Q: What’s going on with my young weeping cherry tree. Planted this past spring, it did pretty well, but seemed to drop leaves early. If I rub the white off, I get a bright yellow mustard-colored stain. It’s sticky, but not gooey.
A: It is an infestation of scale insects. I don’t think I’ve ever seen this many so densely packed on one tree!
As you probably know, scale insects suck the sap of the tree and weaken it.
Since you are noticing them in December, the scale will be pretty easy to control by spraying horticultural oil (click for sources) on them now, again in late January and once more in early April.
The April spray is particularly important because that’s when the young scales emerge from under the covering of their mother’s wax.
In May, wrap some masking tape, sticky side out, around a few limbs close to where the adult scales were growing. Examine the type every few days to see if there are any yellow baby scales caught on the glue.
If so, spray again with horticultural oil.