Unfortunately more info is needed to say what is wrong.* You don't say where you live and that might tell me if you are experiencing drought conditions* which could cause what you see.* There are many varieties of Cotoneaster, some are evergreen and some are deciduous.* I'm thinking yours are deciduous and the leaves are turning their fall color now.* That could be from environmental stress, a fungal or bacterial disease or insect pests.* Look closely at the branches to see if there are any growths.* Those growths might even be oozing a liquid. Are the tips of the branches in a hook-like position? * Look for insect pests on the limbs or under the leaves or any webbing.
Let me know what you see upon closer examination and if you are experiencing a drought and we'll see if we can find out what the problem is.* You were smart not to spray anything without knowing what the problem is as you could potentially stress the plants even more.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.