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Late-season soybeans show more fungicide response

A first-year study on large-scale research sentinel plots at Stoneville, Miss., indicates soybeans planted later in the season may respond more to fungicide applications than do early-planted soybeans.

Agronomist Trey Koger, USDA Crop Genetics and Production Research, said the tests with varying fungicide programs were prompted by worries in 2005 over the emergence of Asian soybean rust, which scientists say thrives in warm, moist weather.

“We have speculated that planting soybeans in early April may allow our early-maturing varieties to avoid some soybean disease pressure during the summer months,” he said. “So we took our early soybean production system and planted as it early as possible in April, versus planting in mid-May, which is the planting window our soybean growers commonly used prior to widespread adoption of the early soybean production system.”

Most soybean farmers’ production systems have continually moved to earlier planting dates due to the research findings of the likes of former USDA agronomist Larry Heatherly. Koger noted that beyond drought avoidance, the ability to get the soybean crop harvested and fall tillage done before the typical fall rains hit, and the flexibility of harvesting soybeans before harvesting most of the cotton crop, the early soybean production system may have other benefits such as disease avoidance.

“In all honesty, I was glad that our growers didn’t have to deal with rust last summer,” he said. “While we didn’t get any rust, we still obtained good overall information on fungicide data and overall plant health as it relates to planting date and maturity group. And what we found was that when we planted in early April, we didn’t see the degree of response from fungicides, compared to the response to fungicides when we planted in May.”

Koger said the fungicide responses on soybeans planted in April were subtle and more variety-specific than anticipated from those plants planted in May.

“That is not to say that we did not see a yield benefit from fungicides applied at the R3 to R4 soybean growth stage on Group 4 soybeans planted in early April,” he said. “It is just that we didn’t see as much of a response when planting in early April versus planting in early May.”

He said there was a basic reason for the discrepancy: The Group 3 and 4 varieties planted in early April were planted early enough to circumvent serious disease pressure in 2005.

Koger said for growers, the data has pertinent management implications. “We had a really hot dry summer this last year. That affected our disease pressure: We had less disease pressure last year than we did the past two years, and that was a big impact. Even though we had less overall disease pressure in 2005, we still saw yield responses from fungicides.

“But overall, when we planted a Group 3 in early April, we did not see a response to fungicide application. However, when we planted a Group 4 or 5 in early-April to early-May, we consistently saw a response from a fungicide application at the R3 to R4 growth stage.

“Basically, the later we planted, the more response we saw from fungicide applications.”

Koger said the compromise, however, is that when Group 3 soybeans are planted in early April, those plants did not yield as well as the Group 4 variety planted during the same time period, whether a fungicide was applied or not.

He said in the tests where fungicide applications were made at R3 and R4 soybean growth stage (one-fourth to three-fourth inch-long seed pods developed in any of the upper four nodes) and under the hot and dry weather conditions in 2005, he didn’t think the soybeans would show a huge response to fungicides.

“But what we did see was a response even though it was dry. That leads me to believe that — even in a dry year — a farmer is going to see response from a fungicide application on soybeans with good yield potential planted in April when the fungicide is applied at the R3 to R4 soybean growth stage.”

Of course, accurately predicting weather is impossible, he noted. “In a wet year, like 2004, we may have seen a response from fungicides on the Group 3 planted in early April. And we would probably have seen larger responses from fungicides on our Group 4s than what we saw in the dry summer of 2005.”

Koger said farmers’ most beneficial fungicide programs targeted at controlling some of the more common foliar diseases, such as frogeye leaf spot, do not do a great job at managing late-season diseases, such as cercospora leaf spot, which has emerged as a persistent and more prevalent problem in soybeans grown in the South.

“What we are seeing is that some varieties may be fairly resistant to frogeye but are susceptible to cercospora, while some varieties are fairly tolerant to cercospora but highly susceptible to frogeye.

“Overall we have fungicides that do a good job of managing frogeye and a lot of our other foliar diseases when we make timely applications. Our weakness is that we have not found a fungicide program that does a really good job of managing late-season cercospora.”

What also would help ease that problem, he noted, is finding a fungicide that does a good job of controlling cercospora while also determining when is the best time to spray that fungicide for late-season control of cercospora. “Right now we are searching for a single-pass fungicide application coupled with a disease tolerance package that will provide good season-long control of our most common diseases — such as frogeye — that typically develop in mid-summer and late-season. Applying fungicides at the R3 to R4 soybean growth stage has shown to result in the best yield and economic return. Fungicides applied at R3 to R4 help to reduce late-season cercospora but do not effectively control it.

“If we apply fungicide early we may not account for cercospora. On the flip side, if we spray a fungicide later trying to target cercospora control, then we will not be accounting for some of the other foliar diseases that strike earlier in the growing season.”

Koger said there is a lack of research that addresses the timing issue. “Additionally, we have not found a fungicide that gives us excellent control of late-season cercospora control when applied on our typical application timing windows.”

Koger did note that from the research conducted, the fungicide Headline provided marginal cercospora control. “We saw a 5-bushel to 8-bushel response for a cercospora-susceptible Group 5 variety planted in early April. In that same variety, we saw a 9- to 10-bushel response when planted in early May, Group 5.

“It did give us some control but didn’t completely control it,” he said. “We did not have other fungicides such as Quadris and Topsin M in that test, so we cannot honestly say how well Headline stacked up against other fungicides with respect to cercospora control.”

Future research Koger has planned includes an evaluation of Headline, Quadris, Topsin M and Domark applied at different timings and rating their ability to control late-season cercospora.

Koger said the first-year fungicide study only reinforces farmers’ preference to plant soybeans in early spring.

In fact, he said some farmers are asking how early is too early to plant. “We not only gained valuable rust detection information from our 2005 sentinel plots in Mississippi, we also began wondering exactly how early is too early to plant soybeans based on what we saw in some of our earliest-planted sentinel plots,” he said. “Growers began asking how well the earliest-planted sentinel plots did from a yield standpoint. We didn’t carry them out to yield, but did notice our earliest-planted sentinel plots (planted in February) came up to a full stand and looked pretty good during the growing season.

“Granted, in most years it is too wet to get in the field in February and most of March, and based on everything we know right now, our optimal planting window in Mississippi is early- to mid-April, especially for Group 4s and 5s.”

However, he said determining what date is too early needs to be resolved. “Twenty years ago, we were planting soybeans in July, now we are planting soybeans as early as March. The window of planting in late-March through April may remain our optimal planting window, but we need to know how early is really too early.” Koger thinks that not even the threat of Asian soybean rust will likely impede the early planting trend.

“We won’t go back (to planting in later spring) to avoid rust. We will continue to plant early, and we’ll manage rust with fungicides. We have the tools from a fungicide standpoint to manage Asian soybean rust and hopefully in the future we will have resistant varieties, but we don’t have much in our arsenal to avoid typical late-summer droughts except for planting early.”

Koger said that some sentinel plots planted immediately following Valentines Day last year ultimately made a stand.

“How early can we plant? We still don’t know how early we can. There remains a lot of interest (on what constitutes too early) but not many answers yet.”

e-mail: [email protected]

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