Answer: Cotoneaster is generally problem free, but there are two diseases that might affect it:
Fire blight causes wilting and blackening of shoots and
the dead leaves hang on the twigs. There is no chemical
control. Prune out infected twigs well below the infected
Scab causes dark areas on the leaves that turn yellow,
then brown, then drop off. No chemical controls are
listed. Rake up and destroy infected leaves.
Aside from these two problems, there may be something going on within the soil that is affecting your plants. I don't think a small amount of concrete dumped in the bed would affect them, but I wonder if the new walkway has changed the drainage in the area? Dig down to see what's there, and make sure that the soil drains quickly (cotoneaster detests wet feet!).
Q&A Library Searching Tips