The outbreak of sudden death syndrome in Iowa soybean fields in 2010 has shown that many SDS management recommendations need more thought. That's what X.B. Yang, an Iowa State University Extension plant pathologist, told a large crowd of farmers, crop consultants and others attending ISU's recent Integrated Crop Management Conference at Ames. 

"From talking to farmers, seed company people, co-op agronomists and others, I have received several reports from them suggesting that the way SDS occurred last summer was inconsistent with what the text books have to say about this disease," says Yang. "At Iowa State's ICM Conference in early December, people shared their unique observations and asked some excellent questions."

Following is a summary of what Yang and his ISU colleagues learned about SDS, based on their experience and that of farmers whose soybean fields were hit with the disease during Iowa's very wet 2010 growing season.

Herbicides and SDS—is there a connection?

Growers are asking questions about glyphosate herbicide application and SDS occurrence. Some farmers believe that the use of glyphosate is the reason for SDS outbreaks. That's an issue which was raised more than a decade ago when Roundup Ready soybeans were first introduced to the marketplace.

"We addressed that issue at the time," says Yang. "Our data show that plots with glyphosate had levels of SDS about the same as in plots with Pursuit, a popular herbicide before Roundup Ready soybeans came along. However, the use of two times the recommended label rates of Pursuit had more SDS occurring than the normal application rate. Currently, some growers apply a higher dose of glyphosate than what the label calls for. They use the higher application rates of glyphosate for weed control because they fear weed resistance — they want to make sure they kill the weeds."

This should be taken into consideration for SDS management. If you apply a higher than recommended rate of glyphosate, you may increase the likelihood of SDS occurring. "Keep in mind that in the 2010 growing season, high SDS levels were found in fields with Roundup Ready soybeans and in fields of non-Roundup Ready soybeans," says Yang.

The use of lectofen (Cobra herbicide) applied specifically for disease control has been suggested to reduce SDS. "Our three years of studies in 1997-1999, which were published in leading plant pathology journals, showed that SDS was reduced by Lectofen application in the greenhouse," says Yang. "However, it was not effective in three years of field trials."

A revisit of this issue in 2008 with multi-location trials in eastern Iowa produced the same results. In 2010 Cobra applications on soybeans for the purpose of trying to control SDS produced a yield penalty compared with control applications. That was due to lack of white mold disease in 2010, which is consistent with our results in 1998." 

Can you use seed treatments to control SDS?

Use of seed treatments to control SDS has been tried for years, since the disease was a problem first discovered in Arkansas, without success. "No seed treatments on the market are effective," says Yang. "This approach to try to control SDS is a very difficult task, as we have learned. There are new chemicals in the research and development pipeline that are being tested by various companies. However, you need to see good results from using the products in the field before committing to production use on your farm."

Does improving the drainage in a field help control SDS?

Drainage has been listed at the top of SDS management recommendations by some institutes after this year's outbreak. Tiling a field requires major investment and such investment may not pay off in terms of SDS management.

Effectiveness of this measure varies depending on the years and this measure as a way to control SDS is effective in years with normal weather conditions, he points out. Observations from a flood year show that many well-tiled fields had severe SDS infestations and the disease was often more severe in-line with the drainage pipes.   

Multiple diseases in the same field are another concern 

"Keep in mind that SDS may not be a major problem next year although this disease has been widespread three years in a row," says Yang. "Types of disease outbreaks are determined by the types of extreme weather events, and the changing climate has made weather events often unpredictable."

Besides SDS, white mold is on top of the disease list in many Iowa fields in recent years. Other important soybean diseases in Iowa include viruses, foliar diseases and Phytophthora, notes Yang. This past summer, brown spot was very severe in Iowa and surrounding states. Fungicide applications at ISU's Nashua research farm in northeast Iowa with chemicals from different companies had four to nine bushel per acre yield increases compared with a "no spray" check.

This year the level of Phytophthora occurrence in mid-summer was the highest in recent years although its damage was incomparable with SDS, he says. The major damage caused by Phytophthora was seedling blight which led to replanting of soybeans. Use of a seed treatment that includes a fungicide that specifically controls this disease is an effective way to manage this risk. Consider using a seed treatment for this disease if you plan to plant a soybean variety that does not have good Phytophthora resistance in its genetic package.   

If you have SCN in a field, you will likely have SDS too

There is no question that soybean cyst nematode can enhance the severity of SDS, as proven by numerous scientific studies. But what happened in many fields in Iowa in 2009 and 2010 shows that SDS can also strike without SCN, says Yang.

"Weather the last two seasons suppressed the occurrence of SCN, as the 2009 summer was one of coolest in history and 2010 was a flood year," he notes. "However in both years, SDS was prevalent with a low level of SCN. This suggests that SCN management may help reduce SDS in years which have normal weather conditions, not a year like 2010 or 2009."

Soybean variety selection is your number one defense

"Do not plant a soybean variety susceptible to SDS unless the coming growing season is likely to be dry," he advises. "SDS has been prevalent in recent years and the 2010 epidemic in Iowa shows that the pathogen is present in every soybean field." It is true that a susceptible variety yields a few bushels higher when it is grown in a disease-free situation. However, a one-year strike by SDS disease can cut your profit so much that it will take several years to make up.

"Farmers should plant an SDS resistant soybean variety as a major option for SDS management," says Yang. "But you should do this with the expectation that resistance to SDS will not be as reliable as resistance to Phytophthora. Breeding resistance to SDS into  soybean varieties by breeders in both the private and public sectors has made progress over the years, but there is still a long way to go because of the nature of SDS."

NOTE: XB Yang is an Iowa State University professor of plant pathology with research and extension responsibilities in soybean diseases. Yang can be reached at (515) 294-8826 or by emailing [email protected].