My globe willow's leaves are turning yellow and falling off on one side of the tree. What's going on?
I can't tell for sure without seeing the tree, but there are three possibilities that come to mind.
The first two are that you have a disease affecting it. There are two that we fight in western Colorado that can cause this.
The first is a yeast-like organism called frothy flux. It actually ferments the inner bark of the tree. It usually causes a bubbly, foamy ooze from the infected site. It doesn't always ooze, or it can for a while then stop only to start again the next year. It can kill off enough vascular tissue in the tree to cause leaf yellowing and drop.
If you catch the infection early, it usually can be cut it out. If that's what you have, what you do is to mix up a bucket of 10 percent bleach water (one part bleach to nine parts water). Get a sharp, strong knife and cut out the infected bark around where it is oozing. You need to make sure to remove the bark completely down to the hard wood of the tree.
Don't cut out any more bark than you have to, and look at the cut edge of the bark. Infected tissue will usually have a brown discoloration while healthy bark is greenish white to ivory.
If you have to remove more than one half of the circumference of the bark around a branch, you're better off cutting it off completely.
Once all the infected bark is removed, thoroughly soak the wound with bleach water to disinfect it. Then just leave the wound alone. Don't paint it or wrap it or cover it. Leaving it open to the dry air will really help.
If you happened to miss a bit of the infection, it will usually start foaming at that point, and you will need to repeat this process.
The second disease is a more serious one called cytospora. This is an airborne fungus that attacks trees that are weak or under stress for one reason or another. The symptoms of this are very subtle, but the easiest thing to look for are small pinprick-sized bumps on the smaller, dead, dried out twigs that still have thin and smooth bark. They may not be everywhere, but there are usually patches of them.
If this is what's going on, I'm afraid you'll probably eventually lose the tree and there isn't a whole lot you can do about it. If the infection is confined to an isolated branch, you can cut it off, but usually by the time we notice something is going wrong, the disease has spread through the vascular tissues into the trunk of the tree and then there's no cutting it off.
The last possibility is that you have spider mites feeding on the leaves of your tree. These tiny little guys are barely visible with the naked eye, but they make up for their lack of size with numbers. They'll swarm in the thousands and thousands.
Spider mites like hot dry weather, so we start seeing their damage about now. Honestly, the best control for them is to repeatedly spray the tree with a hard jet of water. You're looking to knock them off of the leaf where they'll die. In addition, cooling and wetting the plant makes the environment less favorable for these little monsters. Spray the tree every day or every other day for a couple of weeks. It's amazingly effective!