See:
Pea (Pisum sativum) - Root Rots
Notes There have not been a lot of reports of root rot but the following have been reported in the USA: Aphanomyces euteiches, Phytophthora cactorum, Pythium oligandrum and Rhizoctonia solani. The first three organisms are water molds that thrive under soil saturation conditions. Water management is essential. A root rot due to Fusarium was one of the few diagnoses by the OSU Plant Clinic.