I have a Eucalyptus tree and also a bush. I pruned the bush in winter and this has resulted in some die back, which I have noted from your pages. The tree, however, seems to have started to lose its leaves for no apparent reason. They have become discolored, turning yellow then gradually going brown and dying. It is difficult to tell if the whole tree is affected or just the lower ones. We have had the tree two years and it has previously flourished.
Eucalyptus Die Back and Leaf Discoloration
Certified GKH Gardening Expert
The weather is likely the culprit. While it may have been fine in the past, things like extra moisture in the soil, extra winds or even slightly colder temperatures can harm the tree, especially if it had recently been pruned.
If it is alive, it will regrow its leaves. This article will help you determine if it is still alive: https://www.gardeningknowhow.com/plant-problems/environmental/how-to-tell-if-a-plant-is-dead-and-how-to-recover-an-almost-dead-plant.htm