Infection rarely progresses into the interior until very advanced stages. Root disease caused by Aphanomyces can occur as two distinct forms in sugar beets. The early season, or seedling phase, is commonly referred to as black root. It does not rot the seed or affect initial stand establishment, but can significantly affect plant stands by inducing damping-off of seedlings several weeks after emergence. Symptoms on infected seedlings begin as grayish, water-soaked lesions on stems near the soil level.
Lesions soon progress from gray to black, causing the stem to become thin and thread-like.
Cotyledons seldom wilt before advanced stages of disease kill the seedlings. Plants may recover at night and regain turgor, but are more prone to wilt again in the heat of the day. Leaves also may take on a scorched appearance and become brittle. In severe cases, the entire root may disintegrate, leaving only strands of vascular elements intact. The aboveground symptoms at this stage can be deceiving.