Why is my eucalyptus tree turning yellow?

Hi Kate - according to this site, it could be insects or a disease. This site has lots of info on eucaluptus trees: https://www.gardeningknowhow.com/ornamental/trees/eucalyptus/eucalyptus-tree-problems.htm

The environment could also factor in. Too much water/moisture can cause leave to turn yellow, then brown and fall off. Another thing to consider is if other trees in close proximity are also not having deeply green leaves. Light colored leaves are a sign that there's not enough iron in the soil. If that's the case, get an ironite soil additive that gradually dissolves. The package will tell you what to do.

Visit your local nursery to get more information for your area and project. Take pics and leaves to get the most help.

Too much water can result in yellowing of leaves on nearly any plant/tree. I suggest you take a couple of pictures of the tree. One from a distance and another up close of the yellowing leaves and go to your County Extension Agent's office. They can likely diagnose the problem and offer you advice about what you can do to nurse your tree back to health. The service is free.

Hi Kate, Too much water/moisture can cause leave to turn yellow, then brown and fall off. Another thing to consider is if other trees in close proximity are also not having deeply green leaves. Light colored leaves are a sign that there's not enough iron in the soil.