Garden Pests and Diseases: Wine & Roses Weigela yellowish leaves, 1 by pegoplant

pegoplant wrote:
For the past two springs, about half of my branches on my Wine & Roses weigela have brownish-yellow leaves (not dead, just that color) instead of olive green. They all bloom normally. I think last year they all went back to being olive green later in the season.

Both this spring and last were proceeded by a relatively harsh winter, but most of the plant is somewhat protected by our second-story deck. Last summer was mostly cold and wet.

You can see from the photo I have attached (bad-looking leaves on left, normal on right) that they bloom normally. Researching online has suggested root nematodes but I haven't found a photo to confirm it's the same condition. Anybody know if that is what my problem is?