Diagnosing Soybean Stem Diseases

Alison Robertson, Faculty, Plant Pathology


Stem diseases of soybean are economically important in Iowa, and their incidence is on the increase.  Despite their threat to soybean production, many agricultural consultants do not have a very good understanding about the general biology of each stem disease, and diagnosis.  It is important to correctly diagnose problems in the field each season so that informed decisions can be made regarding management practices (cultivar selection, planting date, fungicide usage, etc.).


To educate those in agribusiness on the biology of the seven most common stem diseases of soybean in Iowa (namely Pythium damping-of, Phytophthora root Rot, Rhizoctonia, Sudden Death Syndrome, Brown Stem Rot, White Mold and Stem Canker) and use this information to aid diagnosis of stem diseases.


Soybeans were planted at ISUs Field Extension Education Laboratory in Boone, Iowa.  Plants were artificially inoculated with each of pathogen being discussed.  In addition, greenhouse grown and inoculated plants were also grown for a training session, Soybean Stem Diseases, which was part of the Crop Diagnostic Clinic Series.  The disease cycle of each disease was discussed.  Important biological facts that related to disease development where emphasized, e.g., infection by Phytophthora sojae only occurs in warm (~ 70F) soils, thus Phytophthora root rot is not an early season problem since soil temperatures are between 55 and 65F.  Participants had the opportunity to look at and dissect diseased plants prepared in the greenhouse, and those plants in the field.  Key characteristics of symptoms of each disease were pointed out.

To document impact of the training session, participants were asked to fill out a Post-then-Pre Evaluation Form.  Mean scores for After (present score) and before (pre score) were compared using a paired t-test.


Evaluation forms were received from 128 participants.  All participants improved their knowledge of soybean stem disease biology and diagnosis during the session (Table 1).  Significant improvements (P<0.05) were made in understanding the disease triangle and the importance of the environment in disease development; disease cycle biology and aspect that could be used in disease diagnosis; overall diagnosis of stem diseases; and differentiating between seedling diseases caused by Pythium and P. sojae. 

Table 1.  Mean scores of a Post-then-Pre Evaluation Form filled out by participants at the Soybean Stem Disease session at the Crop Diagnostic Clinic held at ISU Field Extension Education Laboratory, July 10-20, 2006.

How would you describe your knowledge/understanding of the following   

Knowledge/understanding---------------------- (mean)a P valueb       After   Before        
1.  Soybean disease development and the disease triangle      3.49    2.65    0.005 
2.  Disease cycles and how they can be used in disease identification   3.45    2.4     0.038 
3.  Identification of soybean stem diseases     3.43    2.25    0.001 
4.  Diagnosing seedling diseases caused by Pythium and Phytophthora sojae       3.41    2.31    0.043 
5.  Diagnosing Sudden Death Syndrome, Brown Stem Rot and Stem Canker    3.51    2.55    0.411 

a 1 = no understanding/knowledge; 2 = fair understanding/knowledge; 3 = good understanding/knowledge; 4 = excellent understanding/knowledge.
b Differences between means were compared using the paired t-test.

Some additional comments about the training that were received include:

One of the best disease sessions I've attended as far as understanding the information and associating it with the disease

Encouragement of discussion and questions kept the students involved

Great samples

September 2006

142 - Integrated Pest and Crop Management

Page last updated: October 19, 2006
Page maintained by Linda Schultz, lschultz@iastate.edu