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Crop Tech: Rootworms Grow Up Resistant

02:49AM Aug 27, 2011
corn rootworm

Rootworms are rascals. The latest evidence of this comes in the first documented case of resistance by a field population of western corn rootworm in northeastern Iowa.

An Iowa State University team led by Aaron Gassmann responded to producer concerns that Bt hybrids expressing the Cry3Bb1 protein were exhibiting significant larval damage. Gassmann’s report in the PLoS ONE scientific journal reveals how he collected western corn rootworm adults from those fields and found that the progeny from those parents able to survive on Bt corn hybrids expressing the Cry3Bb1 toxin at levels higher than western corn rootworm from fields not exhibiting significant larval damage.

Hybrids with the Bt Cry3Bb1 toxin include Monsanto Company’s VT3 technology and SmartStax hybrids marketed by Dow AgroSciences and Monsanto. The good news is the progeny from the corn rootworm adults remained susceptible to the Cry34/35 Ab1 protein (also found in SmartStax and Herculex hybrids), suggesting there is an absence of cross-resistance.

Todd DeGooyer, Monsanto’s corn traits lead, says the company is working with Gassmann to assess how the lab results impact field scenarios. Farmers had used Bt hybrids expressing the Cry3Bb1 protein for three or more consecutive years in the fields in question.
"While we don’t like to see this kind of situation arise, it is not surprising given the rootworm pressure in that region and the selection pressure given a continuous Bt corn scenario," DeGooyer says.

"We encourage growers to follow an integrated approach to rootworm control that includes crop rotation. Those dedicated to continuous corn should use stacked hybrids containing multiple modes of action to protect against belowground pests, such as Genuity SmartStax," he adds.

Flash in the Field

Here’s a news flash … those lime green or yellow soybean leaves on the top of the crop late in the season weren’t an illusion. "Yellow flash" is a temporary chlorosis of newly emerging soybean leaves that sometimes follows an application of glyphosate to glyphosate-resistant cultivars.

yellow flash
Yellow flash or yellowing of emerging soybean leaves sometimes occurs after herbicide application.

University of Illinois weed scientist Aaron Hager says the flash typically shows up when applications are made late in the season, when the soybean is in the reproductive stage of growth. It’s usually noticed most in areas of sprayer overlap or where growers fire up the boom.

A few days after glyphosate is applied, all leaves on the soybean plant remain green except the newest leaves at the top. These leaves continue to grow, but chlorophyll production is reduced, leaving a yellow color caused by carotenoid pigments becoming visible in the absence of chlorophyll.

Mark Bernards, a Western Illinois University agronomist, says the temporary yellowing usually happens when the plants have undergone rapid growth and environmental conditions are warm.

"The symptoms generally last for about a week to 21 days after [glyphosate] treatment and no decreases in yield have been reported," Bernards says. If it is dry when the yellow leaves appear, those leaves might stay yellow until the crop resumes growth after rain releases the crop from stress.

There are other reasons for yellowing of leaf tissue in soybean plants. Nitrogen deficiency, manganese deficiency, potassium deficiency and soybean cyst nematode have been shown to play a role in turning soybeans yellow, but all are separate issues with yield consequences.

The good news is that yellow flash is largely environmental and, while not advisable to try to induce, is more of a curiosity than a concern.

Infection-Fighting Peptides

Soybean rust has shown an uncanny ability to outsmart genetic defenses. University of Missouri–Columbia researchers have identified peptides that have the potential to protect against the devastating disease.

The breakthrough was made by researchers Frank Schmidt and James English. They first used peptides to protect the roots of crops such as cucumbers, melons, tomatoes and squash against invasion by Phytophthora capsici, a pathogenic microorganism. Schmidt says the same concept is helping plant scientists understand a variety of diseases, including soybean rust. The disease was first identified in the U.S. in 2004 and, to date, has caused the most yield losses in Southern states.

Proteins and other molecules on the surface of a fungal pathogen contribute to the growth and development of the organism or provide a means of monitoring and detecting environmental signals that contribute to development. This also represents significant targets for pathogen disruption by defense peptides.

Schmidt and English culture the disease organism and monitor which peptides bind to the pathogen most effectively. They then test the effects of the most interactive peptides on the growth and development of the pathogen. Schmidt says 20% to 50% of the peptides that bind strongly produce an inhibitory reaction and could be useful in defending against soybean rust.

"We are using biomolecules that inhibit the growth of the pest," Schmidt says. "This is a biotech strategy that delays the onset of the soybean rust. It doesn’t kill rust, it buys a longer window of time. The next step is to get the soybean to express it in the plant to provide protection from infection."

Schmidt believes the results of the research could engineer resistances for plants that are more environmentally safe solutions for fighting infections.

Goss’s Wilt Spreads

Corn plants dying early in the field became more than a sign of dry weather in parts of the Corn Belt this year. A severe outbreak of Goss’s bacterial wilt was confirmed in areas of Illinois, Iowa, southern Minnesota and Nebraska by late July.

Planting tolerant hybrids, managing residue and preventing transfer of infected materials to currently uninfected fields are critical steps to limit the potential spread of the disease.

Goss Wilt
States west of the Mississippi have corn fields found to be susceptible to Goss’s bacterial wilt this season. Severe outbreaks can result in significant yield losses.

"This is the first time we’ve seen Goss’s wilt in Illinois this widespread and this early," says Todd Thumma, Syngenta agronomist. "Goss’s wilt has typically been more prevalent in the western Corn Belt, but until now it had been very sporadic in Illinois."

"Bacterial diseases require some type of wounding to infect a plant," explains Suzanne Bissonnette, director of the University of Illinois Plant Clinic. "Goss’s wilt, caused by the bacterium Clavibacter michiganense subspecies nebraskensis, finds easy infection from tissue damage after hail, high winds and heavy rainfall."

Goss’s wilt is often misdiagnosed as drought stress or other foliar diseases, especially in areas where it is not typically prevalent. The symptoms of the disease can include gray to tan lesions on the leaves that follow the leaf veins, with small dark green or black "freckles" within the infections. Another telltale sign of Goss’s wilt is a glossy appearance to the affected leaves.

Once Goss’s wilt is present in a grower’s field, it will always be present and with suitable environmental conditions the disease can recur in following seasons. Planting hybrids that are Goss’s wilt tolerant is the most effective way to manage the disease. Bissonnette says research indicates dent corn inbred A632 and hybrids in which this and related inbreds are used are highly susceptible.

Fungicides will not control the bacterial disease. Fields that are corn-on-corn; fields that have
detected or undetected Goss’s wilt from previous seasons; fields with high corn residue; and fields with weed hosts, such as green foxtail or shattercane, are at a higher risk for infection.

To ensure tolerance, Thumma says, Syngenta screens both potential and current hybrids for Goss’s wilt tolerance using in-field disease conditions. This rigorous rating system, along with diverse genetics, enables Syngenta to bring to market hybrids that deliver strong tolerance to Goss’s wilt and other yield-robbing diseases.

Watch the Monsanto breeding pipeline to produce new sources of genetic resistance to Goss’s bacterial wilt. Although still early in the breeding program, the product concept is anticipated to be delivered in the 2015 time frame.