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Wheat Rust Invasion is On; Should You Spray?

Leaf and stripe rust has been confirmed in spots around the Plains as the region's winter wheat crop rounds into the homestretch toward harvest. And, as it continues to spread, yield potential is taking an increasing hit. So, should you go to the expense to treat for rust in your fields?

There are several factors to consider in making the decision. First and foremost, the weather. Rust thrives in wet conditions, those common around the Plains right now, and the diseases are likely to spread like wildfire if the spores are present when the rain's falling. And, a lot depends on how far along your crop is when the rust pops. That goes a long way to determining the economic return of applying a fungicide, especially considering how damp conditions also favor other potentially harmful diseases this late in the game, says University of Nebraska Extension plant pathologist Stephen Wegulo.

"The fungicide spray should be timed to protect the flag leaf. The concept of a threshold level of stripe rust above which a fungicide should be applied is not very helpful because in general, plant diseases are best controlled preventively. In addition, because rust spores are numerous and microscopic and it takes 7 to 10 days from infection to appearance of pustules (the incubation period), waiting until a certain threshold is reached based on appearance of pustules gives the pathogen more time to infect and produce more spores which spread and cause new infections," Wegulo says. "In fields in which stripe rust appears at the heading growth stage, or when heading is starting, it is better to apply a fungicide at full heading that will control stripe rust as well as Fusarium head blight (scab). One of the conditions favoring stripe rust (wetness) also favors Fusarium head blight."

If you're still on the fence on the agronomic benefits of knocking down the rust, consider the dollars and cents as the ultimate decision factor. Especially if your fields are rust-susceptible, the net return of spraying should be the final arbiter.

"Research conducted by the author in 2007 in Nebraska (wet growing season with foliar diseases — rust and leaf spots) provided estimates of how much yield increase or net profit to expect from spraying at the beginning of stem elongation (growth stage Feekes 6) compared to spraying at flag leaf (growth stage Feekes 9). Averaged across five fungicides (Quilt, Headline, Tilt, Quadris, and Stratego) and four locations, spraying at Feekes 6 resulted in a net yield increase of 19 bu/ac and a net profit of $67/ac. Spraying at flag leaf resulted in a net yield increase of 22 bu/ac and a net profit of $85/ac," Wegulo says. "In a dry growing season with little disease (2006), yield increase was the same (6 bu/ac) from spraying at Feekes 6 or Feekes 9 and net profit was also about the same (about $5/ac) from spraying at Feekes 6 or Feekes 9."

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