A weed control mentality “is not the approach to take” in dealing with Asian soybean rust, says Tom Allen.
“If you find ASR in a soybean field and think, ‘Well, I’ll wait until I find more and then take care of it,’ you’re just asking for trouble,” he said at the annual meeting of the Mississippi Agricultural Consultants Association.
“This thing can move at lightning speed, given the right environmental conditions — and could make you gasp at how rapidly it can spread across a soybean field. It scared the heck out of folks from Iowa when I showed them photos and data on what it can do.”
Allen, Mississippi State University Extension and Research Assistant Professor at the Delta Research and Extension Center, says, “We need to respect this disease much more than we have in the past and not become complacent about its presence.”
Had it not been for all the late-season rains last year and subsequent seed rot, he says, “I think we would have had more extensive losses from ASR.”
In 2009, Allen notes, Mississippi had 2.2 million acres of soybeans, and all 82 counties in the state had ASR-infected plant material, whether on kudzu or soybeans. There were 22 sentinel plots of early-planted soybeans in key areas to detect the disease as it develops, helping the soybean rust team to keep growers informed about potential movement throughout the state and to make recommendations for protecting their crops.
Mississippi, Louisiana, and Arkansas had a cooperative scouting program involving 1,658 unique locations, he says, and detected ASR in 213 counties and parishes. In 2009, 36.7 percent of the Asian soybean rust reported nationally was from the Mid-South states of Arkansas, Louisiana and Mississippi.
Statewide, in Mississippi he says, estimates of losses to ASR ranged from much less than 1 percent to 25 percent of total production in at least one field.
“In a Noxubee County soybean field checked in September, the grower had massive defoliation that he thought was caused by charcoal rot, but ASR was identified as the initial disease. Charcoal rot was a secondary infection. He harvested 45 bushels per acre from this field. A neighbor had applied a 4-ounce rate of a strobilurin fungicide to his soybean field, and it turned out he made the right decision. He got a 15 bushel return from that application, preventing a 25 percent yield loss, based on his yield compared to that of his neighbor — a pretty good return for the chemical cost.”
Allen suggests that a 6-ounce rate would be better and likely still be cost-effective. “I wouldn’t skimp on it.”
Not all fungicides are labeled to prevent ASR, he notes. “I’m in the process of completing a list of suggested materials that should be available this spring.”
Additionally, fungicides can help manage the disease. In Louisiana field trials, specifically to manage yield loss from ASR, a single fungicide application resulted in a 6.8 percent to 31 percent response in harvested yield.
Since the disease was first detected in Louisiana six years ago, “we’ve made a lot of progress in learning about how it develops and spreads,” Allen says.
The kudzu vine, which grows rampantly in southern summers, is estimated to cover 300,000 acres in Mississippi and thousands more acres in Arkansas and Louisiana, is additionally a host for ASR.
“Only 11 counties and parishes in the three states have been determined to not contain kudzu as of 2009,” Allen says.
Research has previously been conducted on the differential response of kudzu to infection by the ASR fungus. In Mississippi, during 2006 and 2007 seed was collected from 51 kudzu sites throughout the state and it was determined there were variable responses to challenge from the fungus in greenhouse studies conducted by the USDA at Fort Detrick, Md.
Allen notes, there is a wide variation in response in part due to “the incredible range” of genetic variation in kudzu. “One kudzu plant may be absolutely covered with ASR, while another, growing a short distance away, has none.”
To aid in future scouting, kudzu was focused on late in the season since ASR was so widespread.
“In Mississippi, 91 percent of the 231 kudzu sites we looked at had observable, sporulating pustules of ASR,” Allen says. “Infection ranged from light to very heavy.
Overwintering potential for ASR in Mississippi is still being evaluated, Allen says. “In December, a laundromat at Batesville, Miss., had some kudzu growing against the building and it was covered with ASR. In instances like that, it’s possible it could overwinter given the warmer temperatures provided by the laundromat.”
Many growers have expressed concern that ASR seems to be more widespread, Allen says.
“Yes, there is more rust out there, but we’re also much better at finding it.”
He urges growers and consultants to frequently monitor the Soybean Rust Hotline (866-641-1847), funded by the Soybean Promotion Board and BASF, for updates on ASR findings and management suggestions.
And, Allen advises, “Don’t pay attention to coffee shop talk. There are a lot of false reports and misinformation. Check the hotline to get the facts.”
Leaf spots and fungicides in cotton
While fungicide applications can be cost-effective for ASR in soybeans, that may not necessarily be the case for leaf spots in cotton, Allen says.
“In most cases, the underlying cause of fungal leaf spots is a stress-related nutrient deficiency. Nearly all leaf spot diseases are related to potash deficiency in the leaves. From a plant pathology standpoint, there is a varied range of leaf spots, with names that often change. They’re pretty much opportunistic “trash” fungi, which usually don’t cause much of a problem on their own — they need some other stress.
“We saw tremendous amounts of leaf spots in cotton last year, and there was some talk that they were caused by generic glyphosate or adjuvant burn, but that’s not the case. Last year’s crop was shallow-rooted and there was a lot of weather-related stress. It was just a case of these trash fungi jumping on plants because they could, because the plants were stressed.”
Based on the products currently labeled for use in cotton, a fungicide application just prevents further spread of the fungi, Allen says. “You’re not going to recoup leaf areas already affected or lost. And it’s certainly not worth the cost of the fungicide.”
In some cases, as in 2008, the leaf loss could “actually be considered to be a good thing because it allowed better air circulation through the lower plant canopy late in the season and prevented severe boll rot and hardlock.”
Studies have been conducted for the past two seasons to consider the role of fungicides in increasing harvested lint — or as a “plant health” benefit. In studies that included “fairly heavy rates” of foliar fungicides, there was little response, Allen says. “The best response was two applications of Headline, a total of 12 ounces, which returned 74 pounds of lint. If you run the math, it just isn’t cost-effective. You’d need to get quite a bit of response to recoup the application cost.”
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