Surviving the Green Bridge
The wheat curl mite vectors three viruses to winter wheat throughout the Great Plains. This serious virus complex includes wheat streak mosaic, Triticum mosaic, and High Plains wheat mosaic viruses. Management of this virus complex must include the control over-summering hosts of this mite during the green bridge period between wheat harvest and emergence of the new crop wheat in the fall.
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[00:00:05.325]Surviving the Green Bridge.
[00:00:08.100]The wheat curl mite is a vector
[00:00:09.470]of a complex of three viruses in wheat,
[00:00:11.516]the most common being wheat streak mosaic virus.
[00:00:14.556]In the Great Plains, this complex is estimated to cost
[00:00:17.523]winter wheat producers 2 to 3%
[00:00:19.551]of their production each year,
[00:00:21.437]amounting to millions of dollars in lost production.
[00:00:24.458]Mites become a problem when they are numerous in the fall
[00:00:27.334]and carry the virus into the new crop wheat.
[00:00:30.152]But, to be able to do this, they must survive the summer
[00:00:33.902]on a living bridge host.
[00:00:36.129]As wheat nears harvest,
[00:00:37.952]mites reach their peak population density of the year.
[00:00:41.521]Mite populations just prior to wheat harvest
[00:00:44.051]can easily reach 1,000 mites per head,
[00:00:46.989]thus exceeding about two billion mites per acre.
[00:00:50.222]However, once wheat is fully mature,
[00:00:53.114]they can only survive about a day
[00:00:54.874]if they are unable to find a new host.
[00:00:57.726]As these mites move off the maturing wheat,
[00:01:00.268]they must find a new host, or a green bridge,
[00:01:03.550]to enable their survival and buildup through the summer
[00:01:06.586]and subsequent movement to the new crop wheat in the fall.
[00:01:11.249]Because the mites reproduce rapidly in wheat
[00:01:13.824]and the viruses are very active in wheat,
[00:01:16.458]the ideal host for the mite virus complex
[00:01:19.566]is volunteer wheat that emerges before wheat harvest.
[00:01:23.157]This is referred to as pre-harvest volunteer wheat.
[00:01:27.251]Emergence of volunteer wheat prior to harvest
[00:01:29.648]most often results from hail storms
[00:01:31.676]during the early to late dough stages in wheat,
[00:01:34.602]about three weeks prior to harvest.
[00:01:37.390]The presence of this volunteer allows mites
[00:01:40.047]to move to the volunteer from the existing wheat.
[00:01:44.499]Mite populations, along with viruses,
[00:01:47.139]build up in pre-harvest volunteer wheat through the summer.
[00:01:50.384]And if left uncontrolled, they will disperse by wind
[00:01:54.376]into the newly-planted wheat crop in the fall,
[00:01:57.112]thus completing the green bridge
[00:01:59.199]and beginning the virus infection in the wheat crop.
[00:02:03.312]The risk level for pre-harvest volunteer wheat is very high.
[00:02:07.153]Thus it must be controlled before fall planting.
[00:02:12.544]So there are two main methods that we can use
[00:02:14.033]to control that volunteer wheat out there
[00:02:15.713]and break that green bridge.
[00:02:17.587]The first method that folks can use is tillage.
[00:02:20.826]And tillage can be a really effective,
[00:02:24.911]if you use a disk or some sweeps.
[00:02:27.034]Something that will lift that wheat
[00:02:29.640]up and out of the soil and dry it out, is very effective.
[00:02:34.691]That only works, though,
[00:02:35.591]if a farmer or producer is using tillage.
[00:02:38.452]The other option would be if folks are using no-till,
[00:02:43.300]they would have to rely on herbicides.
[00:02:46.697]The herbicides that most folks would use
[00:02:48.911]would be a glyphosate.
[00:02:52.792]It's fairly cheap, and it's also a broad-spectrum herbicide
[00:02:56.765]that would do well on other weeds out there
[00:02:58.043]that folks are trying to control as well.
[00:03:02.079]The other option would be some grass-specific herbicides,
[00:03:05.933]like Assure II or Select Max.
[00:03:08.168]Those are both effective at controlling volunteer wheat.
[00:03:12.211]Really, no matter what folks choose,
[00:03:13.693]it's important that they get out there
[00:03:16.094]at least a month or so
[00:03:17.512]before they intend to plant that wheat
[00:03:20.366]so that the tillage and the herbicides can take effect
[00:03:23.637]and break that green bridge at least two to three weeks
[00:03:25.431]before we intend to plant that wheat
[00:03:26.861]and take care of the wheat curl mite out there.
[00:03:30.591]There are numerous grasses
[00:03:31.665]that can host mites and viruses.
[00:03:33.613]However, hosts present as wheat is maturing
[00:03:36.550]have the greatest risk.
[00:03:39.719]One green bridge host that poses less risk
[00:03:44.234]is late-maturing corn.
[00:03:46.833]When wheat is planted next to corn
[00:03:49.912]that remains green well into the fall,
[00:03:52.618]mites move from the corn onto the wheat and transmit virus.
[00:03:57.693]Foxtail millet that remains green until wheat emerges
[00:04:02.276]can also serve as a host for the mites
[00:04:04.851]and can result in the mites moving onto the emerged wheat.
[00:04:09.796]One other summering host that holds a high risk
[00:04:12.879]for developing virus problems is volunteer wheat
[00:04:16.078]that emerges in a summer crop such as sunflower or corn.
[00:04:21.092]If this volunteer emerges at wheat harvest,
[00:04:26.716]mites infest it readily.
[00:04:29.496]And if it's not controlled,
[00:04:31.250]it can pose a significant risk in the fall.
[00:04:36.077]Some summer annual grass weeds, such as barnyard grass,
[00:04:41.725]also can serve as a good host for the mites and viruses.
[00:04:44.846]The density of these annual grass weeds
[00:04:47.470]determines the level of risk for mite and virus movement
[00:04:51.732]onto the fall-planted wheat.
[00:04:54.491]Volunteer wheat and wheat stubble
[00:04:55.978]is often widespread.
[00:04:57.462]However, little mite movement occurs after wheat harvest.
[00:05:01.606]So it is important to establish a two-week break
[00:05:04.624]in the green bridge following harvest.
[00:05:07.456]Pre-harvest volunteer wheat
[00:05:08.786]has by far the greatest risk for virus development.
[00:05:12.397]Therefore, managing this green bridge host is critical
[00:05:15.804]to reduce the risk from this virus complex.
[00:05:19.618]It is important to identify
[00:05:21.093]the presence of this volunteer at harvest
[00:05:23.838]and focus efforts on eliminating this serious threat
[00:05:26.951]and avoid harvest losses.
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