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I'd look towards root damage, first. Maybe small animals such as moles or other rodents have made homes a little too close to that one. If you don't see any signs, then it is definitely possible that it just happens to have succumbed to disease. Sometimes, even plants in the same species can have varied "immune systems".

Keep an eye out for small animals, root rot, or spreading to other plants. The best and most accurate way to know what is going on here will be to take samples to your local extension service. This article will give you more information, as well as a link to find the closest to you:

https://www.gardeningknowhow.com/garden-how-to/info/what-is-extension-service.htm