Scouting for Phytophthora Root and Stem Rot in Soybean
Phytophthora root and stem rot is a common soilborne disease caused by a water mold, Phytophthora sojae. In poorly drained soils, Phytophthora can have a severe economic impact.The most common symptom is areas of the field with no stand from early season damping off and on older plants, a dark brown lesion on the lower stem that extends from the root into the upper portions of the plant (Figure 1). During the early stages of soybean development, when pre- and postemergence damping off occur, symptoms are indistinguishable from Pythium, another water mold. Later in the growing season, symptoms on more mature plants include chlorosis of leaves and wilting, stunting, and death. Mid- to late season symptoms may be confused with Diaporthe stem canker or white mold.
Figure 1. Characteristic Phytophthora root and stem rot lesion extending up the stem from the root.
Figure 2. Phytophthora root and stem rot disease cycle.
Planting into saturated, warm (>60°F) soils with a history of Phytophthora root and stem rot poses the greatest risk for disease. Disease is most common in low-lying areas that are poorly drained and prone to saturation or flooding. Planting into soils that are no till also poses a risk for disease development due to increased water retention and higher inoculum density in the top 2 to 3 inches.
Phytophthora sojae can infect the plant at any growth stage from seedling through maturity. Scouting for Phytophthora root and stem rot should be done following prolonged periods of rain when soil temperatures exceed 60°F. Finding patches of diseased plants in low-lying, poorly drained portions of fields is common (Figure 3).
While scouting for Phytophthora root and stem rot, it is important to note that there are other diseases that have similar symptoms, such as stem canker and white mold. The key difference between the three diseases is that the brown stem lesion characteristic of Phytophthora will develop from the base of the stem upward and plants have rotten roots. With the other diseases, brown lesions develop at a node and rarely progress down stem to the soil line (Figures 4-6).
Figure 3. Phytophthora root rot usually occurs in patches in low-lying areas of field.
Figures 4-6. Phytophthora symptoms and similar diseases
Diversity of Phytophthora sojae
P. sojae is classified into pathotypes (which used to be referred to as races) based on its ability to cause disease on soybean lines that contain single Rps resistance genes. Within a single soybean field, the population of P. sojae is made up of hundreds of individuals called isolates. From two to as many as 50 different pathotypes have been classified from isolates recovered from soybean plants from a single field.
During a 2012–2013 survey of 208 fields in the Midwest, more than 870 isolates of P. sojae were recovered from either soil through a baiting procedure or diseased plants. More than 200 pathotypes were identified in the region based on the few isolates that were evaluated from each field. Among these isolates, more than two thirds were able to cause disease on two or more Rps genes. Soybean lines with Rps1a, Rps1c, and Rps1k were susceptible to 43 to 68% of the isolates, while 15% or less of the isolates caused disease on Rps3a, Rps6, and Rps8.
The best management tool for P. sojae is the use of resistant varieties.There are two forms of resistance: race-specific genes (Rps genes) and partial resistance. Rps genes will provide complete resistance to certain P. sojae populations. The most common Rps genes are Rps1a, Rps1c, Rps1k, Rps3a, and Rps6. Partial resistance offers some resistance to all populations of P. sojae. However, partial resistance is not expressed until the first true leaves have developed, so emerging seedlings are still vulnerable to infection by P. sojae early.
Other disease management options include some form of tillage and/or tiling to improve drainage and seed treatments with active ingredients mefenoxam, metalaxyl, or ethaboxam.
Yuba Kandel, Iowa State University; Alison Robertson, Iowa State University; Anne Dorrance, The Ohio State University; Ed Zaworski, Iowa State University; Martin Chilvers, Michigan State University; and Nathan Bestor, Iowa State University.
Suzanne Bissonnette, University of Illinois; Carl Bradley, University of Kentucky; Emmanuel Byamukama, South Dakota State University; Loren Giesler, University of Nebraska-Lincoln; Doug Jardine, Kansas State University; Dean Malvick, University of Minnesota; Samuel Markell, North Dakota State University; Daren Mueller, Iowa State University; Adam Sisson, Iowa State University; Damon Smith, University of Wisconsin; Laura Sweets, University of Missouri; Albert Tenuta, Ontario Ministry of Agriculture; and Kiersten Wise, Purdue University.
All photos were provided by and are the property of the authors and reviewers except Figure 6 courtesy Craig Grau, University of Wisconsin.
This project was supported by Agriculture and Food Research Initiative Competitive Grant no. 2011-68004-30104 from the USDA National Institute of Food and Agriculture. Monsanto Company provided funding for the 2012–2013 Midwest survey. 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 States Department of Agriculture - National Institute of Food and Agriculture and the Grain Farmers of Ontario for their support. Contributors to this series come from land-grant universities in the North Central states and Canada.
This information in this publication is only a guide, and the authors assume no liability for practices implemented based on this information. Reference to products in this publication is not intended to be an endorsement to the exclusion of others that may be similar. Individuals using such products assume responsibility for their use in accordance with current directions of the manufacturer.
This publication was developed by the Crop Protection Network, a multi-state and international collaboration of university/provincial extension specialists and public/ private professionals that provides unbiased, research-based information to farmers and agricultural personnel.
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