University of Nebraska-Lincoln Extension, Institute of Agriculture and Natural Resources

July 21, 2006

Phytophthora root and stem rot showing up in many soybean fields

Phytophthora root and stem rot on soybean.
Even though we’ve been very dry is some parts of the state, we are seeing quite a bit of Phytophthora is soybean fields in those areas that caught heavy rains. Most of the samples have come in over the last two weeks. Not all of the symptoms match with typical Phytophthora symptoms (dark lesions that run up the stem from the soil line). The symptom we’re seeing consistently is internal browning of the crown and lower stem, which is visible when stems are split. In some instances it looks like Brown Stem Rot (BRS), but we have not isolated this pathogen from any of these fields.

In most cases symptoms started to develop within a week of a heavy rain. One interesting issue regarding Phytophthora is that we do not know how drought stress interacts with the pathogen. We know that heavy rains and saturated soil conditions favor disease development, but exactly how drought stress on the plant interacts with these conditions prior to moisture availability has not been investigated. My hypothesis is that drought stress could reduce the plant’s defense mechanisms and then when the high moisture availability comes, the plant is more susceptible. This is definitely an area that needs further investigation.

Producers should keep records on which fields they identify with Phytophthora so that they can select soybean varieties with resistance and/or tolerance to the disease for those fields. This disease will not affect corn. Fields with a long-term history of Phytophthora may require a different resistance gene if you are noticing Phytophthora-killed plants when you grow resistant varieties. In addition, fields with a history of Phytophthora will require additional levels of Mefenoxam chemistry above the standard rate. See the Extension NebFact, Management of Phytophthora Diseases of Soybean” (NF02-518). When we have randomly sampled soybean fields in the past years we have found Phytophthora in 12% of Nebraska fields.

Loren J. Giesler
Extension Plant Pathologist

You've got mail!
To receive Email notification when the latest Crop Watch is posted to the Web, use this form.

Copyright 2006 by the University of Nebraska Board of Regents. All rights reserved.
Published by University of Nebraska-Lincoln Extension in the Institute of Agriculture and Natural Resources Cooperating with the counties and the U.S. Department of Agriculture.
The University of Nebraska-Lincoln does not discriminate on the basis of gender, age, disability, race, color, religion, marital status, veteran's status, national or ethnic origin, or sexual orientation.