ISU Extension Agronomists Advise: Watch Corn For Seedling Disease

ISU Extension Agronomists Advise: Watch Corn For Seedling Disease

"Damping off" is an increasing disease problem on young corn plants in southern Iowa, ISU Extension agronomists say.

Damping off is a seedling disease problem that may be more associated with soybean plants, but it also affects corn plants. It's occurrence on corn has been increasing in southeast Iowa the past several years. It's something to keep an eye on, especially if you grow corn in southern Iowa.

WATCH FOR SEEDLING BLIGHT: Seedling blight, commonly referred to as "damping off," is a disease condition you normally hear about on soybeans. But it's an increasing problem on corn in southern Iowa, southeast Iowa in particular. Damping off has shown up in more and more fields the past few years and ISU plant disease specialists have an on-going research project focusing on it --looking at possible resistance to fungicide seed treatments.

"We have seen more damping off of corn seedlings, causing more farmers to have to replant fields in recent years," says Mark Carlton, an Iowa State University Extension field agronomist at Albia. "It's not a statewide problem so far -- it's mostly occurring in the southeast part of the state, where it's becoming more widespread." Carlton's colleague, ISU Extension field agronomist Virgil Schmitt, covering the southeast corner of Iowa, has also visited a number of farmers' fields the past few years to diagnose damping off on corn.

During the spring of 2012 thousands of acres of corn in southern Iowa had to be replanted in late May due to poor stands caused by seedling disease. Many of the fields affected were planted between April 23 and 27, 2012. From April 28 through May 8, 2012, 2 to 4 inches of rain fell across southern Iowa and southeast Iowa, respectively, and soil temperatures dropped below 55 degrees F for four or five days. Approximately one week later, many fields with damped off seedlings were reported.

The Iowa Corn Promotion Board and two companies, BASF and Valent, are funding a research effort led by ISU Extension plant pathologist Alison Robertson to investigate this epidemic. The study aims to find the cause and search for solutions to corn seedling blight due to the Pythium pathogen.

Searching for solutions -- study funded by Iowa Corn Board in southeast Iowa

Especially following wet springs, corn seedlings can die off and may result in fields having to be replanted resulting in increased cost and lost production. The goal of this research is to identify strains of Pythium present in these affected fields and find management practices that provide control.~~~PAGE_BREAK_HERE~~~

All of the seed corn farmer's plant is treated with a fungicide, notes Carlton. The cause of the seedling blight outbreak is likely a strain of a soil fungus that today's fungicides aren't effective at providing protection against.

"Last year we visited 25 of those affected fields and collected corn seedlings with damping off symptoms, and recovered nine species of Pythium," says Robertson. The most prevalent species recovered was P. torulosum, a known pathogen of corn. Tests in the lab and growth chamber at ISU established that cool, 55 degree F soil temperatures favor both seed rot and root rot caused by P. torulosum.

Fungicide seed treatments protect germinating seed from pathogens. Metalaxyl (active ingredient in Allegience) and mefenoxam (Apron) have excellent activity against pythium species, says Robertson. Strobilurin fungicide seed treatments such as azoxystrobin (Dynasty), trifloxystrobin (Trilex) and pyraclostrobin (included in Acceleron) also have some activity against this group of pathogens.

Resistance by some strains of pythium disease to fungicide seed treatments is a possibility

A few years ago researchers in Ohio reported resistance to all these fungicides among Pythium species that they had recovered from diseased corn and soybean seedlings in Ohio. "We tested the Pythium species we recovered in 2012 in southeast Iowa and also found that they differed in sensitivity to metalaxyl, azoxystrobin, trifloxystrobin and pyraclostrobin," says Robertson. "Some isolates continued to grow in the presence of the fungicide. Resistance to metalaxyl and mefenoxam and also the strobilurins has been reported for numerous pathogens."

Valent is expecting registration of a new fungicide, ethaboxam, in 2013. It will be combined with metalaxyl or mefenoxam and marketed as the AP3 Fungicide System. Ethaboxam is highly effective against Pythium and Phytopthora sojae, and belongs to a different chemical group than metalaxyl and mefenoxam.

Robertson and her ISU plant pathology team evaluated ethaboxam and metalaxyl alone and in combination in controlled environment trials using soil collected from four fields in southeast Iowa in which stand loss occurred during May 2012. All treatments reduced root and mesocotyl rot and improved emergence. In collaboration with Valent and BASF ISU is testing ethaboxam and other experimental compounds on farmers' fields in southeast Iowa during the 2013 growing season.

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