The Enid News and Eagle, Enid, OK

October 26, 2013

Crop disease may ‘spring’ up: Volunteer wheat helps spread virus that can hurt yields

Staff and wire reports
Enid News and Eagle

ENID, Okla. — Wheat producers who didn’t get rid of volunteer wheat before their latest crop emerged could be in for problems next spring with wheat streak mosaic.

“The yield impact of the virus is pretty significant,” said 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.”

The disease is a threat to wheat in Kansas and Oklahoma, 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.

“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.

Wheat streak mosaic virus complex causes stunting and yellow streaking leaves, said Roger Don Gribble, Oklahoma Cooperative Extension Service northwest area agronomist.

“In most cases, infections can be traced to a nearby field of volunteer wheat, although there are some likely other hosts of the mites that transmit the virus,” he said. “Other hosts can be corn, millet and many other annual grasses, such as yellow foxtail or even prairie cupgrass.”

Shoup said 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 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 said virtually all of the wheat streak mosaic issues he’s seen are 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 said. “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.”

Volunteer wheat also will harbor Hessian flies, which may cause the wheat plant to lodge at harvest; aphids, which could be vectoring barley yellow dwarf; or even chinch bugs, which feed on developing wheat as it emerges, Gribble said.

Volunteer wheat also uses soil moisture that is a valuable asset for good growth and development of the 2014 harvested wheat crop, he said.

According to U.S. Department of Agric-ulture’s National Agri-cultural Statistics Ser-vice, 61 percent of the state’s wheat crop has emerged.

So far, 84 percent of the crop has been planted, according to the NASS report released Oct. 21.

So far, things are looking good for the newly emerged crop. NASS rates 11 percent of the crop in excellent condition and another 58 percent in good condition. Twenty-seven percent is rated fair, while only 4 percent is rated poor or very poor.For some wheat growers, another upcoming chore is to get GreenSeeker strips established, Shoup said, noting use of the optical crop sensing system is increasing.

“The driving factor has been the lodged wheat problems we’ve had,” he said. “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 said, 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-foot strip across the field at a rate that is 120 percent of normal — probably 120 to 150 pounds of nitrogen 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 nitrogen-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 also can 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 nitrogen sources.

“There are some challenges,” Shoup said. “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.”

Mark Parker writes for Farm Talk, of Parsons, Kan., and contributed to this story.