Agronomy Talk

Agronomy Talk: PHYTOPHTHORA ROOT AND STEM ROT

Published on Monday, November 04, 2019

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Understanding Phytophthora 

Phytophthora root and stem rot is a common soil-borne disease in soybeans that is caused by the watermold, Phytophthora sojae. Phytophthora often occurs in poorly drained soils; it is most economically damaging in low-lying areas and fields that are prone to flooding.

Phytophthora can infect soybeans during any growth stage; however, planting into saturated, warm soils with a history of Phytophthora poses the greatest risk for infection. The disease overwinters as spores in soil and crop residue. Planting continuous soybeans, particularly into no-till fields, increases the risk of development due to increased water retention and higher inoculum densities. 

When the soil is saturated and the soil temperature is higher than 60°F, the spores germinate and produce more spores, called zoospores. Zoospores are mobile; they can swim through soil water, and are attracted by soybean root exudates. Symptom severity will vary based on the variety’s susceptibility to the disease.

Early-Season Symptoms

Symptoms occur about 5 to 7 days after a rain event and include seed rot and pre-/post-emergence damping off. During the early vegetative stages, stems have a bruised appearance and are soft. Leaves may become chlorotic, which is usually followed by wilting and plant death. Symptoms during this stage are nearly indistinguishable from Pythium root rot, another watermold, and can only be differentiated by a laboratory examination.

Late-Season Symptoms

Low-lying areas of the field facing premature plant death should be scouted for Phytophthora. Plants are often killed within a row before adjacent rows are affected.

Scouting Notes:

  • Leaves are chlorotic between the veins
  • Wilting and plant death
  • Affected trifoliates remain attached to the plant
  • Dark brown lesion on the lower stem that extends up from the taproot 

The key to differentiating Phytophthora from other diseases like stem canker is by the characteristic lesion. The defining symptom is a dark brown lesion that will often reach several nodes up and will end up girdling the stem, which will stunt and kill the plant. The Phytophthora lesion develops from the roots of the plant and moves upward; whereas, the other diseases develop at a node and rarely progress down the stem into the soil.

Management: The best way to manage Phytophthora root and stem rot is by planting resistant varieties. 

Phytophthora root and stem rot is a genetically diverse disease.

  • It is classified into races based on its ability to overcome resistance.
  • Any single field may contain many races.
  • Phytophthora resistance genes and tolerance ratings are published in the Beck’s Product and Program Guide.

Ongoing Research:

During a 2012-2013 survey on over 200 pathotypes throughout the Midwest, more than 67% could cause disease on multiple Rps genes. Soybean lines with Rps-1a, Rps-1c, and Rps-1k were susceptible to 43-68% of the isolates, while 15% or less of the isolates caused disease on Rps-3a, Rps-6, and Rps-8. This study is currently being repeated to update information on the activity of Rps genes.  

Other Control Methods:

The best way to manage this disease is to choose varieties that have both Rps genes and a high field tolerance rating. Other cultural practices include tillage and/or tiling to improve drainage, and crop rotation. Chemical fungicide seed treatments are available for early-season protection. Two of the most common chemistries with activity on watermolds are metalaxyl and mefenoxam. Metalaxyl is a component of Beck’s Escalate® yield enhancement system on soybeans. 

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Camille Lambert
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Camille Lambert

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