June 1998
Scouting, Controlling Scab and Midge Crucial at Heading
By Dr. Michael Peel, NDSU Extension Small Grains Specialist
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Prairie Grains is the
official publication of
the Minnesota
Association of
Wheat Growers,
North Dakota Grain
Growers Association,
South Dakota Wheat,
Inc., and the
Minnesota Barley
Growers Association.
Most talk about the development of small grains is relative to leaf stages, mainly because that is when most herbicide applications are made. However, scab in wheat and barley becomes a problem when affected crops enter reproductive developmental stages. This change occurs at flowering when pollination or anthesis occurs.
Wheat and barley are fundamentally different at this stage; while wheat flowers two to three days after heading, barley flowers while the head is still in the boot. In both crops this can vary depending on the variety and the environment they are grown in. Moisture or heat stress can result in wheat flowering one to two days earlier.
Wheat and barley are most susceptible to scab shortly after head emergence. Scab infection occurs first as an infection on dead anthers and then spreads rapidly to the developing head.
Scab control with a fungicide has only proven effective when completed shortly after head emergence in barley and during flowering in wheat. This is because fungicides are meant to prevent the disease; once infection has occurred the fungicides will not eliminate the disease.
Moisture and a source of inoculum are the important factors for scab development. The organism that causes scab, Fusarium graminearum, survives on residue of wheat, barley, and corn. If spring wheat, durum, or barley were planted in last year's small grain or corn stubble, then there is a ready source of inoculum. Combine this with rain during flowering and a forecast that predicts more rain or continued high humidity, and there is a high potential of disease development. Under these circumstances control measures are warranted.
There is good evidence that a fungicide application is justified when applied correctly. Research by Marcia McMullen, NDSU extension plant pathologist, shows reduced scab, and yield increases above 30% in two of three treatments at Fargo in 1997 (Table 1). Recent research indicates application technique dramatically affects fungicide efficacy. NDSU extension agronomist Terry Gregoire elsewhere in this issue talks about application techniques that can greatly enhance the effectiveness of a fungicide application.
Fungicides registered for use during heading include: Tilt at 4 fl oz/A applied at Feekes growth stage 10.51; Benlate at 0.5 lb/A plus mancozeb at 1.0 lb/A applied at growth stage 10.51; and mancozeb at 2.0 lb/A applied once at Feekes growth stage 10.0 and 10 days later or stage 10.51. A Section 18 emergency exemption for Folicur has been approved; it should be applied at 4 fl oz/A. Improved control with ground applications has been observed when higher volumes of water are used, e.g. 20 versus 10 gallons per acre. n
Monitoring Wheat Midge
Wheat growers should continue to be on the alert for the orange wheat blossom midge. This pest lays eggs on wheat from head emergence to early flowering; fortunately, it is not a problem on barley. Using insect Degree Days (DD), NDSU can predict when the midge will begin to emerge. By knowing how many insect DD were accumulated when wheat or durum was planted, it can be determined if there is potential for a midge problem.
Insect DD start accumulating for the wheat midge as soon as the temperature reaches 40o F. Wheat planted between 200 - 600 insect DD will head during peak midge activity, putting that wheat at the highest risk of midge infestation. For northeast ND, 200 DD was reached about April 28. When this was written, 600 DD is expected to occur around mid to late May. Accumulated DD can be obtained through NDSU Extension services NDAWN system at www.ext.nodak. edu/homepages/regeberg/ndawn/. Use DD with 40o F as the base temperature. If you do not have access to the Internet or need help in evaluating insect DD readings, contact your county extension agent.
Female wheat midge will start emerging at about 1300 DD and will be complete by 1600 DD. Once 1300 DD are reached, field scouting should be conducted in areas where midge has been a problem in the past. Figure 1 on the next page is the wheat survey map of North Dakota for the 1998 growing season showing the concentration of overwintering larvae from 1997. Growers in areas with the highest potential for infestation must be particularly vigilant in scouting for the midge.
Wheat midge can only be observed in the field during the evening from about 9 p.m. to midnight when wind speeds are below 6 mph and temperatures above 59o F. Sticky traps (a paper plate coated with cooking spray such as Pam works well) can be used to detect the presence of midge, but are not used to make treatment decisions. Sweep coated plates through wheat heads to determine if midge are active and if it's time to start scouting. Wheat fields should be monitored from head emergence until flowering is 80% complete.
The adult wheat midge are small orange flies about one half the size of mosquitoes. Larvae feed on developing kernels and are also orange-colored. Infested wheat heads show no visible symptoms but larvae are easily observed when developing kernels are inspected.
Threshold levels for treatment are one adult midge per four or five heads. This must be determined by examining heads at several locations throughout a field. Wheat is attractive to midge from heading to early flowering. Treatments at later stages will prove ineffective.
The only product registered in North Dakota with wheat midge on the label is Lorsban 4E-SG at one pint per acre. Use the following as a guide for application when threshold levels have been reached: If 30% headed, wait up to 4 days to treat; if 70% heading to flowering optimum control can be achieved; if 30% to 60% of heads have visible anthers, spray immediately; if 80% of heads have visible anthers, treatment will be ineffective. Lorsban 4E-SG should be applied before 9 a.m. or after 6:30 p.m. for optimal treatment results.