The problem with continuous soybeans
Ohio State University researchers don't recommend growing soybeans year-after-year in the same field. The list is endless why this is not a good thing to do, but we also know that commodity prices, planting restrictions, landlords, etc. force this option on almost one-third of our field crop production acreage. This production practice is less than optimal primarily due to the build-up of pathogens in either the soil or on crop residues. If pathogen populations are too high, then the losses can be very substantial. Case and point, Soybean cyst nematode (SCN), frogeye leafspot and Sclerotinia white mold. There are several cases in Ohio, where low levels of a particular disease were found in a field at the end of one growing season and the same variety was then planted back into the same field the following year, which resulted in an outbreak of disease and greatly reduced yields. The key to preventing this from happening is to go look at those fields - now. In addition, the R6 growth stage is a very key time to tell if you chose the right variety for that particular field (resistance to Sudden Death Syndrome (SDS), SCN, White mold, Stem Canker, Frogeye) as well as if the pathogen population is increasing. Road side scouting/truck scouting doesn't work in these cases, you need to wade into the field and look into the canopy. Here is some of what we have been finding for ourselves and hearing about from others.
Soybean cyst nematode (SCN). Over the next few weeks the fields will start to mature. Where SCN populations are very high we have observed that the SCN "hot spots" will mature much faster than the rest of the field, giving it a patchy appearance. For those pockets which are maturing early, dig up the plants and check the roots for the white pearls of the SCN females. They will be the size of the head of pin. If you can easily find them, this is a good field to put wheat or corn in for 2012.
Sudden Death Syndrome (SDS) and Brown Stem Rot (BSR). Both of these diseases have similar foliar patterns. I tend to think that SDS has a brighter yellow cast to the leaves than BSR. BSR can also give a "greasy" appearance to the base of the stem. The best way to separate these two is to compare the crown and the pith. For SDS the internal tissues of the crown are gray and sometimes the blue-green color of the spores are on the surface of the root tissue; the pith is white and healthy. In contrast, BSR, the crown is white but the pith is chocolate brown color. For BSR, this discoloration of the pith may be the whole length of the plant or just at a few nodes. Soil pH plays a key role in symptom development for BSR, so that is something else to make note of. SCN also plays a role in the severity of the symptoms for SDS, again check those roots for signs of the SCN females.