We have 3 pine trees along a bank and a cedar tree in front of them. They were planted over 22-25 years ago and are about 25-30' tall. One is 90% brown and looks incredibly bad. It seems that over the last 2 weeks this has happened and it is dying rapidly. I don't want the other pines and cedar to die also so what could be causing this and what can I do??
If 90% of the tree is gone, you wlll have to replace it or plant another species of tree. We were not able to identify the tree from the photos. It looks like it may be a leyland cypress or arborvitae.
Leyland cypresses are, unfortunately, susceptible to disease and pest problems. Look for bagworms, a defoliator of evergreens. http://extension.umd.edu/hgic/bagworms-trees-and-shrubs
They can can also get winter injury, which can cause branch die-back. There may have been a lot of root competition from the pines for moisture and nutrients. (https://extension.umd.edu/news/brown-leyland-cypress-winter-damage)
Seiridium canker is a very common fungal disease in Leyland cypresses. The small browning tips will eventually drop out. You can also prune out larger branches that have died back. There are no fungicides that are effective for this disease. Here is further information about it: https://extension.umd.edu/hgic/plants/seiridium-and-botryosphaeria-canker-leylands-trees
Here is information on our blog about leyland cypess https://marylandgrows.umd.edu/2018/04/11/why-is-leyland-cypress-turning-brown-winter-took-its-toll/
Arborvitae - They do not have a lot of disease issues. They can decline due to drought, poor planting techniques, root competition for moisture and nutrients, mechanical root damage, etc.
Here is our publication on these types of problems http://extension.umd.edu/sites/extension.umd.edu/files/_docs/programs/hgic/HGIC_Pubs/TreesandShrubs/...
They can be susceptible to bagworms. You will have to look for this.
Here is our publication on When to Remove a Tree http://extension.umd.edu/node/2273