Recently documented resistance of the common sunflower, Helianthus annuus, to a widely used soybean herbicide could have big implications for commercial sunflower producers in years to come.
Last year, Kansas State University weed scientists were alerted to a field at Rossville (near Topeka), where soybean grower Doug French was experiencing problems controlling wild sunflower with the postemergence broadleaf herbicide “Pursuit.” French, producing soybeans under a monoculture system on that particular field, had treated with Pursuit for at least seven consecutive years.
KSU weed physiologist Kassim Al-Khatib took seed samples from a number of wild sunflower plants in French’s field and, after some difficulty, was able to initiate germination and growth in a university greenhouse. He and his colleagues then began spraying the sunflower plants with various rates of Pursuit — from one-fourth of the standard rate all the way up to 32 times the labeled treatment rate.
The KSU researchers discovered that about 25 percent of the treated sunflower plants were still susceptible (i.e., controlled by Pursuit); another 50 percent were heterozygous (showing partial resistance) — and the final 25 percent were completely resistant to the herbicide. “We did not kill them, even at the 32x rate of Pursuit,” Al-Khatib reports
That news has generated considerable interest within the sunflower industry, where a lack of postemergence broadleaf herbicides continues to hamper weed control for many producers — including those growing the crop under no-till or solid-seeded systems.
This past winter, Fargo, N.D.-based USDA-ARS research geneticist Jerry Miller received a packet of those northeastern Kansas seeds from Al-Khatib. Using a special dormancy-breaking protocol developed by colleague Gerald Seiler, Miller was able to germinate the majority of seeds and initiate greenhouse growth.
At about the four-leaf stage, the young plants were taken to a special spray chamber at North Dakota State University and treated with Pursuit at up to 16 times the labeled rate. As in the Kansas tests, the North Dakotans witnessed three general reactions: Some plants were completely susceptible to the herbicide; others survived vegetatively, but the growing point (reproductive parts) was killed; and the remaining 28 plants displayed complete resistance.
This spring, pollen from those resistant plants will be crossed with that of the maintainer lines HA 89 (oil-type) and HA 292 (confection). Less than two weeks later, Miller will use a technique called “embryo recovery” to bypass the entire seed maturation and dormancy process, thereby producing new seedlings in a much shorter time. When the resulting plants reach the four-leaf stage, some will be sprayed to determine whether the herbicide resistance is a dominant or recessive trait. (Hopefully it’s dominant; if not, there’ll be additional crossing to make sure both parents of the hybrid end up with the resistance.)
The next step will be to self-pollinate one generation of the Pursuit-resistant wild sunflower to remove the dominant “branching” characteristic which exists in the wilds. Again using embryo culture, Miller will produce a new generation of plants, of which about one-fourth should have the desired single head. “We’ll save the resistant and single-headed plants,” he says. “They’ll produce flowers, and we’ll back-cross them to the line we want.” That step will occur in late summer or early fall and will be followed by another back-cross procedure. The Pursuit-resistant lines should be ready for field testing by the summer of 1998.
“We’ll be using every sophisticated technique we know to progress this as quickly as possible,” Miller emphasizes.
While it’s premature to predict how soon this resistance could end up in commercial hybrids, the potential implications are obvious for those sunflower producers who are searching for more help with broadleaf problem weeds such as kochia, pigweed, mustard, nightshade and cocklebur.
On the downside, Pursuit is a herbicide which can carry over in the soil and injure certain succeeding broadleaf crops for up to three years or more. So rotational restrictions would be an important consideration if it does indeed become available for use on sunflower. Also, its long residual and sharply focused mode of action make it more prone than many other herbicides to development of resistance.
For now, KSU’s Al-Khatib has initiated a project (with financial support from the National Sunflower Association) to answer exactly why those northeastern Kansas wild sunflower plants did develop resistance to this particular herbicide. He’s also studying whether sunflower could have a tendency to develop resistance to additional herbicides with modes of action similar to that of Pursuit. Kansas State University likewise plans to conduct surveys (some in conjunction with other states) to determine whether this resistance may exist in additional wild sunflower populations around the region. — Don Lilleboe
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