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It Pays to Control Rust in Confection Sunflower

Thursday, December 15, 2005
filed under: Disease

Research in Colorado suggests that rust can have a big negative impact on sunflower yields, and that timely control can make a big difference in crop return.

A side-by-side plot trial in 2005 on the Ryan Weaver farm near Burlington, Colo., showed a yield difference of 767 lbs between sunflower treated with Folicur® (tebuconazole) and untreated. The trial was irrigated, planted with a confection hybrid known to be susceptible to rust, and treated around mid to late pollination, or about R6. The Folicur treated site yielded about 2,000 lbs/ac, untreated about 1,200 lbs/ac. Seed size was also larger in the treated compared to untreated.

Assuming a contract price of $20/cwt, the difference in gross return between treated and untreated in this trial was about $140/ac – about a six fold increase beyond the total treatment cost (product + aerial application) of about $20/ac.

Many oil-type hybrids have adequate resistance to rust and therefore, hybrid selection is the first line of defense. Some confection hybrids have shown better rust resistance than others as well. And as this trial indicates, confection hybrids are more likely to respond to fungicide application for rust control.

“We knew what rust could do to confection sunflower, and that a treatment helps control it, but this really shows how much,” says Weaver. Treatment timing is key, however. “If you wait too long, not only will you lose treatment effectiveness, but you run into label concerns about spraying too close to harvest.”

Fortunately, treatment timing coincides with insecticide spraying. “You’re spraying for seed weevil and head moth anyway, so you might as well include the fungicide if it’s needed.” While there is no label restriction against tank-mixing Folicur with insecticide, it’s prudent to cross-reference the label of the insecticide being used for any label restrictions in tankmixing a fungicide. Consider a jar test to determine compatibility of product mixtures.

Rust was a problem this year in some sunflower fields in parts of Kansas, Colorado, Texas and Nebraska. Disease development is typically favored by warm temperatures, cloudy days and wet foliage for at least 8-12 hours, and extended wet periods of 3 to 4 days can cause serious damage.

Colorado State University extension agronomist Ron Meyer, who coordinated the Folicur treatment trial on Weaver’s farm, points out that there are numerous strains of crop rust, and that they are often crop specific, able to infect one but not another crop type. For example, it is the sunflower rust strain Puccinia helianthi that is specific to sunflower, but sunflower isn’t affected by either Asian soybean rust or wheat rust. So rust in the previous year’s wheat crop doesn’t mean an increased chance of rust in the following year’s sunflower crop, and vice versa.

“You need three conditions present for there to be a rust problem: the crop host, the rust strain specific to the crop host, which usually arrives with south winds, and the right environment for infection to proliferate,” says Meyer. All three conditions must be met for a disease outbreak to occur, which explains why the disease is spotty from year to year and amongst locations.

Sunflower rust pustules are found on the underside of sunflower leaves, beginning on the lower leaves of the plant and moving higher in the canopy under favorable conditions. Rust can be yield impacting when the majority of the plant’s upper leaves and bracts are covered with rust pustules prior to the plant reaching growth stage R-6, which is when the ray flowers (petals) are wilting. If rust pustules are found on the upper leaves of confection sunflower at budding, then the crop may benefit from a fungicide application.

Folicur 3.6F received a Section 18 Emergency Exemption in Kansas and Colorado in 2005 to control sunflower rust. According to the label for use in 2005, Folicur should be applied at the rate of 4 fl. oz. per acre at the earliest sign of rust pustules or when weather conditions are conducive for rust development. A second application may be made 14 days later if needed. The product label in 2005 allowed a maximum of 8 fl. oz. of active ingredient that may be applied per acre per year. Folicur may be applied up to 50 days before harvest.

Another fungicide, Headline, was registered for use in sunflower in 2005, and rust was listed on the pests controlled section. However, Headline® provides no curative (systemic) activity. It is a strobilurin fungicide and must be applied prior to infection (preventative activity only) to have any effect. Thus, in cases where rust symptoms are already present, there would be little benefit from a Headline application.

Growers whose confection sunflower were affected by rust in 2005 should consider hybrids that offer better genetic rust resistance, and/or budget for a fungicide application, as well as consulting with an agronomist for more specific product and application information. – Tracy Sayler

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