June 18, 2010

Phytophthora root and stem rot on soybean.

Over the last several weeks we have started seeing seedling diseases in soybean. The most common fungi involved in seedling diseases in Nebraska are species of Fusarium, Phytophthora, Pythium, and Rhizoctonia. All four are capable of killing soybean seedlings or at least causing damage sufficient enough that it affects the ability of the plant to achieve its full yield potential. In many cases this year, the cause of poor stands and plant damping has been Phytophthora or Rhizoctonia. These two diseases are favored by completely different weather conditions.

Phytophthora, which is favored by wet conditions, and Pythium are referred to as water molds. In some fields, the seed was treated with a fungicide and yet seedling disease is still developing. This can occur with very wet conditions and when the wrong treatment or product rate is used. The most common example of a product rate issue is when mefenoxam or metalaxyl is put on at a rate too low for good Phytophthora control.

More information on product rates and management for Phytophtora is in NebGuide G1785, Management of Phytophthora Root and Stem Rot of Soybeans..

Rhizoctonia is a disease typically found in drier conditions or sandy fields. Rhizoctonia is sometimes referred to as a stress pathogen because it often is worse after hail or sandblasting (high speed winds). Rhizoctonia is a soilborne fungus that is in many fields, just waiting for the right stress conditions to occur. In fields with a history of Rhizoctinia use a strobilurin-based fungicide seed treatment for future plantings. See a complete list of seed treatments in NebGuide G1852, Seed Treatment Fungicides for Soybeans.

Management

At this time in the season there are no treatments for this problem. I encourage you to get a diagnosis of the problem in your fields so you can take proper management actions in the future. These include seed treatment and the use of resistant varieties (for Phytophthora), based on the field history. The only action that can be taken now is to ensure the crop does not undergo any moisture stress if and when things dry out more.

If heavy rains occur, we will continue to see more Phytophthora in some fields. This pathogen can kill plants at any stage of development. Pythium, on the other hand, typically does not kill plants much past the V5 growth stage.

Additional information on Phytophthora and Rhizoctonia diseases of soybean can be found on the Plant Disease Central website. If you’re not sure which fungus is causing the problem this year, I encourage you to send a plant sample to the UNL Plant and Pest Diagnostic Clinic so you know what to plan for in the future.

Loren Giesler
Extension Plant Pathologist, Lincoln