The short guide to sudden death syndrome in soybeans

First discovered in Arkansas in 1971, sudden death syndrome (SDS) now affects nearly every state where soybeans are grown. SDS is an important disease because it can overwinter in soil, creating larger infected areas with each growing season until most of the field is diseased. SDS is most severe when soybeans are planted early into cool, wet soils and when heavy mid-summer rains saturate the soil.

Sudden death syndrome showing up on earlier-planted soybeans. Replanted soybeans crossing rows show no SDS. Daren Mueller, Iowa State University, Bugwood.org

Cause

SDS is caused by the soilborne fungus Fusarium virguliforme. SDS overwinters in crop residue and freely in the soil. When the warm weather hits in the spring, fungi germinates and infects nearby soybean roots.

Symptoms almost never appear before flowering. Heavy rains during reproductive stages are conducive to SDS development. The fungus in the wet soil produces toxins that translocate to leaves and produce foliar symptoms. The fungus itself does not climb the stem more than a few centimeters above the soil line.

Symptoms

Symptoms first appear with the yellowing and defoliation of upper leaves. These first symptoms are often confined to a few sporadic areas throughout the field – often in wetter or more compacted zones.

As the disease progresses, other symptoms include:

• Diffuse chlorotic mottling and crinkling of the leaves
• Leaf tissue between major veins turning yellow then brown
• Leaflets dying and shrivelling
• In severe cases, leaflets dropping off, leaving only the petioles attached

Soybean leaf showing foliar symptoms of SDS. Leaf tissue between major veins is dying. Daren Mueller, Iowa State University, Bugwood.org

Diagnosis

There are three strong indicators that a plant is infected with SDS:

1. Tan to light brown discoloration and streaks on the interior of the lower stems and taproot when compared to a healthy plant (Abney, Shaner, Westphal, and Xing 2)
2. Small, light blue patches of spore masses on the taproot surface (Abney, Shaner, Westphal, and Xing 2)
3. Lack of secondary roots on an infected stem compared to a healthy plant (Abney, Shaner, Westphal, and Xing 2)

These roots are infected by SDS. Notice the white and light blue spore masses on the exterior of the taproot. Daren Mueller, Iowa State University, Bugwood.org

Management

Resistance

There are soybean varieties with low field tolerance all the way to full resistance.

Planting date

Planting early predisposes the soybean to infection. Always plant the fields last where SDS has been found in previous years.

Compaction

Correcting soil compaction and water permeability problems may reduce the risk for SDS (Abney, Shaner, Westphal, and Xing 3).

Fungicides

Fungicides applied to foliage have no effect and fungicides applied in furrow during planting or as seed treatments have limited effects.

Rotation

Rotation does not appear to significantly reduce SDS.

Yield loss

The extent of yield loss depends on the severity and timing of the disease relative to plant development. Early season development of SDS means the flowers and young pods might abort. Later season SDS development means fewer seeds per pod or smaller seeds. The earlier the prevelance of SDS, the more yield will be reduced. (Abney, Shaner, Westphal, and Xing 1).

Note: When a field has both SDS and soybean cyst nematode present, SDS is much more severe.

What else could it be?

• Brown stem rot darkens the pith, but there is little discoloration of the cortex (Abney, Shaner, Westphal, and Xing 2).
• Brown stem rot, charcoal rot, chemical burn, and stem canker will exhibit similar foliar symptoms but the dead leaflets usually remain attached to the petioles (Abney, Shaner, Westphal, and Xing 2).
• Stem canker is often associated with spore mats within the stem that are visible as longitudinal, net-like striations (Abney, Shaner, Westphal, Xing 1).
• Charcoal rot gets its name from the fine layer of ‘charcoal’ dust that develops within the stem tissue.
• With chemical burn, roots will likely appear completely healthy; whereas with SDS, the roots are visibly damaged.

Sources

• Westphal, A., T.S. Abney, L.J. Xing and G.E. Shaner. Sudden Death Syndrome of Soybean. 2008. The Plant Health Instructor. DOI:10.1094/PHI-I-2008-0102-01
• Westphal, A., T.S. Abney, L.J. Xing and G.E. Shaner. Diseases of Soybean: Sudden Death Syndrome. BP-58-W.