Diagnosing Common In-Season Issues in Soybeans

Brown Stem Rot - Disease Facts

• Caused by Phialophora gregata, a fungus that survives in soybean residue.
• Fungus infects roots early in the season, but symptoms of vascular system damage usually appear in mid-summer, during reproductive development.
• Brown stem rot (BSR) development is greatest between 60 and 80 ºF and when soil moisture is near field capacity.
• BSR may be more severe in fields where SCN is also a problem.

Brown Stem Rot - Identification and Symptoms

• BSR infection causes vascular and pith tissues to turn brown to reddish-brown, which is a distinguishing symptom of BSR.
  • Split stems longitudinally to inspect for BSR, checking at and between nodes near the soil line (Figure 1) and (Figure 2).
  • When disease is severe, discoloration is continuous from the base of the plant upwards.
  • When disease is less severe, discoloration only occurs at nodes, with healthy, white tissue between nodes.

Management

• Select Resistant Varieties: Pioneer® brand soybean varieties have been continually improved for resistance to BSR (Figure 3).
• Crop Rotation: Effective in reducing disease inoculum – two years away from soybeans is more effective than one.
• Tillage: Some tillage may be necessary to bury infected residue – the rate of inoculum decline is directly related to the rate of soybean residue decomposition.
• Manage SCN: Plant varieties resistant to both SDS and SCN.

Figure 1. Split soybean stems showing BSR symptoms mid-season (top) and in a mature plant (above).

Figure 2. Split soybean stems showing BSR symptoms in the plant on the left. The pith is dark brown while cortex remains green in infected plants.

Figure 3. Brown stem rot symptoms on a susceptible soybean variety (left) compared to a resistant variety (right). Note wilting, premature defoliation and lodging. Symptoms occur after pod fill begins and are more severe with dry soil conditions.

Phytophthora Root and Stem Rot

Disease Facts

• Caused by the soil-borne fungus Phytophthora sojae (also known as Phytophthora megasperma f. sp. glycinea).
• Pathogen has many races, and multiple races may occur in a field.
• Disease is favored by extended wet field conditions and temperatures between 60 and 80º F (15 to 27º C).
• May infect soybeans at any time during the growing season.
• Above-ground symptoms may not be evident for several weeks after initial infection.

Identification and Symptoms

• Symptoms begin in the root.
  • Brown, discolored taproot and secondary roots and less root mass.
  • Nodulation is often minimal, leading to chlorotic, N-deficient plants.
  • Affected plants may be stunted, so fields have an uneven appearance.
• Symptoms may spread to the stem (Figure 4).
  • Brown discoloration develops at the soil line.
  • Dark-brown to red-brown lesion may progress up the stem (key diagnostic feature of the stem rot phase).
  • Diseased tissues quickly become soft and water-soaked, and wilting and plant death may soon follow, especially during stress periods (Figure 5).

Figure 4. Split stem showing brown discoloration due to Phytophthora infection compared to a healthy stem.

Figure 5. Wilted plants surrounded by healthy plants are a common sign of Phytophthora.

Management

• Variety selection and seed treatments are the most effective means of managing Phytophthora.
• Corteva Agriscience uses molecular breeding to develop varieties with resistance genes and field tolerance to Phytophthora.
• Improve field drainage and remediate compaction and hardpan layers if possible.

Sudden Death Syndrome

Disease Facts

• Fungal disease caused by Fusarium virguliforme.
• Fungus colonizes only crown and roots of the plant.
• Above-ground symptoms are caused by a toxin produced by the fungus and translocated throughout the plant.
• Cool, moist conditions early in the growing season often result in higher disease incidence.
• Favorable disease conditions may result from early planting, high rainfall and/or low-lying, poorly drained or compacted field areas.
• Infection occurs early in the season, but symptoms usually do not appear until mid-summer.

Identification and Symptoms

• A blue coloration may be found on the outer surface of taproots due to the large number of spores produced.
• These fungal colonies may not appear if the soil is too dry or too wet.
• Splitting the root reveals cortical cells have turned a milky gray-brown color while the inner core, or pith, remains white.
• General discoloration of the outer cortex can extend several nodes into the stem, but its pith also remains white (Figure 6).
• Leaf symptoms first appear as yellow spots (usually on the upper leaves) in a mosaic pattern.
• Yellow spots coalesce to form chlorotic blotches between the leaf veins (Figure 7 and Figure 8).
• Affected leaves twist and curl and fall from plants prematurely.

Management

• Select SDS-resistant varieties.
  • Pioneer has developed elite soybean varieties with improved SDS resistance.
  • Soybean breeders have selected for genetic resistance in multiple environments with high levels of natural SDS infection.
• Manage soybean cyst nematode (SCN).
• Improve field drainage and reduce compaction.

Figure 6. Split soybean stem on top shows stem symptoms of sudden death syndrome infection. Split stem on bottom is healthy.

Figure 7. Soybean leaf showing symptoms of sudden death syndrome infection. Drying of necrotic areas can cause curling of affected leaves.

Figure 8. Soybean plants infected with sudden death syndrome. Necrotic areas of leaves dry rapidly. Leaves drop from the plant prematurely, but leaf petioles remain firmly attached to the stem.

Soybean Cyst Nematode

Nematode Facts

• Soybean cyst nematode (SCN; Heterodera glycines) is a small plant-parasitic roundworm that attacks the roots of soybeans.
• Beginning in the 1990s, the widespread availability of soybean varieties with PI88788 SCN resistance provided a largely effective management tool for SCN in North America.
• The PI88788 source of SCN resistance no longer provides effective control in many fields, meaning that SCN once again poses a significant threat to soybean yield that requires grower attention and management.

Identification and Symptoms

• Aboveground symptoms of SCN are not distinct and can be mistaken for soil compaction, nutrient deficiency, or other factors.
• SCN damage can appear as stunting and yellowing, often in circular or oval-shaped patches in the field (Figure 9).

Figure 9. Strips of SCN-resistant and non-resistant soybean varieties in a SCN-infested field, showing damage to the non-resistant varieties.

• SCN is best diagnosed by carefully digging up plants and looking for females and cysts on the roots (Figure 10).
• The bodies of the females are white and easily visible during early and mid-summer but turn brown and become more difficult to see late in the season. 

Management

• Samples should be taken and sent to a diagnostic laboratory to determine the HG type of the SCN population in the soil.
• Rotate resistant varieties – the most common source of resistance other than PI88788 is PI548402 or “Peking” resistance.
• Rotation to a non-host crop to reduce SCN pressure. 
• Nematicide seed treatments with activity against SCN are currently available and can provide added protection when used with an SCN-resistant soybean variety.

Figure 10. Lemon-shaped cysts of SCN visible on soybean roots.