Diseases of alfalfa (
Phytophthora root rot
Phytophthora root rot is caused by the fungal-like organism Phytophthora megasperma. It is soil borne and is a pathogen only of alfalfa. Disease is most common in cooler weather in very wet soils where conditions are favorable for production of motile spores (zoospores). These spores are attracted to alfalfa roots and infect anywhere on the tap root. P. megasperma causes a root rot of seedlings or mature plants (photo 1). Phytophthora root rot occurs most commonly in poorly drained soils.
Phytophthora root rot is controlled by using tolerant varieties of alfalfa and cultural practices such as good field leveling, avoidance of heavy soils and rotation.
1 
June 4, 2007