Seedling Diseases of Soybeans

With a cool, wet start to the growing season, growers are now racing to get the crop in. After planting, be sure to scout seedlings for disease. The following are some of the prevalent pathogens in our area that can affect your seed – whether conditions are cold and wet, or hot and dry.

Types of Seedling Diseases, Causal Agents, and Symptoms

The three main categories of soybean seedling diseases are seed rot, seedling mortality, and root and stem decay:

Seed rot (V0-VE growth stages):

• Fungal pathogens: Pythium, Phytophthora, and Phomopsis
• Typical symptoms: soft decay of the seed and missing seedlings in the row or poor emergence

Seedling mortality (damping-off or seedling blight) (VE-V4 growth stages):

• Fungal pathogens: Pythium, Phytophthora, Rhizoctonia, and sometimes Fusarium
• Typical symptoms: hypocotyl appears pinched and rotted, as shown in the above photo. Yellowed and wilted leaves later turn brown. The leaves remain attached to the stem after plant death, shown below.

Photo courtesy of NDSU

Root and lower stem decay (VE-V6 growth stages):

• Fungal pathogens: Phytophthora, Rhizoctonia, and sometimes Fusarium
• Typical symptoms: brown to reddish-brown necrotic lesions on stem and rotted roots. The photo below shows characteristic reddish lesions on the stem of soybean plants infected with Rhizoctonia.

Photo courtesy of NDSU

Pathogen Identification

Identifying the specific pathogen causing disease will assist with management decisions for the field in future growing seasons. This may include planting resistant soybean products or using fungicide treated seed. Consider the following information for each field to help distinguish which pathogen is present:

• General soil temperatures
• General soil moisture (saturated, wet, or dry)
• Soybean product resistance or tolerance to Phytophthora
• Plant growth stage

Distinguishing between Pythium, Phytophthora, and Rhizoctonia damping-off

Pythium

• Hypocotyl will appear rotted. Leaves of infected seedlings will turn brown before dying. Stems are soft and watery.
• Common in wet, poorly drained, compacted soils.
• More common in cooler temperatures (50-60 degrees Fahrenheit).

Phytophthora

• Symptoms are very similar to Pythium infection.
• Symptoms usually appear above the soil line.
• Common in wet, poorly drained, compacted soils.
• More common in warm temperatures (80 degrees Fahrenheit).

Rhizoctonia

• Localized reddish-brown lesions can be seen in the cortical layer of the main root or hypocotyl. Stems remain firm and dry.
• Wilting symptoms usually occur in dry soil conditions.
• Symptoms usually appear below the soil line.
• Common in moist (but not saturated) soil conditions.
• Common in warm temperatures (80 degrees Fahrenheit).

For assistance with disease identification, contact your state Plant Disease Clinic:

• Minnesota – University of Minnesota Plant Disease Clinic: 612.625.1275
• North Dakota – NDSU Plant Diagnostic Lab: 701.231.7854
• South Dakota – SDSU Plant Diagnostic Clinic: 605.688.5545

Management

Diseases are difficult to control once they develop. Therefore, the most inexpensive, efficient, and effective disease control measures are preventative. Remember, any factor that delays germination or seedling emergence provides increased opportunity for disease infection.

Management of soybean seedling diseases includes:

• Avoid planting in environmental conditions that promote disease (temperatures below 68 degrees Fahrenheit, wet and/or compacted soils).
• Plant high quality, professionally grown seed that is disease free.
• Use fungicide seed treatments to provide control of Pythium, Phytophthora, Rhizoctonia, and Fusarium.
• Plant resistant or tolerant soybeans (Phytophthora only).
• Rotations with a grass crop for several years can help to reduce disease inoculum in the soil.
• Fungal pathogens can survive by overwintering in plant debris and in the soil; therefore, diseases may be more problematic in fields where no-till or reduced-till is practiced and in fields with high amounts of plant residue.
• Use good agricultural practices to reduce plant stress.

In summary…

• Several pathogens can cause soybean seedling diseases, which can slow germination and plant growth, and even cause seedling death.
• Cool temperatures and wet, poorly drained soils favor disease development.
• Fungal pathogens can be difficult to manage once they are present within the environment, so preventative measures provide the best control.