"Yellow Wheat" - Possibly Barley
Yellow Dwarf

In a previous CropPest Ontario article (May 10, 2007 - Issue #3) the three of the most common wheat virus diseases in Ontario were discussed (Soilborne Mosaic Virus, Soilborne Spindle Streak Mosaic Virus and Barley Yellow Dwarf). Identifying virus diseases in cereals is difficult since the symptoms are often subtle and can be less dramatic then you would see in other crops such as legume crops (soybeans or dry beans) and requires specialized lab tests (serological). The University of Guelph's Pest Diagnostic Clinic has the capabilities to test for the various viruses.

Over the past few weeks, many winter wheat samples have come into the office labeled "yellow wheat - WHY?" Many of the "yellow wheat" samples were due to frost or cold temperature injury, manganese deficiency, or a combination of other factors. A number of samples
however were most likely virus and most likely barley yellow dwarf since the primary symptoms were stunting and yellowing, reddening or purpling of leaf tips. These "virus-like" samples were sent to the diagnostic lab and the majority did test positive for at least one of the many closely related viruses that make up the barley yellow dwarf group. To be more specific they tested positive for the two most common BYDV strains (subgroups) that occur in Ontario. They are BYDV- PAV and BYDV - RPV (Table 1)

Table 1. BYDV strain subgroups and preferred insect vector

How common is BYDV in Ontario? A lot more then you probably thought! That is one of the conclusions that Jocelyn Smith who recently completed her masters (University of Guelph, Ridgetown Campus) on this topic. As part of Jocelyn's thesis, winter wheat fields and fields containing volunteer wheat were sampled by her with assistance from OMAFRA Ridgetown (Albert Tenuta and Cheryl Van Herk) in order to determine the incidence and prevalence of BYDV stains in southwestern Ontario.

BYDV was detected in 20 and 29 percent of winter wheat samples even though these plants did not show any visual signs of infection (non-symptomatic) and in 10 and 25 percent of volunteer wheat samples over two years, respectively (2004 and 2005). BYDV-PAV was the most prevalent strain detected, followed by BYDV-RPV. The study supports volunteer wheat is an important reservoir of BYDV in south-western Ontario.

The PAV strain of BYDV was found in all positive samples, and the RPV strain was also detected in a small number of samples (Table 2). This level of infection is slightly higher than found by Paliwal (1982) in the late 1970's and early 1980s, where 9 to 22 percent of plants (again not showing symptoms) were infected with BYDV.

Just as Jocelyn observed, Paliwal found PAV to be dominant strain followed by MAV and RMV strains. The big difference though is Jocelyn found less variation in strain incidence, as only RPV was also detected (Tables 2-3).

The prevalence of the PAV and RPV strains in winter wheat corresponds with the dominance or prominence of the bird cherry oat aphid (R. padi) in southwestern Ontario wheat fields. The bird cherry oat aphid is the most efficient vector of the PAV and RPV strains of BYDV and these results suggest that BYDV remains a problem in Ontario winter wheat even in the presence of low cereal aphid populations.

Presently, there are few control options available for BYDV. The best strategy is still the simplest - "avoid early planting". Early planting allows the aphids more time to infect the plants in the fall. Off course an extended fall and mild winter can allow the aphids a longer feeding period. Recommended or optimum planting dates for winter wheat take into consideration BYDV and hessian fly and promote a vigorous plant to maximize winter hardiness.

Chemical sprays to control the aphid vectors are not practical or economical since scouting or detecting the aphids is very difficult. By the time populations reached detectable levels, virus transmission has most likely already occurred. Preventative sprays would not be economic as BYDV is unpredictable. There maybe more help in the future with potential BYDV resistant varieties.

So if you are looking at yellow stunted wheat, do not rule out barley yellow dwarf virus. For more information about virus testing or other problem samples, call the Pest Diagnostic Clinic at (519) 767-6256 to receive a sample submission form and fee schedule.

Table 2. Incidence and strains of BYDV detected by DAS-ELISA in winter wheat in 2004

Table 3. Incidence and strains of BYDV detected by DAS-ELISA in winter wheat in 2005

Table 4. Incidence of BYDV-PAV detected by DAS-ELISA in volunteer wheat field samples in 2003 and 2004.

Figure 1 - Field displaying typically barley yellow dwarf symptoms which include yellow and stunted plants. (Tenuta, OMAFRA)

Figure 2 - It is typical to see various degrees of stunting due to BYDV infection. The left plant is showing a greater degree of stunting then the plant on the right but both are stunted compared to the health parts of the field (Photo - APS Press).

Figure 3 - Yellowing and browning of leaf tips often first sign of BYDV infection (Montana State University)

Figure 4 - Reddening or purpling of leaf tips is common for BYDV (Tenuta, OMAFRA).

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