Farm Talk

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October 15, 2013

Controlling volunteer wheat key to shutting down wheat streak mosaic

Parsons, Kansas — Wheat streak mosaic can shut down wheat yields and defund next summer’s profits quicker than Congress in a nasty mood.

“The yield impact of the virus is pretty significant,” explains Doug Shoup, Kansas State University Extension agronomist. “If you have 80-bushel yield potential, wheat streak mosaic can cut that down to 20 bushels per acre.

“We used to think of it as a western Kansas disease but it’s been a growing issue for southeast Kansas producers for the past three years and killing volunteer wheat is the only way to control it.”

The 1/100th of an inch long wheat curl mite is the vector for the virus, hosted by volunteer wheat and piggybacking the virus from plant to plant on the wind. In the more wide-open spaces of western Kansas and western Oklahoma, the mites can travel a mile or more.

For southeast Kansas and the surrounding area, a quarter- to half-mile is more likely, Shoup says. That, however, means it’s not just the field you’re planting that’s vulnerable — the wheat curl mite can sail in from adjacent fields as well.

“Killing volunteer wheat is the key to control,” Shoup says. “You want it dead two weeks before your wheat comes up.”

Symptoms, which resemble those of the barley yellow dwarf virus, appear in early spring.

“Last year we started picking it up in Cherokee County in February,” Shoup notes. “We didn’t confirm wheat streak mosaic in every county but I would bet that there was some every county of southeast Kansas last year.”

The K-State agronomist explains that there is no chemical treatment for the virus and none of the area’s commonly grown wheat varieties offer significant resistance to the mite or the virus.

That means that disrupting the life cycle of the wheat curl mite by removing its host is the one and only remedy. The mite has a seven- to 10-day life cycle and will die in the absence of a host, making it critical to kill volunteer plants two weeks prior to the new crop’s emergence.

It is possible for corn or fescue to host the wheat curl mite but Shoup says virtually all of the wheat streak mosaic issues he’s seen in southeast Kansas were traceable to volunteer wheat plants.

“Be aware of the volunteer wheat on your place and within a quarter-mile of where you want to plant,” he recommends. “You can eradicate it on your farm but if you have a neighbor nearby with a lot of volunteer wheat that’s not controlled, I’d strongly consider not planting wheat in that field — I’d definitely think twice about it.”

Nitrogen-Rich Benchmark Strips

For some wheat growers, another upcoming chore is to get GreenSeeker strips established, Shoup says, noting that use of the optical crop sensing system is increasing in southeast Kansas.

“The driving factor has been the lodged wheat problems we’ve had,” he explains. “In a dry year, following corn with wheat, there’s going to be some leftover nitrogen that the wheat can utilize. When you go ahead and apply what you consider a normal sidedressing rate, you can end up with too much nitrogen.

“The GreenSeeker system gives the producer a much better estimate of how much nitrogen is actually needed in the spring. It won’t necessarily increase your yield but it can save you money on nitrogen application.”

The essential first step in taking advantage of GreenSeeker, Shoup points out, is to establish N-rich strips in the field to provide a benchmark for nitrogen utilization. Because of possible field variability, he suggests applying nitrogen in a 10-ft. strip across the field at a rate that is 120 percent of normal — probably 120 to 150 pounds of N for most growers.

In the spring when the wheat is at 5 on the Feekes scale — after greenup, prior to jointing — a GreenSeeker unit emits bursts of red and infrared light, measuring the amount of each that is reflected by the plants in the N-rich strip and locations in the rest of the field.

The strength of the light indicates the health of the plants by measuring biomass and greenness and is calibrated to data predicting wheat yield. Using the reading as a benchmark, the grower can then determine how much nitrogen is needed in the remainder of the field to meet yield expectations.

Handheld GreenSeeker units are available and the sensors can also be mounted on applicators and used with most variable rate controllers to apply targeted rates in variable fields by taking real time data from the crop canopy.

With appropriate software, a variable rate can be applied with granular or liquid N sources.

“There are some challenges,” Shoup adds. “Primarily, it’s frequently wet when we’d want to do this but it can be an excellent tool and it’s definitely a lot better than guessing and spending money on nitrogen you don’t need — or not having enough to produce good yields.” £

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