June 23, 2011                       Issue No. 8

Low Densities of Cereal Aphids Present in Both ND and MN this Past Week

Most of our small grain aphids don’t overwinter here in ND and MN. They re-populate every year via migrants coming in from states to the south.

Populations in most areas are still very low, but there may be an issue of concern other than yield loss from aphid feeding.  This year, small grains were planted late in most areas making fields especially susceptible to Barley Yellow Dwarf Virus (BYDV).

BYDV is vectored by aphids, especially the bird cherry oat aphid, one of the most common aphids that we find in our wheat.  The bird cherry oat aphid is olive green with a brownish patch at the base of cornicles. Two other common aphids in our area include the English grain aphid (bright green with long black cornicles) and greenbug (pale green with a darker stripe down the back).  The bird cherry oat aphid feeds primarily on leaves in the lower part of the small grain plant. When aphid populations are high, BYDV can spread quickly through small grain fields. At greatest risk are later-planted fields which attract migrating aphids that are moving from more mature fields.

BYDV has already been confirmed by the NDSU Diagnostic Lab in winter wheat samples in ND.  In other areas, some symptoms that look like BYDV have been reported.  Given that we've just started to see aphids showing up, we suspect this means that aphids are arriving already carrying virus.

Because we're dealing with late-planted small grains, and the long term forecast calls for cool weather through June, small grains in the area are at a greater than normal risk for BYDV if there is an aphid infestation. As a result, producers/crop consultants should be proactive with their cereal aphid control in late-planted wheat. The typical economic threshold for aphids in wheat is when 85% of the stems have more than one aphid present prior to complete heading. However, knowing that most of our late-planted wheat is at high risk for aphid infestation and BYDV transmission, we recommend that producers/crop consultants be more preventative against aphid infestations and treat as soon as aphids are detected or BYDV is observed in the area.  Field scouting should begin earlier than normal (typically before stem elongation) and continue up to the heading stage of wheat to prevent economic yield losses. Due to the high risk status, best pest management of aphids may require more than one application of insecticides this year to prevent yield losses from aphids and BYDV. Even if early aphid populations require treatment because local BYDV symptoms have been reported, uninfected wheat may still require treatment for aphids later in the season if aphid populations rise to yield limiting numbers.

 Pyrethroid or organophosphate insecticides registered in small grains should provide adequate control.  A listing of insecticides registered for cereal aphid control in small grains is available from the ND Field Crop Insect Management Guide 2011, E-1143, NDSU Extension Service.

http://www.ag.ndsu.edu/pubs/plantsci/pests/e1143w1.htm

Janet J. Knodel - janet.knodel@ndsu.edu

Ian MacRae - macrae@crk.umn.edu

Correct Stage for Applying Fungicides for Scab Control in Wheat and Barley

Early planted winter wheat in southern North Dakota is at or near flowering, the critical stage for Fusarium Head Blight (FHB or Scab) infection. With the recent rains, the risk of Scab being problematic on winter wheat fields flowering this week is quite high.

Several fungicides and fungicide combinations have been shown to be effective in reducing Scab development and DON levels in wheat and barley when applied at the appropriate growth stage. 

Timing in wheat

Early flowering is the optimum growth stage for applying fungicide for scab control in all classes of wheat (winter, spring and durum).  Applying fungicide during early flowering helps to protect vulnerable florets during fertilization and early grain-filling.  The center spike in the accompanying photo is at the ideal stage for applying fungicide.  The spike on the left has emerged from the boot, but has not yet started to flower (there are no visible anthers extruded from the glumes) and will likely be at the optimum stage in about two days.  The spike on the right is past the optimum stage; the anthers are bleached and dried, unlike the turgid, yellow anthers in the center spike.  The length of time from head emergence to the beginning of flowering usually takes about three days.  Experience has shown that it is better to apply fungicide too early rather than too late. 

Timing in barley

Flowering in barley begins just before head emergence, so barley florets are not overly susceptible to scab infection.  Therefore, Scab infections do not generally impact yield in barley.  The scab fungus, however, is able to infect the glumes of barley and produce DON which impacts its market value, particularly if it is being sold for malt.  The optimum stage for applying fungicides to protect the glumes of barley from infection is when the spike is fully emerged from the boot.  In the accompanying photo, the spike third from the left demonstrates the optimum stage for treating barley with fungicides, with those further to the left too early and the one on the right too late.  With barley the appearance of the first spikelet from the boot is a good indication that the best stage for spraying is only a few days away.

Joel Ransom - Joel.ransom@ndsu.edu

Flowering Winter Wheat at Risk for Fusarium Head Blight (=Scab)

Winter wheat producers in ND may be at high risk of Fusarium head blight infection, if their crop is near or entering flowering. A very large area in eastern and north central ND is showing moderate to high risk of infection today for head scab susceptible varieties.

For moderately susceptible varieties, the risk is only slightly lower.  If possible, winter wheat in flowering stage should be treated with a triazole fungicide now in risk zones indicated on the North Dakota map.  Triazole fungicides need a minimum of one hour drying time before rain, but 4 hours of drying time is preferred. 

Although Folicur (4 fl oz per acre) and other tebuconazoles may be very low $ cost per ounce this year, the preferred products for head scab control are Prosaro at 6.5 fl oz per acre or Caramba at 13.5 fl oz per acre.  Prosaro and Caramba provided approximately 20-30% better reductions in scab severity and DON (vomitoxin) accumulation than Folicur in studies averaged over three locations (Carrington, Langdon, and Fargo) and five years (2005-2009).  Yield responses with Prosaro or Caramba were about 4 bushels greater than with Folicur over these same five years, and were 5-8 bushels better than with Folicur in 2005-2007, years more conducive to head scab than 2008 and 2009.  

Marcia McMullen marcia.mcmullen@ndsu.edu

Odds of Controlling Bolted Plants?

I have received several calls about controlling large weeds, many of which are biennial or winter annual plants that have progressed from rosette to bolted and flowering with seed-set imminent.

Once biennial/winter-annual plants bolt they become much more difficult to control even with herbicides that normally provide adequate control. These plants have entered the final phase of their life cycle with seed-set left to complete. They usually have a large plant mass and foliage, which also takes more herbicide to desiccate than a young plant a few inches tall with a few leaves.

With a realistic expectation of what herbicides can and can’t do at this advanced stage, it may be productive to realize that plant death may not occur but you can destroy most of the weed seed by applying translocating herbicides. Glyphosate, Pursuit, Raptor, 2,4-D, dicamba are all phloem mobile herbicides and move from ‘sink-to-source’ accumulating in areas of high metabolic activity – in other words they move to where seeds are forming and can disrupt normal seed development. This is why we state in the weed guide under glyphosate preharvest application for various crops – Do not apply on crop seed grown for seed because reduced germination may occur.

Rich Zollinger - r.zollinger@ndsu.edu

Sprayer Cleanout

Now that herbicides containing alternative herbicide sites (modes) of action are being mixed with glyphosate and a greater diversity of herbicide resistant crops are being grown, proper sprayer cleanout and knowing the type of herbicide resistant crop planted in a specific field is VERY important.

Sprayer cleanout is especially important before spraying extremely susceptible crops such as sugarbeet.  Herbicides can become attached to sprayer parts, especially tanks and other parts made of plastic.  Growth regulator and ALS-inhibiting herbicides are the most likely herbicides to adhere to plastic material.  After these herbicides become attached to the plastic they can be removed by herbicides and adjuvants used in future sprayer loads.  The worst case scenario of herbicides adhering to plastic tanks and sprayer parts is when a mixture of a growth regulator herbicide is left in a sprayer tank overnight or for multiple days before being emptied.  To compound the problem if the next spray mixture has herbicides and adjuvants that act as tank cleaners and this spray mixture is left in the tank for multiple hours, the attached herbicide can be removed and becomes mixed with the current spray mixture.  For additional information about proper sprayer cleanout consult page 77 of the 2011 NDSU Weed Control Guide.  This can also be found online at:

 http://www.ag.ndsu.edu/weeds/weed-control-guides/nd-weed-control-guide-1/wcg-files/11-Gen.pdf.

Jeff Stachler

Field Perimeter Weed Management

Usually the inside field perimeter has a greater weed density, diversity, and plant growth compared to the center of the field. Allowing weeds to produce seeds along the outside field perimeter is one reason for the greater weed density in the inside field perimeter.

Another reason for greater weed density on the inside field perimeter is the increased likelihood of late-season weed germination due to greater sunlight penetration to the soil surface from the edge of the field.  Another field perimeter concern is the build-up of herbicide-resistant weeds due to a reduced herbicide rate from the last nozzle on the spray boom.

Some suggestions of how to manage field perimeters are as follows:

1.     Apply a preemergence herbicide around the entire inside field perimeter to reduce early-season weed densities and plant growth.  Then apply postemergence herbicides over the entire field at the same time.  Scout to determine need for a second postemergence application along the inside field perimeter.

2.     If no preemergence herbicide was applied, apply postemergence herbicides around the inside perimeter of the field one week prior to the rest of the field or at the proper weed stage and height since the weed along the field perimeter are usually larger than inside the field.  Apply a second postemergence herbicide application to the inside field perimeter at 14 to 21 days after the first application to control weeds emerging after the first application and include a residual herbicide when available to control late-season weed germination.

3.     Apply postemergence herbicides to the inside field perimeter at the same time as the rest of the field.  At 14 to 28 days after the postemergence herbicide application and/or at the last possible stage of the crop, cultivate the field perimeter to remove weeds germinating after the herbicide application.

4.     Mow the outside field perimeter as low as possible every 30 days or just before any plants begin to flower.  One mowing may work, but it depends upon the weed species diversity.  The goal is to not allow any weeds to produce seeds and to use mechanical means of control to reduce the selection of herbicide-resistant plants.

Jeff Stachler

South-Central ND

Winter wheat field stages range from flag leaf to flowering. Spring wheat and barley seeded during the first week of May are jointing. Generally, small grain fields in all growth stages continue to be in excellent condition.  Early May planted corn is in the 5- to 6-leaf stage. Stages of soybean range from plant emergence to 2- to 3-trifoliate leaves (planted in early May). Late-planted row crops stands have improved with recent rains. Disease threat is high in crops due to wet conditions and moderate air temperatures. Tan spot in wheat is commonly found by Extension crop scouts and crop advisers, but also wheat streak mosaic virus is becoming more common and we are monitoring for barley yellow dwarf virus. Sunflower rust has been found at the Carrington REC and foliar disease in pulse crops has been observed.

Greg Endres gregory.endres@ndsu.edu

North Central ND

Small Grains: Barley Yellow Dwarf Virus Confirmed

We have positive confirmation from the NDSU diagnostic lab for Barley Yellow Dwarf Virus in a spring wheat field in McLean County and a winter wheat field in Ward County.  If this virus is found in wheat or barley fields, aphids should be controlled much sooner than the current threshold (85% of stems with at least one aphid). Cereal aphids have been present in some fields in the area but are at low levels currently.  In addition to seeing a lot of tan spot and bacterial blight, we did see some powdery mildew in a winter wheat field in Ward County.

Canola Update

Many of the canola fields in the area are still in the susceptible stage for flea beetle damage.  Most of the fields in the area are below the economic threshold.  Diamondback moths and tarnished plant bugs (Lygus lineolaris) have also been caught in our flea beetle traps over the last couple of weeks. 

Daniel Waldstein Daniel.waldstein@ndsu.edu