CPN-1011 — February 2016
Sudden death syndrome (SDS) is one of the most recognizable diseases in the North Central soybean-producing areas of the United States and Ontario, Canada. However, the disease’s importance varies greatly throughout the region. In some states, SDS may be present in the majority of fields and cause statewide yield reductions. In others, the disease is regional or rare.
In North America, the fungus Fusarium virguliforme causes SDS. SDS symptoms are similar to those caused by some other soybean pathogens, making it difficult to diagnose in the field. The presence of other pathogens, such as soybean cyst nematode (SCN), can also exacerbate disease.
This publication examines the symptoms of SDS, describes conditions favorable for the disease, explains how SDS differs from several other plant problems that may look like SDS, and recommends management practices.
The first observed SDS symptoms are often early leaf symptoms, including yellow spots between leaf veins (Figure 1), known as interveinal chlorosis. These lesions may expand and turn brown as the spots expand between veins, which is called interveinal necrosis. Leaf veins remain green (Figure 2).
As the disease progresses, leaves die and prematurely fall from the plant, while petioles remain attached to the stem (Figure 3). Pods and seeds may be aborted. Foliar symptoms of SDS can resemble those of many other diseases (see Table 1).
Plants infected by F. virguliforme can have discolored and decaying roots even if there are no noticeable foliar symptoms of SDS. The woody tissue in the taproot (or cortex) can be brown to dark gray, while the upper portion of stem pith remains white (Figure 4).
Several soybean diseases may cause similar foliar symptoms to SDS, which makes it important to closely examine the stem and root for an accurate diagnosis see Table 1). If the stem pith is brown and discolored, the disease might be brown stem rot (BSR, caused by Cadophora gregata) rather than SDS; however, it should be noted that plants can be infected by both SDS and BSR fungi. When soils are wet, the SDS fungus can reproduce on the root surface, sometimes producing a mass of spores that are purple-blue (Figure 5).
The fungus that causes SDS survives the winter in crop residue and soil (Figure 6). Early in the season, the fungus infects soybean roots, sometimes within a few days of seed germination. Infection is favored by cool, wet soil conditions.
The SDS fungus produces toxins that plants transport through their water-conducting vessels (xylem) up to the leaves, which results in foliar symptoms. Typically, these symptoms occur after flowering, but they have been observed during early soybean growth stages on highly susceptible varieties or when conditions are cool and wet after emergence (Figure 7).
Hot and/or dry conditions in the spring or summer can halt SDS development. When SDS symptoms are severe, plants may defoliate prematurely, but petioles will remain attached to the stem.
Cool, wet field conditions shortly after planting favor early root infections and disease establishment. Frequent or heavy rains midseason can favor early symptom expression. Irrigated fields are at higher risk for SDS.
The presence of soybean cyst nematode (SCN; Heterodera glycines) in the same field where SDS is found can increase the severity of SDS symptoms (Figure 8). Therefore, fields with SDS should be tested for the presence of SCN. Research has linked the presence of SCN with higher levels of SDS and increased yield loss. However, SDS can occur independently of SCN, and vice versa.
Some crop production practices may also increase the risk of disease development. Fields with poor soil nutrients, low pH, poor drainage, or moderate to severe soil compaction are at higher risk of SDS.
Yield losses due to SDS can be highly variable and depend on several factors, including the environment, the timing of symptom development, the susceptibility of a soybean variety to the disease, and the levels of SCN present in the field. SDS can weaken plants and cause premature defoliation — infected plants also may produce fewer and smaller seeds (Figure 9).
Yield losses can be substantial in years when susceptible soybean varieties are planted in cool, wet fields with a history of SDS and SCN. If symptoms develop later in the season, or weather is not conducive for disease can be minimal. Losses can be minimal. Losses are rarely fieldwide since the disease often occurs in patches (Figure 10). If foliar symptoms appear at or after growth stage R6, yield loss may be minimal.
Diagnosing SDS can be challenging, because other diseases and disorders cause symptoms similar to SDS (see Table 1). Foliar symptoms alone are not enough to diagnose SDS. It is important to inspect plants closely, dig and inspect roots, and split soybean stems open.
After splitting stems, the pith of a plant infected with SDS will be white even if roots are severely rotted. The cortex may have brown to gray discoloration. You may also need a laboratory diagnosis to distinguish SDS from other soybean diseases and disorders.
SDS management strategies include planting soybean varieties with SDS resistance, managing SCN, using effective fungicide seed treatments, improving soil drainage in fields with recurring SDS, rotating crops, avoiding or reducing soil compaction, and maintaining proper pH and fertility levels. Growers must plan on implementing an integrated SDS management strategy, because a single management tactic alone is not likely to adequately control the disease.
There are no soybean varieties completely resistant to SDS, but partially resistant varieties are available (Figure 11). Planting the most resistant varieties available is the foundation of an SDS management program. Keep in mind that planting partially resistant varieties does not ensure complete control of the disease, but it will minimize yield loss.
Weather and field conditions influence disease severity, even on partially resistant varieties. Most soybean breeding programs have concentrated on evaluating resistance based on foliar symptoms. Less is known about resistance to root rot, and the correlation between root rot and foliar symptoms is not well understood.
SCN populations have been associated with SDS and may increase disease severity. Practices that decrease SCN — such as planting SCN-resistant varieties and rotating crops — may delay SDS onset and reduce disease severity.
Minimizing soil movement may slow the introduction of the SDS fungus to new fields. Improving drainage and soil structure and reducing compaction can also help to alleviate SDS symptom severity.
Tillage in compacted areas may reduce disease severity; tillage can also allow soils to warm more quickly during spring. However, some research indicates that SDS is reduced in no-till fields. Always consider the impact of tillage on soil erosion and quality when using it to manage SDS.
Rotating to corn does not typically reduce SDS in subsequent soybean crops. Corn and several other crop and weed species can maintain or even increase the SDS fungus in the soil. A number of other legume species can be hosts of the SDS fungus and suffer root rot. These include alfalfa, pinto bean, navy bean, white clover, and red clover.
One study in Iowa suggested that a more diverse rotation can reduce SDS. In this study, adding a third year into the corn-soybean rotation (where either alfalfa or an alfalfa/red clover mixture were planted) reduced the amount of SDS that developed in the subsequent soybean crop (Figure 12). Incorporating small grain crops into the rotation can also reduce disease. More research is needed to understand the effect of crop rotation and cover crops on SDS.
A traditional SDS management method is to plant fields with a history of the disease last, allowing the soil to warm and dry, thereby reducing conditions that favor disease. However, research shows that delayed planting can decrease yield potential and may not reduce SDS. Therefore, it is recommended that delayed planting, or planting high-risk fields last to reduce SDS only be considered in fields with a severe history of the disease.
Soil fertility levels may affect SDS occurrence and severity, but results from published research are somewhat unclear about how different fertilizer applications and soil fertility levels affect SDS. Maintaining proper soil fertility levels is important in reducing plant stress and maintaining overall plant health, which may help reduce the occurrence of soybean diseases such as SDS.
High soil moisture levels within a few days after planting increases the risk of infection by the SDS pathogen, and irrigation during the mid-late soybean reproductive stages may increase SDS foliar symptoms. Farmers should recognize that over-irrigation during the soybean reproductive stages can increase the risk of SDS.
Foliar fungicides and most seed treatments are not effective in reducing SDS. However, the fungicide fluopyram (ILeVO®, Bayer CropScience) has efficacy against SDS. As a seed treatment, fluopyram has reduced SDS severity and protected yield on susceptible varieties compared to a base seed treatment in several research trials conducted by extension plant pathologists in the North Central United States and Ontario, Canada.
Thiabendazole (Mertect®, Syngenta CropProtection) is also labeled for SDS management but has not reduced SDS to the level fluopyram has in research trials. Additional fungicide seed treatments are being developed and evaluated for their effect on SDS.
Brown stem rot (BSR) foliar symptoms include yellowing and necrosis between veins. The stem symptoms include brown, discolored pith tissue, especially near nodes of soybean stems.
BSR-infected plants will have discoloration in the pith of the stem. BSR does not cause rotted roots.
Plants with Fusarium wilt have brown vascular tissue in the roots and stem. Leaves turn yellow and die but remain attached to the stem. Plants eventually wilt and possibly die. The disease is frequently observed in Ontario, Canada, but may be present in other northern states, especially in wet years.
Fusarium wilt does not cause interveinal chlorosis on leaves and the leaves stay attached to the plant after they die.
Stem canker symptoms can include leaves with necrosis between leaf veins or completely wilted leaves and stems.
A plant infected with stem canker often has a characteristic red lesion on the outside of the stem. The lesion will typically start at a node above the soil line and extend upward.
White mold is typically more of a problem in years with rainy and cool environmental conditions that occur at flowering. Lesions develop on the nodes and expand up the stems. Leaves often die and remain attached to the stem.
Plants infected with white mold have white fungal growth on the outside of stems. In addition, the white mold fungus produces sclerotia that are hard and black.
Patches of soybeans with yellow leaves may appear in soils that have low potassium levels. Soybean cyst nematode and Diaporthe spp. may increase symptoms. Symptoms include yellowing that begins at the leaf margins of the uppermost trifoliates.
The foliar symptoms of potassium deficiency are different from SDS symptoms. Stems and roots with potassium deficiency will not have symptoms.
Foliar SDS-like symptoms can appear after applying certain triazole fungicides, especially when conditions are hot and dry.
Check the pattern of symptom development and field activity prior to symptom development. Symptoms will only appear on leaves.
To learn more about SDS, visit the visit the NCSRP Soybean Research Information and Initiative (SRII) website (www.soybeanresearchinfo.com) or consult your land-grant institution or OMAFRA. Other publications in the Soybean Disease Management series are available by visiting the SRII website, your land-grant institution’s website, or the Grain Farmers of Ontario website (www.gfo.ca).
Daren Mueller, Iowa State University
Carl Bradley, University of Kentucky
Martin Chilvers, Michigan State University
Anna Freije, Purdue University
Loren Giesler, University of Nebraska
Adam Sisson, Iowa State University
Damon Smith, University of Wisconsin
Albert Tenuta, OMAFRA
Kiersten Wise, Purdue University
Emmanuel Byamukama, South Dakota State University
Anne Dorrance, Ohio State University
Doug Jardine, Kansas State University
Dean Malvick, University of Minnesota
Sam Markell, North Dakota State University
Laura Sweets, University of Missouri
All photos were provided by and are the property of the authors and contributors except for Figure 4 by Tom Hillyer, Figure 5 by Dan Sjarpe, Figures 8 and 13 by Craig Grau, Figure 9 by Greg Shaner, Figure 12 by the Leopold Center for Sustainable Agriculture, Figure 14 by John Kennicker, Figure 15 by Alison Robertson, and Figure 18 by W.J.Mansour.
The Soybean Disease Management series is a multi-state collaboration sponsored by the North Central Soybean Research Program (NCSRP, www.ncsrp.com).
The Crop Protection Network (CPN) is a multi-state and international collaboration of university and provincial Extension specialists, and public and private professionals that provide unbiased, research-based information to farmers and agricultural personnel.
This project was funded in part through Growing Forward 2 (GF2), a federal-provincial territorial initiative. The Agricultural Adaptation Council assists in the delivery of GF2 in Ontario.
The authors thank the United Soybean Board and Grain Farmers of Ontario for their support.
Design and production by Purdue Agricultural Communication
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