Our small snowball bush has some curled leaves. I don't see any evidence of any bugs eating it or damaging it. What could be causing it? What can I do to treat/fix it?
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Check the information in the link - whether you've seen them or not, Snowball Aphid (ceruraphis viburnicola) is the most likely problem, given the appearance of the leaves, the fact that its a Viburnum opulus (Snowball bush or tree) and your location. They probably over wintered on your plant, have hatched out and are now causing these problems because they penetrate the sap stream, and that's what causes the distortion; they can cause stems and twigs to curl and twist as well as the leaves.
Treatment should have been started early on, when the leaves started to break or appear, but its worth having a go now, it should help. You can try spraying with Neem oil, but you'll need to repeat at least 3 times over 3 weeks, maybe more. Don't be concerned if sooty mould appears - its a side effect connected with the honeydew the aphids produce. It can be washed off, but its important to control the aphid infestation. There are systemic insecticides available if you don't mind something a bit more heavy duty - sprays containing imidacloprid for instance (Merit, Bayer Advanced), though this particular insecticide has been linked with damage to bee colonies.
I could be wrong, but it looks like a fungal infection, similar to peach curl on peach trees. If true, I would avoid getting the leaves wet. Water at the base of the plant. Remove damaged or fallen leaves. You can treat it with some kind of fungicide, but I'm not sure which ones off-hand. Make sure it gets lots of bright sunlight.
Giving it some rockdust (for the silica and calcium) together with a little extra potassium might help it out for the future by increasing disease resistance.
This and additional information can be found here.