Soybean samples have been arriving almost daily at the Iowa State University Plant & Insect Diagnostic Clinic this summer. Early season problems were primarily damping off diseases and problems with herbicide carryover or sometimes a combination of the two. Now leaf disease symptoms are showing up.
The following information comes from Laura Jesse, the ISU Extension specialist who identifies the diseases on the crop plant samples people send in to the lab at Ames. Daren Mueller, an ISU Extension plant pathologist who assists Jesse, also helped provide the following information and observations.
Properly identify common summer soybean diseases
As Iowa's 2014 soybean crop moves into mid-season with environmental stresses (hail, too much rain), there have been more soybean diseases popping up this summer compared to the past few years. These diseases include the following:
Brown spot – A common foliar disease that starts in the lower canopy, it is fairly easy to find in most soybean fields in Iowa.
BACTERIAL BLIGHT: This common foliar disease of soybeans often shows up after thunderstorms.
Bacterial blight – This is a common foliar disease that often appears after thunderstorms, which Iowa has had plenty of in recent weeks. While this disease is usually one of the more simple diseases to identify, some fields have so much disease it makes us doubt what we are seeing.
Sudden death syndrome (SDS) – Yes, it is a bit early to see SDS showing up, but there have been scattered reports of SDS in some fields across the state. Look for yellowing between the veins. Split the stem open lengthwise or look for lesions on the outside of the stem to confirm it is SDS.
BROWN STEM ROT: You can split the stem open and look at the pitch inside. It should be white and healthy, not brown and diseased.
Brown stem rot (BSR) – Only one report of BSR has come into the ISU clinic so far this summer. Split stems open and look for the brown pith to properly identify this disease.
PHYTOPHTHORA ROOT ROT: This one is known as the "wet foot" disease of soybeans.
Phytophthora root rot – With the overly wet fields in the last half of June and into July, there have been scattered reports of this disease across Iowa. Look for the purple lesion on the lower stem that extends below the soil line.
STEM CANKER : Iowa has had a very wet spring this year, so this disease will likely be showing up.
Stem canker – With the very wet spring, be on the lookout for this disease in summer 2014, as it will show up a bit later than the others. Stem canker will cause a distinct lesion on the stem (thus the name). At times, you may see similar foliar symptoms as SDS or BSR, but eventually the plants will die and the leaves will remain attached.
White mold – When you have cool, wet weather this time of year (last half of June and first half of July) such conditions are conducive for white mold development. Look for scattered dead plants or patches of dead plants with white, fluffy fungal growth on the stem. A more extensive article on white mold was written in early July by ISU specialists.
While many of these diseases can be accurately identified in the field, for some it helps to have a second opinion to confirm the disease. Samples can be sent to the ISU Plant and Insect Diagnostic Clinic, where we have the tools in place to confirm specific diseases.
Package samples with care when sending to the lab
Soybeans do not travel in the mail as well as corn so care must be taken when shipping samples to the clinic for identification. Following these tips will help ensure the ISU specialists receive a good sample. When samples arrive dried out or overgrown by mold it can be very difficult to isolate any plant pathogens. By definition plant pathogens feed on live plant tissue and once tissue is dead it can be very hard to diagnose problems.
Collect at least 3 plants (including roots) showing symptoms. If plants are large it is ok to fold them so they fit in a box.
Collect plants that are not wet with rainfall or dew as they will not travel as well.
Keep plants chilled or in a refrigerator before you send them. Try not to let them sit in a hot car after collecting.
Seal plants in a plastic bag prior to shipping. Do not seal the bag if plant leaves are wet.
Ship samples early in the week so they arrive before the weekend. Both FedEx and UPS will ship samples directly to the clinic. Overnight or second-day delivery is advised especially if weather is forecast to be hot.
Please include our form with a detailed description of what you are observing, the patterns in the field, a history of chemical use in the field, and anything else that might help us understand the situation.
Pictures of the field and symptoms can also be very helpful.
"When samples arrive in the clinic we examine the sample, read the form and often consult with other specialists," says Jesse. "Diagnosing plant disease requires we consider the whole situation. Often stressed plants are more prone to disease problems, so we need to take into consideration possible stressors when diagnosing a disease problem and making any management recommendations."
Some disease can be diagnosed fairly quickly by examining the symptoms the disease causes in the plant or by looking at the spores or other signs under the microscopes. However, many diseases require additional testing. "To confirm many diseases, particularly early season damping off diseases, we have to 'plate for it' which means that we take part of the plant tissue and place it in a petri dish with some type of agar that helps certain pathogens grow," Jesse explains. "Depending on the disease, this process usually takes about 7 to 10 days."
Samples can be dropped off at the campus office located in Bessey Hall, room 327. Clinic hours are 9:00 a.m. to 4:00 p.m., but it is best to call ahead, 515-294-0581.
Laura Jesse is an entomologist with the ISU Extension Plant and Insect Diagnostic Clinic. She can be reached at [email protected] or by phone 515-294-0581.