I had a sugar snap pea plant growing, and when it reached about 6 inches in length, it just started to turn yellow and eventually died. The soil was well draining and had manure. Why did it die?
Sugar Snap Peas
Certified GKH Gardening Expert
It sounds like Fusarium Wilt. You can read more about it here:
If your pea plant was in a container, dispose of the soil and bleach the pot before using it again. If it was in the ground, you can sterilize the soil by covering it with black plastic and cooking the Fusarium fungus to death.