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PNW CONSERVATION TILLAGE HANDBOOK SERIES
Chapter 4 - Disease Control, No. 13, Fall 1987


BYD: The Aphid-Virus-Corn Link

Roger Veseth

Barley yellow dwarf (BYD) is a viral disease of cereals and grasses worldwide and can cause serious economic losses. The virus can be transmitted between plant hosts by at least 22 aphid species. Five strains of the virus have been identified in the Northwest region. Some are transmitted by only specific aphid species and infect certain crops. Others have abroad range of aphid and plant hosts. The complexity of the aphid-virus relationship and their geographic distribution has made the disease difficult to research and control.

BYD was first reported in Washington on spring barley in 1954. Economic losses have been sporatic, being most widespread after mild winters which allow aphid overwintering. Spring barley losses were especially high in 1958 and 1980, Extensive losses in winter wheat were not reported until 1975. Since then, chronic BYD problems have occurred in some locations, particularly near the irrigated Columbia Basin area.

STEEP Research Effort

The highest BYD levels in winter wheat have been associated with early seeding dates. Since early seeding is one crop management option which helps reduce over winter soil erosion under conventionally-seeded winter wheat, the study of BYD has been part of the STEEP conservation farming research program in the Northwest. Stephen Wyatt, Washington State University plant pathologist, has provided the leadership for the BYD research.

Varietal Resistance

All varieties and classes of winter wheat are susceptible to barley yellow dwarf virus (BYDV), Wyatt and, cooperating plant breeders have research underway to develop BYDV-resistant lines. Progress has been slow and the release of resistant varieties is not expected for some time.

Virus Strains

Extensive research by Wyatt and research associates has determined that the predominant BYDV strain in Washington is aphid-nonspecific and is transmitted by a large number of aphid species, In contrast, several aphid-specific virus strains are present in southern Idaho and Montana.

Sources of Virus and Aphids

No major local sources of BYDV, and aphids to transmit the virus, have been found in the dryland cropping region in eastern Washington. Because of the dry summer climate, Wyatt has concluded that major local sources do not exist. It has been assumed that aphids carrying the virus migrate into the dryland areas from the irrigated Columbia Basin area.

Wyatt points out that corn may play an important role as a summer host for the virus. Through field sampling in the irrigated region, the major crop host of the virus, other than wheat and barley, was corn. In 1981, 58 percent of the corn fields sampled harbored the virus. In 1982, it increased to 65 percent. Infection levels within the fields ranged from 4 to 64 percent.

It is believed that the English grain aphid transfers the virus from infected wheat, barley and grasses to corn fields in June. By August when grasses have dried up, high populations of oat bird-cherry aphid commonly build up on corn. As the corn begins to mature, the aphids eventually migrate to new winter wheat fields, carrying the virus with them. These two aphid species are the predominant species found in monitoring traps in eastern Washington and also have the highest percent transmission efficiencies for the aphid-nonspecific virus strain. Wyatt concludes that this is be reason the aphid-nonspecific virus strain is the predominant strain in eastern Washington. Aphid-specific strains could not take advantage of corn as an over summer host since two aphid species are involved.

Wyatt points out that although corn appears to be a major link in the BYD disease cycle, its importance fluctuates from year to year. Populations of the English grain aphid must build up early enough in the season to infect the corn in the seedling stage when it is susceptible. This does not always occur because of yearly environmental variations, changes in cropping practices and other factors. Also, in some summers the oat bird-then-y aphid does not build up to high populations in corn.

Fig. 1. Field survey results of barley yellow dwarf virus (BYDV) in winter wheat and corn fields in eastern Washington, 1981-86 (Wyatt, WSU).

After the initial high incidence of BYD in corn fields, early in Wyatt's survey program in 1981 and 1982, the disease incidence has declined sharply (Fig. 1). By l986, the incidence of BYDV in corn had dropped to almost zero. The incidence of BYDV in winter wheat has also declined the past 3 years from the fairly constant average of 20 percent from 1982 through 1984. Wyatt adds that from preliminary data, average BYDV incidence in 1987 is also near zero for both corn and winter wheat. However, he notes that a small number of early-seeded winter wheat fields in the sample did have as much as 25 percent infection. The reason for this decline in BYD is unknown, but lower aphid populations or delayed migrations because of climatic variations and changes in crop rotation patterns or management practices are some possibilities.

Corn increased in acreage in the Columbia Basin about the same time BYD became more important in winter wheat. Until the 1960's BYD was not a problem in winter wheat grown in the same area as aphid-infested corn. Wyatt estimates that the adaptation of BYDV to corn, or the introduction of a corn-virulent strain of the virus, probably occurred during the late 1960's, He points out that in plant host-range studies of BYDV reported from around the country and the world indicate that BYDV varies considerably in plant hosts. In many regions of the U.S., the virus has not been isolated from corn.

In a 1983 Washington study, only 50 of the 88 virus samples collected across the state could infect corn. The 38 virus samples not able to infect corn were from the northern counties of Douglas, Okanogan, Ferry and Stevens. This area is believed to have local sources of the virus and aphids throughout the growing season on grasses along waterways, potholes and lakes, Corn is not required as a summer host to complete the disease cycle in this region. The 50 virus samples that were able to infect corn were from south and central counties. Wind patterns in relation to the Columbia Basin appear to be an important factor in the importance of corn as a summer host. Winter wheat seeding date is critical in all areas in determining the potential infection of the wheat crop and influences the potential transmission to spring barley the following spring.

Wind Patterns

Aphid migration studies conducted by Keith Pike, Washington State University entomologist at Prosser, indicate that in the late summer and fall, aphids move from the Columbia River area between Oregon and Washington into the irrigated region of Benton, Franklin, Grant and Walla Walla counties. Then carried by prevailing winds, the aphids fan out northeasterly and easterly into the dryland cropping region.

Seeding Date

Winter wheat seeded early is more likely to emerge during peaks of aphid flight activity and be infected in the seedling stage when damage potential is the greatest. Cooler weather later in the fall reduces aphid activity and consequently also reduces infection potential of later-seeded wheat. However, wheat seeded later in the fall usually has lower yield potential and a greater chance of winterkill and damage from other diseases such as Pythium root rot.

Wyatt points out that seeding date is dictated by many factors which often are not controllable. In Whitman County, for example, precipitation level and crop rotation dictate different seeding dates between the eastern and western portions of the county. In the lower precipitation area of western Whitman County, winter wheat is seeded relatively early (e.g. late August or early September) on fallow while seedzone moisture is adequate for germination. In the eastern portion of the county, higher annual precipitation allows winter wheat seeding in an annual cropping rotation after a spring grain or noncereal crops. Seeding is usually later in September and October after fall rains have replenished seedzone moisture (fall 1987 was an obvious exception).

The difference in seeding dates in the county has influenced BYD incidence. In 1982, Wyatt determined that the BYD incidence was 46 percent in western Whitman County compared to 2.5 percent in eastern Whitman County. He notes that experimental seedings of winter wheat in late August or early September in eastern Whitman County are often highly infected.

Control Considerations

Wyatt concludes that because of the complexity of wind patterns, yearly climatic variations, aphid species, BYDV strain variations, aphid-virus strain relationships, cropping diversity and other factors, it is not surprising that BYD incidence and damage varies considerably between areas and from year to year. The predominant BYDV strain in eastern Washington is technically fragile and disruptable, requiring two aphid species and two crops to complete the disease life cycle each year. However, farmers have few opportunities in selecting management practices which will reduce the BYD incidence. The two main control options that do exist are potentially difficult or expensive, or both. Those options are a delayed seeding date and the use of insecticides to control the aphids.

Seeding date is the major determinant of BYD incidence in winter wheat in all of eastern Washington and probably most of the western region. Although a delayed seeding date generally reduces BYD damage, control of this disease is only one of many considerations in selecting a seeding date. Soil water availability, crop rotation, weed control methods, soil erosion potential, control practices for other diseases and many other factors, all which vary from year to year and geographically, also determine the seeding date. Farmers must weigh the potential for BYD against these other factors when choosing a seeding date. Environmental conditions often allow little flexibility.

Aphid control with insecticides has provided only limited BYD control in Wyatt's observations. Foliar insecticide applications do not prevent BYD infections but do help reduce the aphid-virus injury. Application at low aphid populations is important. Repeated treatments may be necessary for subsequent build-ups in aphid populations. Research is underway on more effective, longer-lasting foliar insecticides which currently are not labeled for this purpose.

Granular systemic insecticides applied with the seed at seeding time also help to reduce aphid populations and infection potential. However, because of the cost, the use of granular systemic and foliar insecticides is more likely to be economic near the Columbia Basin area where high populations of BYDV-transmitting aphids frequently occur or the expectation is high because of an early seeding date.

     
 

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