you can do to mitigate the problem. In late winter or early spring, inspect your juniper. Look for galls. They will be dry and woody, but will turn orange and gelatinous in wet spring weather. You want to prune them out before that happens. On your serviceberry, prune off infected twigs and branches and remove them from your yard.
Good luck with your two not-so-merry plants, and thank you for writing in. We found out that two of Minnesota’s favorite landscape plants do not get along!
Written by U of M Extension Master Gardeners in St. Louis County. Send questions to email@example.com.