I agree with the other answers, you probably have an aphid infestation.
But rather than spraying your trees to get rid of the aphids, you may try putting a ring of glue on your tree, to get rid of the ants.
Without their support, your aphid infestation will probably quickly rescind due to the natural predators.
The trees have probably got aphids. From the RHS website here:
Ants may be found climbing plants with aphid colonies, they tend the
aphids obtaining honeydew as a reward. The ants will remove aphid
Not much you can do about it. If you spray the aphids, you risk killing useful predators such as ladybirds. Best treat it as all part of nature's ...
The problem with not treating for aphids is it's likely to get worse, especially with ants warding off aphid predators. With large trees, it's obviously next to impossible to treat for aphids because you can't reach the top easily, but for your smaller tree, if there are a lot of aphids,I would recommend some treatment, even if that's just you physically ...
It might be instructive to look for the Hawthorn shield bug which feeds on the fruits of both hawthorn and other species including rowan (mountain ash). The fruits are attractive because both are pomes, like apple fruits, with a fleshy, juicy outer cover. The shield bugs pierce the fruit and allow them to bleed juices which contain sugars that the ants can ...
This is known as 'brown center and hollow heart'. It is caused by abrupt changes in growing conditions (think of temperature or watering regime changes).
You can still eat them, but you better remove the affected parts first. Unfortunately it makes it not suitable for sale anymore.
Aphids and ants might like my trees, but they love garden nasturtium.
If there's a nasturtium somewhere close to the trees, aphids, ants and ladybugs will happily move there (here's a related article):
As a bonus, these plants are easy to grow. Every part of it tastes great, provided it's not too rich in protein.
It is very leggy and does need 'surgical intervention' in the form of cutting all the stems down to two or three inches to generate new, bushier growth. The other question is, does it need a bigger pot? Turn it out to see if its got roots winding round and round the outside - if it does, it needs the next size up pot, with a drainage hole, and use fresh ...
Contact a local tree care company. A lot of the times they offer a systemic injection into ground around the root system of the tree. The tree will uptake the treatment and then when the aphids bite onto the tree (usually the underside of the leaves where its soft), they will take in the treatment and die. I have had this done with Elm Scale for my Elm tree.
You didn't say how big the tree is. If you have aphids on roses, you can just brush them off. A tiny minority might make it back to the plant or be carried back by ants, but most wont.
Doable if it's a short sapling, but perhaps not too practical for huge mature tree :).