Q.muscadine vines
I have 2 muscadine vines. They are 4-5 yrs old. One has died and now the other one is. What’s killing them? Leaves started turning yellow and now brown.
I can't tell you what's killing them, but I can give you some ways to evaluate the situation, possibly figure out what the problem is, and where to go for analysis.
The first thing is to look at the maintenance of the grapes, to see if they are missing something that they need. Plants that are healthy are much less likely to contract disease, so it follows that a sickly plant may well have some cultural deficiency. https://www.gardeningknowhow.com/edible/fruits/grapes/growing-grapevines.htm
After that, look at the leaves, the stems, the soil for descriptions of common diseases and pests: https://www.gardeningknowhow.com/edible/fruits/grapes/treating-grapevine-problems.htm
If you still don't clearly see the problem, take some samples of leaves, stems and soil to the nearest county extension service, where they will be able to advise you. This link will help you locate the nearest one: https://www.gardeningknowhow.com/garden-how-to/info/what-is-extension-service.htm
