Performing a CSI on Slow-to-Grow Wheat
May 22, 2009
Unlike many years in the Panhandle, we received good moisture earlier this spring — about 140% above normal here and 143% above normal in the southwest district, according to the USDA National Agricultural Statistics Service. While conditions have been drier in recent weeks, the earlier rainfall and recent warm spell helped wheat green up.<p>
Given these conditions, growers should be concered about any area not greening up — especially one that does not coincide with drill rows. This should warrant a CSI (Crop Scene Investigation) to determine the cause.
Conducting a Wheat CSI
Scout suspect wheat areas to identify the source of the problem and determine your short- and long-term control options. Several likely causes are described below.
Russian Wheat Aphid (RWA) may be found throughout the spring in winter wheat and has been reported in northeastern Colorado. Infestations are likely to be spotty. Sparse aphid populations can build up through mild winters to become a serious problem. The most serious impact occurs when significant aphid populations damage flag (curl) leaves and interfere with normal head emergence and development. Damage symptoms appear as visible white, yellow, or purple longitudinal streaks on the leaves and stems of wheat plants. Sometimes high aphid numbers develop later and impact later maturing crops such as barley or even volunteer wheat.
Aphids survive from one wheat crop to the next on host plants that serve as green bridges. Survival is also aided by wetter than normal summers. A border effect can become evident with Russian wheat aphids as they move from the bridge crop to the new plants and the infestation starts anew. When aphid populations are stressed, succeeding generations (every 7-10 days) can develop wings and travel further with the winds, resulting in more widespread infestations. At this time, growers should be scouting for hot spots — initial aphid infestations. Fall infestations eventually cause the most damage especially with a mild fall and winter. Controlling volunteer wheat and other host plants after wheat harvest is crucial to managing these aphids for you and your neighbors.
Wheat Streak Mosaic Virus (WSMV) infections also will become visible in the spring. Infections often become evident in the spring when, after a warm days of +80° temperatures, wheat begins to turn yellow. Typically, symptoms will develop from the edge of the field, but may continue across the entire field. Wheat streak mosaic begins with a fall infection coming from the bridge crops — adjacent crops actually determine the severity of the infection. An extended warm fall can produce a secondary infestation. Start scouting for wheat streak mosaic in areas that had hail near wheat crop maturity last year. There is no treatment for this disease.
Black Grass Bug. If you notice wheat grass areas such as CRP fields and road ditches turning white, the cause is likely black grass bugs or labops. The black grass bug is native to our grasslands in low numbers, but has flourished in wheat grasses planted in CRP. It damages plants by piercing and sucking contents out of cells and removing the chlorophyll. Working mostly on the upper plant surface, they will start at the tip of the plant and work down. (They are upside down on the plant.) This cell damage is unique and well defined compared to fungal diseases (excluding stripe rust). These can cause a whitish cast to the field, depending on severity.
Damage usually is not a concern in CRP, but it is for those in seed or forage production. In Montana 15 black grass bugs per square foot resulted in 58% forage reduction with the remainder being less palatable. This year damage has been visible in wheat. Labops normally remain along the field edges, but higher numbers could result in deeper field infestation. Any spraying is usually recommended along the field edges. The damaging actions, however, should be short lived with the new leaves showing improvement. The key is still to protect the flag leaf.
ControlEradicating plants, particularly volunteer winter wheat, that can serve as a green bridge from one crop to the next is essential to managing Russian wheat aphids and wheat streak mosaic from season to season and from one neighbor's field to the next.
Extension Educator in Box Butte County