To help you navigate the many diseases impacting soybean plants, we’re providing you with a 2-part series that equips you with guidance to identify and manage nearly a dozen damaging early and late season soybean diseases. For additional identification tips, read the second part of our series, Soybean Diseases Series: Proper Identification.
Types of Soybean Diseases
Healthy plants and Pythium-infected seedlings. Source: Tom Allen, Mississippi State University ExtensionPythium
What to look for: There are more than 30 species of Pythium that can affect soybeans, and they thrive in cold, saturated soils covered in heavy residue. Pythium-infected seedlings are most often found in low pockets of early planted soybean fields or heavy wet areas. Plant death usually occurs randomly or in small patches. Infected seedling tissue appears water-soaked and is soft to the touch, which are signs of wet rot. Plants that do emerge often have leaves that are gray-green and turn brown as they die.
When to watch: Pythium hits the soybean plant early at emergence. Pythium can cause pre-emergence damping-off, or seed rot, as well as post-emergence damping-off of seedlings.
Sclerotinia Stem Rot (White Mold)
What to look for: An early symptom of white mold is leaf wilting in the upper canopy. Leaves may look gray-green or off-color. Cankers may appear on the stems at the nodes.
When to watch: White mold is prone to develop in moderate temperatures (less than 82o F) in the canopy and under frequent rainfall especially as the plants begin to flower and set pods. White mold can cause extensive yield losses in fields that otherwise have high yield potential.
Rhizoctonia infection in a soybean plant. Source: SyngentaRhizoctonia Root Rot
What to look for: Typically, Rhizoctonia is associated with warmer temperatures, hill slopes, or sandy, well-aerated soils. The soilborne pathogen produces sclerotia and can survive in the soil for multiple years. Infected seedlings dampen off after emergence, and the hypocotyls develop a reddish-brown, slightly sunken lesion near soil level. The lesions remain on the stem’s outer surface and can eventually spread to the roots, while the tissue stays firm and dry. The disease caused by the fungus Rhizoctonia can cause seedling blight and root rot in more mature soybean plants. Lower leaves may begin to turn yellow, and plants may appear stunted and less vigorous. When dug up from the soil, infected plants may reveal poorly developed root systems and discolored or rotted lateral roots. The stem may have a brick-red discoloration at the soil line that extends in either direction.
When to watch: Rhizoctonia is a disease that can affect the soybean plant both early and late into the growing season, but it is most severe when it infects young seedlings. The root rot phase of Rhizoctonia typically doesn’t show up until later in the growing season. Severely infected soybean plants that are stressed by hot, dry conditions may die. If cool, wet conditions occur after a Rhizoctonia infection, a flush of secondary roots above the stem may be visible.
Charcoal rot symptoms in a soybean plant. Source: SyngentaCharcoal Rot
What to look for: In advanced stages, leaves turn yellow, wilt and wither but remain attached. After the flowering stage, the taproot and lower stem appear silver or light gray. In later stages of the disease, black specks appear underneath the “bark” that is peeled away from the roots and stem base, hence the name charcoal rot. Seedling infestations of charcoal rot may mimic those of Rhizoctonia, producing reddish lesions on the stem of the seedling. However, if charcoal rot is present, these lesions can be scraped off with light pressure.
When to watch: Charcoal rot typically develops in hot, dry weather in combination with nutrient-deficient soils or unfavorable growing conditions.
Fusarium Leaf Blight
What to look for: There are more than 20 species of Fusarium that can affect soybeans. Conditions favoring disease can range from hot and dry to cool and wet. Fusarium is usually identified by reddish-brown discoloration that causes the seedling roots to appear dry and shrunken. Symptoms are normally restricted to the tap root and aren’t apparent above the soil line. However, leaves can appear burned and may later wilt and fall off plants.
When to watch: Fusarium can cause poor and slow emergence, which can result in stunting and possibly seedling death.
Phytophthora screening at a Syngenta research and development site. Source: SyngentaPhytophthora Root Rot
What to look for: Phytophthora is another root rot that can infect plants early in the season and reduce stands and yield potential. The disease is typically most severe in poorly drained soils with high clay content. Infection is favored by moist to saturated conditions with soil temperatures of 70° to 80° F. If the plant is infected early in the season, symptoms of Phytophthora may resemble those of leaf blight. As plants enter the flowering stage and begin to set pods, symptoms of late-season stem and root rot may develop. Phytophthora symptoms are very similar to Pythium but tend to occur in a continuous, dark chocolate brown lesion that girdles the stem. Infected older plants show reduced vigor or gradually die over the season. There are multiple strains of Phytophthora sojae so knowing which are prominent in your fields is critical to knowing how to manage this pathogen.
When to watch: Look for signs of infection appearing as stem and root rot during and after flowering or as blight early in the season.
Brown stem rot in soybeans. Source: SyngentaBrown Stem Rot
What to look for: Early symptoms include light green to yellow blotches in the interveinal leaf tissue, which can be confused with sudden death syndrome. The vascular tissues and pith of the soybean stem are also affected with a brown discoloration, which can be seen when the stems are split open. Later in the season, this brown discoloration may appear almost continuous within the stem.
When to watch: Foliage symptoms usually develop as soybean plants are beginning to set pods.
Comprehensive Control Strategies
Protect the yield potential of your high-yielding soybean crop with a multiple mode of action fungicide such as Trivapro®, Miravis® Neo or Miravis® Top. Always read and follow fungicide labels to identify the best fungicide for addressing diseases observed in your fields. As you plan ahead to next season, you can reduce the risk of potential outbreaks by treating your soybean seed with a Golden Harvest® seed treatment. This will ensure that your seedlings have a fast, healthy emergence and will be better protected throughout the season. Incorporating these disease control strategies in your production plan will allow your soybean crop to achieve its highest yield potential. For information about more soybean diseases, refer to part 2 of this soybean identification guide. You can also contact your Golden Harvest Seed Advisor with questions or for additional agronomic insights.
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