Windshield Crop Scouting or
Why you should get out of that truck to look and learn!
John Gaska, Shawn Conley, Craig Grau, and Paul Esker
Senior Outreach Specialist, Extension Soybean Specialist, and Extension Plant Pathologists, UW Madison
A lot is happening in the soybean fields we’ve been visiting recently and it is a good time to learn about several common diseases. If you just drive by your soybean fields this time of year, you could easily mistake maturity and natural plant death for several diseases. Just looking across a field you might notice dead plants that look like white mold but could easily be plants killed by stem canker (Fig.1). Plants that are browning from brown stem rot (BSR) could be mistaken for sudden death syndrome (SDS) or just natural senescence.
Fig. 1. Dead plants in the soybean canopy. Only close examination will reveal the cause.
Here’s a brief comparison of some of these diseases and what you might see by looking closer at your crop.
Natural, disease-free senescence of your soybean crop will begin with yellowing of leaves, followed by yellowing of the pods and leaf drop. Careful inspection of these yellowing leaves and examination of the inner stem tissue can confirm whether BSR or SDS is also present. BSR and SDS leaf symptoms are very, very similar. Fig. 2 shows leaf symptoms typical of SDS. Notice the interveinal chlorosis and bright yellowing of the leaves. With BSR, the leaves will exhibit the same chlorosis; however the yellowing may be less intense (Fig. 3).
Fig. 2. Leaf symptoms of SDS on soybean. Note the bright yellow coloration. This bright yellow is often seen with SDS, but further examination is needed to rule out BSR.
Cutting open the taproot and lower stem of a plant with SDS will reveal a brown or gray outer stem and center pith that is white. The stem pith of a plant with BSR will be brown, with browning more prevalent at the nodes (Fig. 4). Careful examination of the stem interior is the one way to confirm which disease is present in the field.
Fig. 3. Interveinal chlorosis and necrosis in soybean caused by BSR.
Fig. 4. Internal stem symptoms (brown pith) of soybean infected with BSR.
SDS is a fairly new disease in WI. It was confirmed in 9 counties in 2006 and will probably be more widespread this year. Also, we are finding more evidence of a link between SDS and soybean cyst nematode (SCN). In most cases where SDS is found, SCN is present as well. Researchers at Purdue have confirmed earlier studies that showed when SCN and SDS are both present, the foliar and stem symptoms of SDS are more severe than if SDS was the only pathogen present. If you find SDS in the fields you scout, be sure to pull a soil sample for SCN. This is where walking your fields, pulling a few plants, splitting the stems and careful examination of the leaves will help you correctly identify diseases (Fig. 5).
Fig. 5. Stem and leaf symptoms of SDS (Top) and BSR (Bottom).
Two other diseases that have similar foliar symptoms, but are very different once you inspect the stems are white mold (WM) and stem canker (SC). White mold is characterized by a white mold found around the lower part of the stem along with sclerotia (black fruiting body) in the pith of the stem and on the stem and branches (Fig. 6)
Fig. 6. White mycelial growth and black sclerotia on stem of soybean infected with sclerotinia white mold.
Symptoms of stem canker include the tops of soybean plant that are dead with leaves still attached on main stem and a reddish brown girdling canker on the lower stem (Fig. 7). Variety selection, tillage and row spacing are all very important in protecting against these two diseases. Without walking through your fields and examining individual plants, these two diseases could be easily misdiagnosed.
Fig. 7. Lesions caused by stem canker on soybean.
Planning for the 2008 crop year requires an understanding of what happened to your fields in 2007. Choices such as variety selection, tillage, and row spacing, just to name a few, can have a significant impact on the prevalence of many diseases. Take note of your management practices and yields this year and make the appropriate management decisions for next year.
More detailed information on these diseases can be found at:
Sudden death syndrome
Brown stem rot
Stem canker
White mold