It has turned into a banner year for sudden death syndrome (SDS) on the soybean crop. Questions have come in from all over the state. My answers to some of them follow. |
How much yield loss can we expect from plants showing SDS symptoms in the top 1/3 to 1/2 of the plant?
For those plants showing the distinctive foliar symptoms of the disease, no further seed development should be expected. So, for example, if you are at 2/3 pod fill, then expect 1/3 yield loss, specifically in those plants. You will also need to consider that seeds from affected plants will be small.
Does the plant die? Will the pods continue to fill if the tops drop their leaves?
The plant will die, and pods will not continue to fill. This has implications for harvest, too, because plants that have dried down much earlier than the rest of the field will likely drop their seed before the rest of the field is ready for harvest.
SDS appears to be worse near the gateways and in areas that are used to park tandems for filling at harvest. Is this related to compaction?
Compaction and other early-season stresses, such as too much water, other root diseases, early planting, and excessive fertility, are important factors that contribute to early-season stress on the plants. These stresses produce a conducive environment for infection by SDS and other early-season root diseases.
How long does it take for a plant to die?
It is hard to be exact about this. The plant has been infected since about 30 to 40 days after germination. Plants showing severe symptoms now will probably not last much more than 2 weeks. It is moot, though, because no more seed development is likely to take place once foliar symptoms appear.
My soybeans were all planted early, and there doesn't seem to be a difference in varieties or brands, or if they were supercoated or not. Why?
Although there is no defined resistance in released cultivars, there are some tolerant varieties, according to Southern Illinois University's variety trials. This means that these tolerant varieties can withstand a susceptible infection but still yield well. The results of those trials can be found at http://www.siu.edu/~soybean/vartest.html. As for seed treatment, research so far indicates that root infection by SDS takes place about 30 to 40 days into the season, well beyond the amount of time a seed treatment would provide protection. Seed treatment certainly can protect your crop from other early-season fungal root rots that are most active near germination and shortly thereafter, which will relieve some early-season stress to the plants.
I noticed that a neighbor's no-till soybeans had SDS in more than 75% of the field. Are no-till soybeans more susceptible to SDS?
This is just a coincidence. We haven't made a strict correlation with no-till. There are likely other factors in the field contributing to the disease such as poor drainage, susceptible variety, high fertility, or possibly other root diseases.
Should we plan to use SCN-resistant soybeans on these fields in crop year 2002?
SCN can certainly be a stress factor. But just as many SCN varieties are susceptible to SDS as noncyst varieties. I recommend a cyst test to determine the population, and if you are above threshold, then use a resistant variety. Why? Mainly because at this point you can get the SCN problem under control, and there are no solutions for SDS yet. Getting SCN under control is a very important factor in relieving early-season stress to the plants.
I looked at 24 Roundup-Ready soybean fields today, and 23 are showing SDS along the roads and in areas of the field known to be wet. Most areas were 50 to 150 feet wide and generally along the edge of the field. Three fields had patches across 25% to 50% of the field. Are Roundup-Ready soybeans more susceptible?
Well, this is an interesting observation, but I caution you not to jump to any conclusions that Roundup-Ready soybeans are somehow more susceptible than traditionally bred soybeans. First, realize that more than 60% of the acreage is now Roundup-Ready, so naturally it may seem as though the disease is more prevalent in those varieties. We would likely be seeing a similar picture if it was all traditional varieties out there. It is worth noting that generally the disease first starts in stressed areas of the fields just as it does in traditionally bred soybeans. Although soybean breeders are working very hard to identify genes for resistance to SDS, a complete package is not available in either Roundup-Ready or traditional varieties.--Suzanne Bissonnette