Plant Health Initiative

Seedling Diseases - Symptoms

Pre-emergence damping off of soybean Photo credit: Dept. of Crop Sciences, University of Illinois

Infection by Rhizoctonia can be identified by dry, reddish-brown stem lesions limited to the cortical layer. Photo credit: University of Illinois

Poor emergence, and rotted, mushy seed is favored by wet soils. Phytophthora sojae can infect soybean seeds before emergence, causing them to rot. Infection by Pythium spp. also causes a soft, wet rot of seed or seedling tissue similar to that of Phytopthora. Seed-borne fungi such as Phomopsis can also rot seed at germination.

Damping off is the collective term for infection below or at the soil surface. Plants infected at the seedling stage will show the typical "damping-off" disease symptoms of bruised, soft stems and rotted roots. Plants frequently wilt and die.  Also called seedling blight.

Stem decay may occur at the seedling stage, or may not be apparant until later in the reproductive stages. In seedlings and older plants, a reddish-brown sunken lesion is a characteristic symptom of Rhizoctonia infection. Unlike infection caused by Phytophthora, or Pythium, the infected stems remain firm and dry.

Infection by the fungus Mycoleptodiscus terrestris causes symptoms that look very similar to Rhizoctonia, and may be misidentified as such. Mycoleptodiscus is active at higher soil temperatures (greater than 80° F) in the summer.

Premature decline of foliage and stems can occur later during reproductive stages, following infection earlier in the seedling stage. The brown stem rot pathogen (Phialophora) and the sudden death syndrome pathogen (Fusarium) cause unique symptom patterns on leaves; a more general decline is probably due to infection by Rhizoctonia or Mycoleptodiscus.

Phytophthora can also kill older plants throughout the season. Dark brown lesions form on the roots and root rot develops. Diagnostic for Phytophthora infection in older plants is a distinct, dark brown discoloration that begins below the soil line and spreads upwards into the stem.

Table 1. Summary of seed and seedling disease symptoms.

 

Disease	Pathogen	Growth Stage	Symptoms
Seed rot	Pythium Phytophthora Phomopsis	V0-VE	Soft decay of seed; missing seedlings in row.
Seedling mortality (damping off, seedling blight )	Phytophthora Rhizoctonia Pythium	VE-V4	Wilting, yellow leaves. Necrotic lesions on stems. Death of seedlings can occur quickly. Leaves remain attached to stem.
Root and lower stem decay	Rhizoctonia Fusarium Phytophthora Mycoleptodiscus Macrophomina	VE-V6	Reddish-brown lesions on taproot and hypocotyl; usually superficial. Phytophthora causes brown lesions on stem above soil-line.
Premature decline of foliage and stems	Rhizoctonia Mycoleptodiscus Fusarium (Sudden Death Syndrome - SDS) Phialophora (Brown Stem Rot - BSR)	R1-R7 but infection occurs much earlier	Wilt, chlorosis, and eventually death of leaves; interveinal tissues progress from yellow to brown, but major veins remain green (SDS & BSR). Internal browning of stems (BSR).

Source: Soybean Seedling Health, University of Wisconsin