Figure 1. Barley yellow dwarf in a breeder nursery at Lincoln, Lancaster County on May 18.	Figure 2. A field with a high incidence of barley yellow dwarf in Jefferson County on May 18.

May 20, 2011

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A survey of wheat fields in south central and southeast Nebraska on May 12 and May 18, respectively, found barley yellow dwarf (Figure 1) to be widespread. Incidence (percentage of diseased plants) was low in most fields surveyed. However, one field (Figure 2) surveyed on May 18 in Jefferson County had barley yellow dwarf incidence ranging from 60% to 100% in some localized areas in the field.

Barley yellow dwarf is caused by barley yellow dwarf virus. It is transmitted by more than 20 species of aphids and has a wide host range including wheat, barley, oats, and many wild and cultivated grasses. Both the virus and its aphid vectors are favored by cool, wet weather. Infections by barley yellow dwarf virus can occur in the fall and continue to occur throughout the growing season the following year. Infections in the fall are more damaging than those in the spring. Once infection has occurred, nothing can be done to cure the plant. Do not apply a fungicide to control barley yellow dwarf.

Dwarfing (Figure 3) is a common symptom of barley yellow dwarf. Other symptoms are highly variable within one cereal crop, differ among cereal crops, and can easily be mistaken for nutrient deficiency symptoms. Typically, leaf discoloration in shades of yellow (Figure 4), red (Figure 5), or purple (Figure 6) occurs from the tip to the base and from the margin to the midrib. Yellowing is the most common type of leaf discoloration.

	(Clockwise from upper left) Figure 4. Yellow discoloration on a flag leaf caused by barley yellow dwarf. Figure 5. Red discoloration on a flag leaf caused by barley yellow dwarf. Figure 6. Purple discoloration on a flag leaf caused by barley yellow dwarf.

Yellowing caused by barley yellow dwarf should not be confused with the general yellowing (see the June 13 CropWatch) that occurs due to nutrient deficiency or root, crown, and leaf spot diseases such as tan spot. Small to large patches of general yellowing affecting most or all the leaves is most likely due to nutrient deficiency.

Yellowing caused by fungal leaf spot diseases occurs mostly on the lower leaves and usually brown to black spots can be seen on these leaves. In contrast, yellowing due to barley yellow dwarf is most conspicuous on upper leaves and especially the flag leaf. Occasionally one can see spots of dwarfed wheat due to barley yellow dwarf in the field corresponding to aphid feeding patterns.

Recommendations

Controlling aphid populations can reduce the incidence and spread of barley yellow dwarf in a field. Effectiveness will depend on knowing which aphid to control and when to apply control measures, since barley yellow dwarf virus is only transmitted by certain aphids. Treating seed with a systemic insecticide can help reduce the spread of aphids and barley yellow dwarf virus.

Identifying and Treating Aphids in Wheat

Aphids have been reported infesting wheat in southeastern and south central Nebraska the last few weeks. Samples of oat-bird cherry aphid (Figure 7) have been identified from these fields. In many years, corn leaf aphids (Figure 8) also may be seen in Nebraska wheat fields. These two aphids generally have a lower damage potential than greenbugs, another aphid that may be seen in Nebraska wheat. In the absence of barley yellow dwarf virus, aphids also may directly reduce yield if they are abundant enough.

Figure 7. Oat-bird cherry aphid.

Figure 8. Corn leaf aphid.

In most years, these aphids do not overwinter in Nebraska in high numbers, but winged forms may migrate in the spring with southerly winds. If conditions are suitable, relatively rapid growth is possible. Often in Nebraska spring aphid populations are controlled by natural enemies such as lady beetles and parasitoid wasps.

To determine if it is worth treating for aphids, determine the identity and number of aphids present.

When scouting, estimate the number of aphids per stem at several locations in a field. Don’t just count aphids in the “hot spots” because you need to get an estimate of the overall aphid population in the field. Treatment threshold levels vary by aphid species and plant growth stage (Table 1). These thresholds do not account for virus transmission by aphids, only the direct effects of feeding.

Insecticides

A variety of insecticides are effective for aphid control on wheat. Check the Department of Entomology website at http://entomology.unl.edu/instabls/waphids.htm for a list of rates and restrictions for suggested insecticides for aphid control in wheat.

Additional information on aphids in small grains, including photos of common species and a key for their identification, can be found in NebGuide G1284, Cereal Aphids.

Stephen Wegulo
Extension Plant Pathologist, Lincoln
Robert Wright
Extension Entomologist, Lincoln
Jennifer Rees
Extension Educator, Clay County