Phytophthora damping-off and root rot is caused by the fungus, Phytophthora megasperma f.sp. medicaginis. Standing water, poor drainage, and generally wet soil conditions favor disease development. This disease can be lethal to seedlings and established plants.
Seedlings may fail to emerge or can be killed after emergence. A conspicuous yellowing of leaves, particularly lower ones, is a characteristic symptom. Infected seedlings often wilt. Seedling tap roots have dark brown to black lesions and may be rotted 2-4 inches below the crown. A yellow discoloration of internal root tissue is often present. Tap roots may appear discolored.
The life cycle of Phytophthora on alfalfa is similar to that on soybean, however, the fungi are completely different and do not attack the other plant host.
Seed treatments are labeled for control of Phytophthora damping off. However, to protect stands beyond the seedling stage, alfalfa varieties with resistance to the fungus should be used.