This disease is especially severe on heavy, poorly drained soils. Although Phytophthora is most damaging to seedlings, mature plants may also be weakened and eventually killed. Cause of the Disease Phytophthora root rot is caused by a soil-borne fungus, Phytophthora medicaginis , which is present in most alfalfa field soils. This fungus survives in organic debris and becomes active in wet soil.
Infection usually occurs on small lateral roots.
From these initial infection points, the fungus gradually grows into the taproot. Stems may wilt or remain upright. In fields with tile drainage, healthy plants are often seen over the tile lines, while plants between tile lines show characteristic symptoms of the disease. Healthy plants on left. These lateral taproots are less efficient during droughty periods than normal taproots since they do not reach deep into the soil profile.