Sudden Death Syndrome of Soybeans

Sudden death syndrome (SDS) is one of the most economically important yield-limiting diseases of soybean in North America. Since its initial discovery in Arkansas in the early 1970s, it has spread from the mid-South Mississippi River basin to infect soybean fields in almost all U.S. soybean-growing states and Ontario, Canada. SDS is capable of causing significant yield loss in soybeans, with reductions exceeding 50% in the most severe cases (Malvick, 2018). In the Midwestern U.S., SDS ranks second only to soybean cyst nematode (Heterodera glycines) in its economic impact (Bandara et al., 2020).

Origin and Spread

Symptoms of what would eventually be called sudden death syndrome were first documented in Arkansas in 1971 (Figure 1). The disease remained unnamed for over a decade (Roy et al., 1997). It was not until 1983, when the capability of the disease to significantly reduce soybean yields had become apparent, that it was given the name ‘sudden death syndrome’ (Hirrel, 1983). The name reflected two important aspects of the disease – first, the rapidity with which foliar symptoms could develop and culminate in premature plant death, and second, the fact that the causal pathogen remained unknown at the time. The term ‘syndrome’ reflected the fact that the disease was defined only as a set of symptoms at this point. It would be another six years before the soil-borne fungus Fusarium virguliforme was confirmed as the causal pathogen. During this time, the disease spread to several additional states. It was confirmed in Mississippi, Missouri, Kentucky, and Tennessee in 1984, and reached the Corn Belt states of Illinois and Indiana in 1985 (Roy et al., 1997). SDS has now been found in at least 29 states, spanning much of the soybean producing area of the U.S. (Figure 2).

Causal Pathogen

In North America, SDS is caused by the fungal pathogen Fusarium virguliforme, formerly known as F. solani f. sp. glycines. The genus Fusarium is a large group comprised of over 1,500 species, including several important plant pathogens. F. virguliforme is part of a subgroup of species within the Fusarium genus called the Fusarium solani species complex (FSSC), which includes over 60 plant pathogenic and saprophytic species (Coleman, 2015). F. virguliforme is believed to be an invasive pathogen in North America, having originally evolved in South America where three additional Fusarium species are known to cause SDS in soybean – F. tucumaniae, F. brasiliense and F. crassistipitatum (Spampinato, 2021). Genomic analysis suggests that these pathogens predate the presence of soybean in the Americas and likely jumped from some other host plant to soybean when it was brought over from Asia in the 18th Century (O’Donnell et al., 2010).

Phytotoxin

The toxin responsible for the foliar symptoms of SDS was first isolated and identified in 2011 (Brar et al., 2011). The fact that F. virguliforme had never been detected in symptomatic leaf tissue, as well as the fact that many Fusarium species were known to produce plant and animal toxins, had long led scientists to suspect that one or more toxins were responsible for the foliar symptoms of SDS. A small protein, given the name FvTox1, was found to be the cause. Scientists found that foliar symptoms only developed in the presence of light, leading them to believe that FvTox1 works by interrupting photosynthesis, resulting in the production of free radicals that cause chlorosis and necrosis of leaf tissue (Brar et al., 2011).

The fact that F. virguliforme undergoes its entire lifecycle in the roots of the plant raises the question of what possible advantage it would derive from causing damage to foliar tissue. Producing FvTox1 would incur a metabolic cost to the pathogen, so it would need to gain some survival benefit from doing so. Scientists believe this is a tactic by the pathogen to weaken the plant by reducing its photosynthetic capacity, thereby reducing its ability to resist infection and allowing F. virguliforme to establish and proliferate more successfully (Brar et al., 2011).

Root and Stem Symptoms

SDS begins as a root disease that limits root development and deteriorates roots and nodules, resulting in reduced water and nutrient uptake by the plant. On severely infected plants, a blue coloration may be found on the outer surface of tap roots due to the large number of spores produced (Figure 4). However, these fungal colonies may not appear if the soil is too dry or too wet. Splitting the root will reveal that the cortical cells have turned a milky gray-brown color while the inner core, or pith, remains white (Figure 5). The general discoloration of the outer cortex can extend several nodes into the stem, but its pith also remains white.

Leaf Symptoms

Though SDS infects soybean plants just after germination and emergence, leaf symptoms usually do not appear until the reproductive stages of crop development (typically mid-summer or later in the Midwestern U.S.). The appearance of symptoms is often associated with weather patterns that bring cooler temperatures and significant rainfall to an area during flowering or pod-fill. First symptoms are often noticed about 10-14 days after heavy rains that saturate soils. Wet soils can increase the production and translocation of the toxin responsible for foliar symptoms.

Leaf symptoms of SDS first appear as yellow spots, usually on the upper leaves, in a mosaic pattern. The yellow spots coalesce to form chlorotic blotches between the leaf veins (Figure 6). As these chlorotic areas begin to die, the leaf symptoms become very distinct, with yellow and brown areas contrasted against a green midvein and green lateral veins. Rapid drying of necrotic areas can cause curling of affected leaves. Leaves drop from the plant prematurely, but leaf petioles remain firmly attached to the stem.

Whole-Plant Symptoms

As plants lose leaf area and roots deteriorate, yield components are affected. Flower and pod abortion are common, resulting in fewer pods and seeds produced. Seeds that do develop are usually smaller. Later-developing pods may not fill, or seeds may not mature. Because plants and pods dry down faster, harvest losses may also increase in SDS-infected plants. Severity of yield reduction is highly dependent on the growth stage of the soybean plant when infection and symptoms occurred. In some cases, premature death of the entire plant can occur without the typical defoliation symptoms, as affected plants yellow and die gradually.

SDS symptoms are usually more severe if SCN is also problematic in the field. SCN increases the stress on the soybean plant and also provides wounds through which the SDS pathogen can enter the roots.

Distinguishing SDS from Other Diseases

Leaf symptoms of SDS can be similar to those of multiple other soybean diseases, including brown stem rot (Cadophora gregata), stem canker (Diaporthe spp.), charcoal rot (Macrophomina phaseolina), and red crown rot (Calonectria ilicicola). However, there are several characteristics that readily differentiate these diseases. To distinguish SDS from the other diseases, first examine the outside of the stem. If the outside of the stem has large brown-black sunken lesions, then it is likely stem canker. The key distinguishing characteristic of red crown rot is the presence of perithecia on the crown and roots just below the soil line, which look like tiny red balls and will give the crown a reddish coloration. Charcoal rot can be identified by scraping the outer stem tissues, which will reveal black, dusty microsclerotia.

If none of these symptoms are present, split the bottom eight inches of the soybean stem. If SDS is the problem, the pith of the stem will be white, and the surrounding cortex will appear grayish brown. In contrast, brown stem rot will cause the pith to be dark brown while the cortex remains green.

Tolerant Soybean Varieties

The first line of defense against SDS is genetic tolerance of soybean varieties. Soybean varieties can differ significantly in susceptibility to SDS infection, with tolerance exhibited primarily as a reduction in symptom severity. For that reason, variety selection is a key management practice to reduce plant damage and yield loss.

To assist growers in choosing tolerant varieties, Corteva Agriscience researchers rate products in multiple test sites with known historical SDS occurrence. These sites, located in states where SDS is problematic, are irrigated and/or planted early to encourage SDS development. Pioneer^® brand soybean varieties are scored on a 1 to 9 scale for SDS tolerance, with 9 being the most tolerant. Continued improvements in breeding for this trait have increased SDS tolerance in commercial varieties over time. For example, Pioneer Z Series soybean varieties, introduced in 2024, scored an average of 0.5 points higher across the entire product lineup for SDS tolerance compared to the preceding A Series.

Seed Treatments

ILEVO^® HL fungicide (active ingredient: fluopyram) is a seed treatment that provides protection of soybean seedlings from F. virguliforme infection, the causal pathogen of SDS. A Corteva Agriscience study conducted at field locations across eight states found that Pioneer^® brand soybeans treated with ILEVO^® HL seed treatment (fluopyram use rate of 0.15 mg ai/seed) yielded 1.8 bu/acre than the base seed treatment across all locations (n=46) and 3.8 bu/acre more than the base treatment at locations with moderate to high SDS pressure (n=12).*

Planting Timing

Newly germinated soybean plants are more vulnerable to infection by F. virguliforme in cool, wet soils. Although growers may be reluctant to delay planting when fields are ready, research has demonstrated later planting to be effective in reducing SDS occurrence. For this reason, growers may consider planting high-risk fields last in their planting sequence. If this results in a 1- or 2-week delay in planting, the impact on SDS occurrence could be significant.

Managing Soybean Cyst Nematode

SCN is a problem requiring management in many soybean fields that are also at risk to SDS. SCN increases the stress on the soybean plant and also provides wounds through which the SDS pathogen can enter the roots. Scientists have also discovered the SDS pathogen can be carried in SCN bodies. This means that managing SCN and limiting its stress on the soybean plant is critical to also limiting damage due to SDS.

Like SDS, SCN cannot be eradicated from an infested field. However, planting SCN-resistant varieties, use of seed treatments effective against SCN, rotating crops, and rotating sources of SCN resistance can reduce SCN populations in the field. Keeping SCN numbers below levels that will cause significant yield loss is the primary goal of SCN management. In addition, any practice that promotes good soybean health and growth will also help against SCN.

Improving Field Drainage and Reducing Compaction

Improving field drainage and reducing compaction go hand-in-hand as wet areas are easily compacted, and compacted areas stay wetter due to restricted soil drainage. Wet, compacted field areas are more susceptible to SDS infection (Figure 8). SDS infection is aided by high soil moisture conditions, and soybean roots already inhibited by compacted and saturated soils are further diminished by the disease.

When stress conditions develop in these fields, yields are often severely reduced due to a limited root system as well as the devastating effects of the SDS toxin on the plant. Growers should strive to improve field drainage and remediate compacted areas as a high priority to reduce the effects of SDS.

Evaluating Tillage Systems

Research findings on the effects of tillage on SDS have been mixed (Westphal et al., 2018), with some studies showing no effect (Kandel et al., 2019). However, several studies have shown that tillage can reduce the severity of SDS by helping soils warm up and dry out in the spring ahead of planting. A study conducted at the University of Missouri showed that no-till systems resulted in much higher percentages of SDS-infected leaves than disking or ridge-till with both May and June planting dates. High crop residue levels are known to result in colder, wetter seedbeds in the spring. In fields with high levels of SDS infection, growers may want to re-evaluate the tillage system they are using.

Reducing Other Stresses

Other plant stresses can render soybeans more vulnerable to SDS attack. These include herbicide stress, nutrient deficiencies, high pH, and pest pressure. Maintaining adequate soil fertility; reducing compaction; and controlling weeds, diseases, and insects all improve soybean growth and plant health and enable the plants to better withstand the effects of SDS.