Established Azaleas Dying

I live in northeast Florida, a warm climate. I have a large bed of azaleas covering an area about 30 feet long and 10 feet wide. There are probably 25 plants in this large area, all planted 35 years ago.
In recent years, I could tell that one area on one end was getting a bit dormant, as the leaves and spring blooms were not lush, and not real green. I monitored the ph level for several years, and it seems normal. These plants get partial sun. Other than this one area of dormant plants on one end, the other bushes were always healthy, green, etc.
However, during this summer, I have started losing some plants. The one area that has looked dormant for a few years has slowly started dying out entirely.... right down to the main stalks near the ground. The leaves are yellow, with rusty looking spots on them, both tops and bottoms of leaves. A group of plants on the opposite end of this flowerbed has now started to yellow, and show the brownish rusty spots on the leaves, and the plants look weak.
These plants have not been over watered, or under watered. During breif periods of drought that we had this year, I made sure they were watered, but I don't think that I overdid it.
In between these obviously dying plants, I still have perfectly beautiful plants that look perfectly healthy, green, and vibrant. Yet, I wonder if these good plants are going to fall prey to whatever is going on with the decaying plants.
Once again, these plants are 35 years old. I wonder if they are like some other species, and just have a life span that cannot be exceeded. For some reason, I thought that azaleas could last even over 50 years.
Before making this post, I have done a lot of research, and there are many diseases and other problems like root rot, but I can't seem to boil it down to one single probable cause. I don't think that I have insect problems, as I don't see any chewed off areas of the leaves.
Anyone have any ideas or comments on this ? Could it be that like me, they plants are getting too old ???
Thanks for any comments or ideas.
James
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I'd layer what you can but my main thought is rip them out and wait a years and start again. We layer constantly as young plant vigor is useful in dealing with mole/voles and droughts and etc. Azaleas and Rhododendrons in a warm clime must be challenging.
Ours do well in zone 5 but this url says otherwise.
http://extension.missouri.edu/publications/DisplayPub.aspx?P=G6825
I thought they like wet cold like Weston found in Tibet.
http://www.google.com/search?q=weston%20Azaleas%20and%20Rhododendrons&ieutf-8&oe=utf-8
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Bill S. Jersey USA zone 5 shade garden
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Bill, what do you mean by "layering?"
james
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Take a low laying branch and cover it with soil. Some folks nick or scar the possible new root area then place a rock on it or a brick and two years later you may have a new plant. Cut it from the mother plant and wait another year. I do this all the time last with some deciduous hollies. Heck if your neighbors have a neat bush approach them and split the bounty. There is also air layering which I am a failure with.
Look for the book "plants a plenty" forget it new $93 but maybe under $5 used or in your libary have fun. It Is out of print.
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Bill S. Jersey USA zone 5 shade garden
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Interesting... thanks Bill !!
James
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You can also take cuttings and root them in pots.
It sounds like this clump of azaleas is suffering from a disease or below ground pest. Have you tried calling your county agricultural extension agent, or local horticultural society or state azalea society?
    Una
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I do plan to call my county extension agent tomorrow.
Thanks
james
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