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Control Now or Pay Later

Thursday, April 1, 1999
filed under: Disease

As the thermometer rises, so does the threat of rust in sunflower fields. Timely destruction of nearby volunteer sunflower plants can help reduce that threat, however, reminds North Dakota State University extension plant pathologist Art Lamey.

Generally cooler weather - which is unfavorable for rust development - has helped hold this disease in check the past several years across much of the sunflower production region. But hot weather late this spring and into the summer could trigger more rust development among those hybrids susceptible to one or more races of the rust fungus.

Rust normally shows up first on volunteer sunflower plants in late May/early June in the Northern Plains, Lamey explains. It appears as the aecial stage, which results from sexual reproduction of the fungus. During this stage, yellow spots - about one-eighth to three-sixteenth inch in diameter - appear on both the upper and lower surfaces of cotyledons and leaves. Up to a dozen tiny cup-like structures form inside each yellow spot. These orange structures produce aeciospores. The aeciospores, as well as any overwintering summer spores, can produce early infections on commercial sunflower plants.

"After the initial infections occur, the repeating or summer spore stage is responsible for most spread of rust in sunflower fields," Lamey notes. "The summer spore stage produces brown pustules with a brown powder, [i.e.], the 'summer spores.'

"It is important to control volunteer sunflower plants in sunflower fields as well as in nearby fields to reduce early season rust," the NDSU plant pathologist emphasizes. "It's also important to control wild sunflower in ditches and along fence rows. Control of sunflower

volunteers and wild sunflower reduces early season infections from the aeciospores."

Such control measures will not actually prevent rust from developing in this year's sunflower fields; but they will help delay its development, Lamey points out - and anything that slows early season rust will help reduce rust severity later in the season. "Controlling volunteer sunflower and wild sunflower as soon as possible will help prevent the

sexual stage from forming - and also will prevent release of the aeciospores, which occurs early in the season," he explains.

A number of herbicides and herbicide combinations are available for controlling volunteer sunflower in small grains following sunflower. Among them are Ally, Amber, Banvel plus MCPA amine, Canvas, Clarity, Curtail, Express, Harmony Extra, Peak, Starane and Tordon. When applied in a tank mix with 2,4-D or MCPA, all these herbicides give good control

of volunteer sunflower that is superior to either 2,4-D or MCPA applied alone, NDSU weed scientists advise. "Sunflower will stop growing shortly after treatment, but may remain green for several weeks, depending on weather and crop competition. Good sunflower control will result, even though control may take a longer period of time as compared

to bromoxynil + MCPA," states the 1999 North Dakota Weed Control Guide.

Bromoxynil, alone or in combination with MCPA, can be used from the three-leaf stage of wheat until just prior to the early boot stage. Tordon combinations should be applied only on hard red spring wheat and on land that will be planted to small grains or flax the following year, says NDSU extension agronomist Duane Berglund. Even at very low rates, Tordon can carry over and injure sensitive broadleaves like sunflower, beans, sugarbeets or potatoes.

Growth stages of wheat that can be safely treated with the above-mentioned herbicides will vary, Berglund adds. Be certain to review herbicide labels for information on timing of applications, he cautions. "Growers applying Amber, Canvas, Peak or Ally plus broadleaf

herbicide combinations should read the label for next year's recropping restrictions," Berglund observes. "Also, after using Curtail, growers should not rotate to any crop except wheat, barley, oats, corn, canola, crambe, grass or sugarbeets within 12 months after treatment."

How important is effective control of volunteer sunflower in the succeeding crop? NDSU research has shown that one volunteer sunflower plant per square foot could reduce wheat yields by up to 16 percent, while three volunteers can rob as much as 35 percent of the wheat

yield. University experiments also indicated that wheat yields begin to be affected when volunteer sunflower is allowed to compete until the wheat is past the five-leaf stage. The NDSU tests showed no yield reduction when the sunflower was adequately controlled while the wheat was still in the three- to five-leaf stage of growth.

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