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Phytophthora Root Rot and Other Soybean Diseases

When scouting for potentially yield-robbing pathogens in soybeans, early season diseases are often overlooked and underestimated. These diseases hit the soybean plant early at emergence and can be missed unless you’ve thoroughly checked your fields. Common diseases such as Pythium, Phytophthora root rot, Rhizoctonia and Fusarium leaf blight have the ability to compromise yields by reducing stand or stunting plant growth. 

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. Pythium can cause pre-emergence damping-off, or seed rot, as well as post-emergence damping-off of seedlings. 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. 

Healthy plants and Pythium-infected seedlings

Phytophthora Root Rot
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. Phytophthora symptoms are very similar to Pythium but tend to occur in a continuous, dark chocolate brown lesion that girdles the stem. There are multiple strains of Phytophthora sojae so knowing which are prominent in your fields is critical to knowing how to manage this pathogen.

Rhizoctonia is a disease that can affect the soybean plant any time throughout the growing season, but it is most severe when it infects young seedlings. Typically, Rhizoctonia is associated with warmer temperatures; hill slopes; or sandy, well-aerated soils. The soilborne pathogen is sclerotia and resides in the soil. 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 tissue stays firm and dry, and can eventually spread to the roots. The root rot phase of Rhizoctonia typically doesn’t show up until later in the growing season.

Fusarium Leaf Blight
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. Fusarium can cause poor and slow emergence, which can result in stunting and possibly seedling death.

These early season diseases are all capable of decreasing the yield potential of your soybean crop. Timely scouting is highly recommended to identify the presence of these diseases in your fields. If you notice these pathogens in your fields, make a plan to manage them next year with a quality seed treatment. Choosing varieties with tolerance to seedling diseases and crop rotation can also reduce infection. Contact your Golden Harvest Seed Advisor with questions or for additional agronomic insights.

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