Sick Soybeans: See What Could be Ailing Your Beans

July 6, 2009 07:00 PM
 

Pam Smith, Farm Journal Seeds & Production Editor
 
When a new species of Pythium was identified in Ohio this spring, it should have been a warning that soybean seedling diseases don't mess around. Plant pathologists are warning that conditions in most of the soybean belt are already proving how problematic early disease can be and now is the time to scout for them.
 
You used a seed treatment? Sure, that may help, but no one seed treatment is effective on all diseases. Scouting is the only tool you have to really determine what's going on and what to plan for in the future.
 
The wet weather and cool temperatures in Illinois and other areas of the Midwest have Carl Bradley, University of Illinois plant pathologist, concerned that growers could see more damping off of late planted soybeans this year. 
 

 
Pythium, a soil borne pathogen that causes seedling blight, likes soil temperatures of 60 degrees or less, he notes. "We generally consider it to be more of a problem when planting early, but this year's cool temperatures make the disease a potential issue. It is the most damaging disease from the standpoint of a loss of stand,” says Bradley. "Early root damage is difficult to overcome as the disease opens the roots to attack by other opportunistic pathogens.” Soybeans with cracked seed coats are more vulnerable to Pythium infections, he notes. The cracks exude sugars that attract the pathogens.
 
Phytophthora is another soilborne pathogen that is similar to Pythium, but likes warmer temperatures.  "In most years when soybean is being planted into cooler soils, we don't generally think of Phythophthora as a seedling pathogen,” Bradley says.  "It comes later when soils are warmer (around 65 degrees), but given the late planting season, we could see more problems with it in seedlings this year too. Plants infected with Phythophthora generally have a brown discoloration extending from the root up the stem after the V2 growth stage.
 
Fusarium is a disease you may associate more with head scab in wheat and corn rot, but it can also lead to general lack of root development and red to brown discoloration in soybean.
 
Rhizoctonia causes different symptoms than other seedling diseases. In this case, the discoloration is usually limited to the cortical layer of the main root and hypocotyl. Infected stems remain firm and dry. Typical symptoms are localized brown to reddish brown lesions on the hypocotyl and lower stem that do not extend above the soil line. This disease causes post emergence damping off as the weather becomes warmer and is often seen in late planted soybean fields. "It's active over a wide range of soil moisture conditions—from dry to wet,” Bradley says.
 
Keep in mind that soybeans can compensate for stand losses better than corn. At this late date, the expected yield loss from the reduced stand must be balanced against the anticipated yield loss from replanting after the optimum planting date.

Watch Carl Bradley describe what's happening in soybean fields:



For More Information
Seedling diseases

How to assess soybean stands
 

 
You can email Pam Smith at psmith@farmjournal.com.

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