Two common foliar diseases of soybeans are showing up in the state:

Bacterial Leaf Blight of soybean is caused by the bacterium Psuedomonas syringae pv. glycinea.

The bacteria can be seed-borne and overwinters in the soil and on soybean residue. Infected seed can lead to seed decay. If conditions are favorable early in the season, infection can occur on cotyledons and can cause seedlings to be stunted or killed. Leaf infection usually occurs early to midseason under moist conditions. The rainy conditions in late July, especially in the northwest portion of the state, and the recent high dew point temperatures the last couple of weeks provided free moisture on leaves allowing the bacteria to spread and increase. Higher temperatures and drier conditions later in the growing season usually check the disease. Bacterial blight is spread when bacteria are moved by wind, rain splash, and by farm equipment passing through the crop. The disease seldom causes appreciable yield loss. However, with more susceptible soybean varieties and with early season infection yields can be reduced as much as 40%.  The disease can begin developing as early as the cotyledonary stage.

Management There is little you can do this season to manage the disease except avoid driving equipment through the infected crop (i.e. cultivating). No doubt it is too late in the season for cultivation but any mechanical damage to soybean plants can spread the disease among other plants and open wounds in leaf tissue allowing bacteria to enter. The bacterium also enter leaves through stomata. Management of bacterial blight for future soybean crops would include sowing disease-free seed, rotating from soybeans to other crops, burying soybean residue, and avoiding the use of highly susceptible varieties. If the variety you planted this year has bacterial leaf blight you may want to avoid using the same variety in that field again. There are more tolerant varieties available. Fungicides won't help since this is a bacterial disease not a fungal disease.

Figure 2. Soybean leaf tattering caused by tissue falling out where bacterial blight lesions coalesce.

Figure 3. Brown spot on soybean leaf.

Brown spot of soybean, also called Septoria leaf spot, is caused by the fungi Septoria glycines.

Brown spot is a very common disease of soybeans occurring most years on the leaves in the lower portion of the crop canopy. Leaves that are at or near senescence are most often affected. In some years, including this year, the disease lesions can appear higher up in the canopy.  The recent warm, moist conditions have promoted brown spot infection of the upper canopy.  Yield reductions usually are negligible but have been reported as high as 15%. Brown spot overwinters in soybean debris and can be seed-borne.

Symptoms

As the name suggests, the disease is characterized by dark brown specks that appear on both sides of the leaf (Fig.3). The specks are irregularly shaped with indefinite margins and range in size from very small to several millimeters in diameter.  The spots often coalesce but unlike bacterial blight do not cause the leaf tissue to drop out. Instead entire infected leaves turn completely yellow and drop from the plant (Fig 4). The spore producing structure called a pycnidia can be seen with a microscope or strong hand lens as a black speck in the dark center of the leaf lesion. Spores produced in the pycnidia are spread by rain splash to uninfected leaves.

Management

There are no sources of resistance although some varieties may be more susceptible than others. Burying debris may reduce disease incidence but research results are not conclusive. Rotation with non-host crops almost always assures that the inoculum load in the soil is reduced over time. Plant seed produced on disease-free plants. Foliar fungicides may be applied between bloom and pod fill to reduce disease. As mentioned earlier, Brown spot seldom causes significant yield loss. 

Figure 4. Brown spot infection on lower leaves that are senescing.

Sudden Death Syndrome (SDS)

This is the time of year to watch for the appearance of SDS. SDS has not yet been officially confirmed in Minnesota. We do suspect that the disease may have occurred last year in several southern Minnesota counties. SDS has the potential to cause significant yield losses in soybeans in the state. To find out more about SDS go to the following web site.