We have young gum trees at the back of our property. We are on rural land and the trees have been in for about six months. Two trees have round lumps about half to full size of a marble. Can you please advise what this is and how to deal with it?
You don't mention where on the tree these bumps are, which is important. Your description sounds like some kind of gall, or maybe a canker. Galls are usually not terribly dangerous. https://www.gardeningknowhow.com/plant-problems/environmental/treating-leaf-gall-on-plants.htm
although some galls, and most cankers, are caused by infectious agents: https://www.gardeningknowhow.com/plant-problems/disease/how-to-fix-crown-gall.htm
For a good diagnosis you can take some samples to the nearest county extension service, where they can give you treatment advice as well as tips on how to keep your trees as healthy as possible. This link will help you find the nearest office: