We typically think of Phytophthora infections to soybean in the spring with seed decay, poor plant stand, damping-off and seedling blight. However, the causal pathogen Phytophthora sojae can infect soybean throughout the season. We observed symptoms of mid-season infections in multiple Missouri fields during 2023 where large areas of seemingly healthy soybean became symptomatic shortly after irrigation. Yield losses as high as 100% can occur in conducive environments when soybean are susceptible, and unfortunately our genetic resistance sources are breaking down over time. A few thoughts about this disease as we enter 2024 include:

1. No water; no infection. The pathogen produces oospores that persist in the soil for years. Under favorable conditions oospores germinate and result in swimming spores known as zoospores. These zoospores are attracted to soybean roots but require water to swim towards the roots. Therefore, this disease is most likely to occur in wet areas including low-lying, poorly drained, or compacted fields. Dry conditions across Missouri in 2023 resulted in a lack of issues until mid-season when irrigation and sporadic rainfalls occurred in northern Missouri. After which soybean began displaying symptoms, which can include: a brown to purple lesion that forms on the stem below the soil line and progresses upward a few nodes (Figure 1); yellowing of lower soybean leaves between the veins and margins; general yellowing of the upper leaves; stunting; and root rot. The entire plant can wilt and die with withered leaves that remain attached to the stem (Figure 2). Reducing or minimizing irrigation in problematic areas may help alleviate disease severity.
   
   

   Figure 1 Brown stem discoloration on an infected plant in the reproductive stage. Image courtesy of Laura Sweets

   

   Figure 2 A soybean plant infected with Phytopthora has wilted leaves that remain attached. Image courtesy of Kaitlyn Bissonnette

2. Contain the problem. Phytophthora is most likely to be spread through a field or fields by equipment. Why introduce the problem to a new field if you do not have to do so? The disease will not be irradicated once it is introduced to a new field; it will be managed. Consider working infected soils last and cleaning equipment thoroughly when moving from an infected field to another site. I can appreciate that this requires time. However, containing the disease to the extent possible may be the most effective and sustainable management strategy.
3. Confirm that Phytopthora is causing the symptoms. Last year the MU Plant Diagnostic Clinic received a record number of yellow soybean seedling samples. Many samples submitted with "Phytophthora symptoms" were actually due to other pathogens. Management strategies can vary based on the disease that is present. Consider submitting suspect samples to the clinic during 2024.
4. Soybean genetic resistance is breaking down. The Rps1c and Rps1k genes (Resistance-to-Phytophthora-Sojae genes) are the most widely utilized resistance genes in commercial soybean varieties and have been in use since 1985. These genes provide complete resistance against the pathogen; however, the pathogen has adapted overtime and is now gaining ability to reproduce on soybean with these genes.
   
   Partial resistance is another type of soybean resistance to Phytopthora sojae. The pathogen is less likely to adapt to partial resistance because multiple soybean genes are required. However, partial resistance is not activated until the plant's first true leaf emerges and will not protect yield to the same extent as Rps genes. Varieties that contain an Rps gene plus a moderate to high level of partial resistance or "field resistance" are ideal for problematic areas. Varieties that contain an Rps gene other than Rps1c or Rps1k should be considered when available. If no options exist, then rotate varieties. There are observations that two different varieties with the same resistance gene and disease ratings can respond differently to in-season infections.
5. Use a seed treatment. Products with metalaxyl, mefenoxam, ethaboxam, and/or oxathiapiprolin have efficacy against oomycetes. The Crop Protection Network updates the fungicide seed treatment efficacy table annually.
6. Improve soil drainage and compaction issues. Research has shown that yield of moderately susceptible cultivars can be improved when soil drainage and compaction issues are resolved.
7. Cultural management practices have weaknesses. Crop rotation may help manage the levels of pathogen present to some extent; however, the thick-walled oospores can persist in the soil for long periods of time. Tillage can bury oospores and help dry out the soil; however, it can also spread the pathogen throughout the field. Utilize another strategy in combination with any cultural practice.

For more detailed biology information on this pathogen or to learn about pathotype information please visit the Crop Protection Network encyclopedia page on Phytophthora Root and Stem Rot.

Header image of irrigation over soybean courtesy of Ryan R Fox via Shutterstock.