UNL CropWatch July 14, 2010: Rhizoctonia and Phytophthora Showing Up in Nebraska Soybeans

UNL CropWatch July 14, 2010: Rhizoctonia and Phytophthora Showing Up in Nebraska Soybeans

July 14, 2010

Over the last several weeks we have seen seedling and early season soybean diseases showing up. 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 to affect the plant's ability to achieve its full yield potential.

In many cases this year, the cause of poor stands and damping off of plants have been Phytophthora or Rhizoctonia. These two diseases are favored by completely different weather conditions.

Photo of Phytophthora root and stem rot on soybean

Phytophthora root and stem rot of soybean

Phytophthora is favored by wet conditions, which is why we refer to it and pythium as water molds. In some fields, the seed was treated with a fungicide, but seedling disease is still developing. This can be the case with wet conditions or when the wrong treatment or product rate is used. The most common example of a product rate issue is when mefenoxam or metalaxyl is applied at a rate too low for good phytophthora control. More information on product rates and management for Phytophtora can be found in Management of Phytophthora Root and Stem Rot of Soybeans (G1785). 

Rhizoctonia is typically found in drier conditions or sandy fields. It is sometimes referred to as a stress pathogen as 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.

At this time we are seeing examples of cortical rot caused by Rhizoctonia. Symptoms will be a dark lesion at the base of the plant that in some cases will girdle the stem. Plants with cortical rot are sensitive to high winds and tend to snap off near the soil line. In fields with a history of rhizoctonia use a strobilurin-based fungicide seed treatment in future plantings. A complete list of seed treatment products for soybean is in Seed Treatment Fungicides for Soybeans (G1852).

Management Options

At this time in the season there are no treatments for these soybean diseases; however, if things do dry out, you may be able to mitigate more extensive damage by ensuring the crop does not undergo moisture stress.

Also, I encourage you to get a diagnosis of the problem in your fields so you can take proper management action in the future. Options would include seed treatment and the use of resistant varieties (for phytophthora), based on the field history.

If heavy rains continue, we’ll 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, 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