Soybean sudden death syndrome showing up
Soybean sudden death syndrome (SDS) caused by the root-rotting, soil-borne fungus Fusarium virguliforme, is beginning to show up in soybean fields across Kentucky. To my knowledge, all of the sightings are in patches of plants or individual plants, and are not field-wide. SDS has been seen in Kentucky each year, to one degree or another, since 1985.
Sometimes this this early symptom can look almost virus like. In most cases, blotches eventually coalesce and result in a yellow and brown discoloration between the veins, but the veins remain green. In severe cases, the leaflets will fall off, but the petioles will remain attached to the plant. Foliar symptoms are the result of a toxin produced by the fungus in diseased root tissue.
The foliar symptoms descibed above can look similar to stem canker, Dectes Stem Bore injury, and even pytotoxicity symptoms caused certain triazole fungicides, so foliar symptoms in and of themselves are not diagnostic for SDS. Additional symtoms include severely rotted roots (F. virguliforme is a root rot organism) and a light brown discoloration of the stem when sliced open with a knife. Healthy stems will appear a cream color. When all three symptoms are evident, you can be quite confident that you have made a field diagnosis of SDS.
This disease is poorly named in that the disease does not usually suddenly appear, nor is the end result always death. In fact, SDS foliar symptoms first appear much like other diseases, a little at a time. It is only "sudden" if you have not been looking. I have seen plants with SDS recover (put out leaves without characteristic foliar symptoms) and in most cases, the disease comes in too late, or does not impact enough of a field, to significantly reduce yields. Severe foliar symptoms can be misleading and often do not indicate that the disease has or will significantly impact yields. In fact, I have seen many instances over the last 27 years where severe foliar symptoms developed across a field (late), but the yield retieved was a record for the field. So with SDS, when symptoms show up relative to crop stage is everything.
Fusarium virguliforme infects roots early in the season and foliar symptoms normally appear in the reproductive stages, as is the case in fields this year. Abundant soil mositure encourages SDS and that is why we are seeing more of the disease this year than we normally do. In a more typical year, a greater extent of SDS is often associated with very early planting; doublecrop beans are rarely affected. However, in a year like this one, planting date associations are blurred and we may find that the disease even ends up developing in doublecrop soybeans.
The best way to control SDS is to plant a resistant variety. They are not that hard to come by. But there is nothing you can do to slow or stop SDS once it is evident in a field. For sure, applying a fungicide WILL NOT HELP.
In summary, SDS often has a "bark" more hurtful than its "bite". Twenty-seven years of experience in Kentucky has shown that people often get all worked up over SDS because of the hyped-up name for the disease and because symptoms usually show up late in the season in an otherwise good looking crop. However, the end result is commonly little or no measureable yield loss, unless the disease is widespread and severe in a field before pod fill is complete, which is rare in Kentucky.