I am hoping you can tell me what is plaguing these yews. They form a hedge...
Your photo is out of focus so we can't say for sure, but this looks like you have a scale insect, most likely Fletcher or cottony camellia scale. They are the white bumps on the woody branches. Scales are sap sucking insects, and they excrete a sweet honeydew which drips on lower foliage where a black sooty mold grows it. Ants are often drawn to the honeydew as a food source.
At this point your yew looks pretty decimated. While we don't usually suggest pruning in the fall, in this case, you surely could as dead material can be pruned out at any time of year. Cut out all the dead wood, all the way to the ground if necessary. If the roots are still alive, the shrub may regenerate itself.
If there are still green branches left, you can apply an insecticidal horticultural oil in the dormant season to kill overwintering nymphs.