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A unique opportunity to find out more about the Phytopthora present in your fields

By Angie Peltier, UMN Extension crops educator and Kathleen Markham, research associate and Megan McCaghey, assistant professor in the UMN Dept. of Plant Pathology.

Phytophthora root and stem rot

Phytophthora root and stem rot (PRSR) is caused by a soil-borne, fungus-like organism (Phytophthora sojae, Ps) called an oomycete, that produces spores that swim in the soil toward soybean roots. The disease is favored by compacted, poorly drained and saturated soils, rain and warm temperatures. With the saturated, warm soils throughout much of Minnesota in 2024, this growing season is proving quite favorable for PRSR.

PRSR can occur as soon as the crop is planted, resulting in rotted seeds or seedlings. Other plants that are infected by Ps don't exhibit severe symptoms until later in the growing season.

Symptoms characteristic of Phytophthora infection

Soil-borne pathogens like Ps are not evenly distributed across a farm field, but rather occur in patches. This is why symptomatic plants tend to be scattered throughout the field or occur in patches.

Early season

Seedling infections can result in plant death before or right after emergence (a symptom referred to as 'damping off'). This can result in 'missing' plants when one goes out to check soybean stands a couple of weeks after planting. It is often difficult to tell what pathogen was responsible for damping off or seed rot as several pathogens can cause these symptoms and still other microbes that eat decaying plant matter (called saprophytes) can move in, complicating diagnosis.

Later in the season

Figure 1. Linear stem lesion and girdling of the stem, characteristic
 of Phytophthora stem rot. Photo courtesy of A. E. Dorrance;
 Reproduced, by permission, from Hartman, G. L., et al., eds.
2015. Compendium of Soybean Diseases and Pests. 5th ed.
 American Phytopathological Society, St. Paul, MN.
What a crop producer or advisor may first notice later in the season is that individual plants or patches of plants begin to wilt with all of the leaves remaining attached to the plant. When one gets a bit closer, one would notice that living plants are located immediately next to or across a row from dead or dying plants. On dying plants there is a chocolate-colored stem lesion that surrounds the stem originating from below-ground (Figure 1). At the lesion, the pathogen restricts water and nutrients to the rest of the plant, causing wilting and plant death.

Current PRR management strategies

There are several seed treatment active ingredients that can provide protection, including mefanoxam, metalaxyl and ethaboxam. Selecting soybean varieties resistant to Ps is also important. There are two types of resistance that are available in soybean varieties: major genes, referred to as Rps genes and partial resistance, which is expressed by many genes all working together to provide tolerance to PRR. Rps genes allow seedlings to detect and completely resist specific Phytophthora sojae races and partial resistance is effective against all races. Because there can be multiple races of Ps in a single field, both forms of resistance are important to have in soybean varieties.

Taking steps to minimize soil compaction by waiting until fields are fit before field operations occur and using controlled traffic to minimize the area of the field susceptible to compaction can help reduce disease risk. Improving field drainage and rotating to crops other than soybean can also reduce risk of PRR.

Unique opportunity for Minnesota soybean producers with PRR

While a crop advisor could visit your field and provide a symptom-based diagnosis of PRR, or the UMN Plant Disease Clinic could provide to you a lab-based diagnosis if you were to send them diseased plants, neither is able to tell you which races of Phytopthora sojae are present in your field.

A researcher on the University of Minnesota's St. Paul campus (Dr. Kathleen Markham in Dr. Megan McCaghey's Lab) is trying to better understand which Phytophthora sojae races are most prevalent throughout Minnesota and so will bait the pathogen from soil or isolate it from infected plants and run a series of tests that will reveal the P. sojae races present. This information will be shared with you to help inform your soybean variety Rps gene selections the next time you plan to plant soybeans in that field.

Contact information to participate

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