In the Nov. 21, 2003, issue of Delta Farm Press I discussed late season cercospora and its effect on the crop this past year, but I did not mention the green effect or lack of leaf drop many experienced this past growing season.
Essentially we observed two extremes. First, we saw Group 4s drop leaves very fast from cercospora, but in many instances stems remained green and some plants retained a few leaves. The second scenario occurred on Group 5s where cercospora caused leaves to desiccate and stick on the plants. In both situations green stems were observed.
Many of us are seeing something we have never seen before. Given the excellent growing condition coupled with early planting, we saw plants growing under better-than-normal conditions. This environment caused many fields of 4s and 5s to want to hang on.
The greening effect can be caused by many things: variety, insects, foliar fungicides and late application of Roundup. However, in years past our soybean crop usually died prematurely.
When we planted later, our crop dried down very uniformly. This was due to widespread disease pressure (which is greater on later plantings) and often a killing frost.
Today, plants are fruiting earlier. The ability to set fruit over an extended period of time has caused plants to hang on. This is very similar to cotton taking on second growth following defoliation.
Other than resistance, we need a good broad-spectrum fungicide to help us address cercospora.
Stem canker is the second disease that caused concern in 2003. Stem canker has the potential to cause more widespread yield loss than any other soybean disease. Sure, there are numerous diseases that cause problems; however, they often occur in one area of a field or they are scattered, not field-wide. Other diseases may be reduced or prevented by the use of foliar fungicides.
Stem canker resistance is available in a number of varieties. Many may remember the old Asgrow variety 5980. It was one of the first high-yielding varieties many ever grew; however, it had one weakness — stem canker susceptibility. Susceptible varieties grown today are not as susceptible as those grown in the past. This is not to say that we should plant susceptible varieties, just that improvements in resistance have been made.
When the problem occurred in 1989, university and company breeding programs jumped on top of it and provided much-needed resistance. Given what we know about stem canker and planting dates, I contend that we could still grow 5980 if we knew how it responded to early planting. I am sure we could plant it behind wheat or in a late-planting scenario with little problem due to stem canker.
Stem canker appears to be most devastating on plantings that occur between April 25 and May 25. If you plant earlier than this or later, it can still affect the crop, just not to the same degree. In addition, earlier-maturing varieties are not affected to the same degree as later-maturing varieties.
Crop rotation will help, but this disease can carry over to a crop of beans following rice. If you are planting in the window I mentioned earlier, do not drop your guard, choose moderately resistant to resistant varieties. The worst case scenario is where you had a problem previously and follow the crop with a susceptible variety.
Early planting and late planting act as an avoidance mechanism. You may see some stem canker on early plantings, but the disease does not have time to fully develop as it does on full-season varieties or varieties planted from April 25 to May 25. We see very little stem canker in double-cropped beans, but you do not need to delay planting to avoid this problem.
Another point to keep in mind is that Southern stem canker was first identified by Billy Moore and Wayne Jordan in 1973 in Clay County, Miss. Given our long history, we have a more-virulent strain than neighboring states. Also, it is more severe in the hill area (when it occurs) than in the Delta.
When making varietal decisions, use Gabe Sciumbato's numerical ratings which are available in the Mississippi variety trial disease reaction tables. The ratings in Mississippi are more representative of our potential problem than other ratings, and I think after this year many of you may have a better appreciation for these ratings.
Every year we collect samples of stem canker and Gabe uses them to obtain isolates for inoculating plants. If someone in Missouri is using a strain from that area, they are not able to accurately evaluate what we have here. That has become very obvious with several popular varieties over the past three years.
The numerical ratings represent a worst-case scenario because they are taken from plants that have been tooth-pick-inoculated, but this should leave no room for surprises. I am not saying that company information is not accurate; just that if they are not using Gabe's source of inoculum, they have not covered all the bases. Gabe's stem canker screening trial is funded by the Mississippi Soybean Promotion Board with your checkoff dollars, a wise investment for your benefit.
We have learned a lot about stem canker since its initial appearance. Given available resistance, there is no need to let this devastating disease wipe out large acreages of soybeans. Log onto Mississippi State University's Internet site for current variety information or contact a county Extension office for a printed copy.
Alan Blaine is the Mississippi Extension soybean specialist. e-mail: [email protected].