Oat (right) and Wheat (left) infected with Barley Yellow Dwarf Virus, BYDV.

Plant infected with BYDV. Notice the yellowing and necrosis starting at the leaf tips.

Wheat infected with BYDV. Leaf tips begin to yellow and progress down the leaf blade.

Barley yellow dwarf virus (BYDV) is one of the most widespread and destructive diseases of small grains worldwide and is the most economically important virus disease of wheat in some parts of the U.S. In Utah, the disease is most important in winter wheat and barley but can cause minor yield losses in spring grains, including oats. The virus infects more than 100 species of cereals and grassy weeds although many of these hosts, including corn, remain symptomless. The virus does not attack broadleaf plants.

The amount of damage caused by BYDV varies, depending on the cultivar of grain, strain of the virus, time of infection, and environmental conditions. However, yield reductions of 5-25% are common, and severely infected crops may yield no grain. Infection of seedlings is the most damaging. In Utah, outbreaks have occurred in which 80% of the plants were infected. However, epidemics are sporadic depending on the weather, and the disease seldom causes these losses.

Symptoms

Barley yellow dwarf infections are difficult to diagnose, especially in wheat, because the symptoms are often vague and easily confused with symptoms of other maladies such as nutrient deficiencies. The presence of numerous aphids in the fall, followed by characteristic symptoms in the spring, is an indication of BYDV, but more stringent tests are necessary to confirm virus infection. Generalized symptoms include leaf discoloration progressing from the tip to the base and inward from margins, usually yellow but often becoming reddish or purple, especially in barley and oats. In barley, the discoloration is typically blotchy and uneven. Infected plants tend to have less flexible leaves and poorly- developed root systems.

Early infections may be lethal, but otherwise result in stunting with reduction in the number of tillers and heads in barley. Older leaves turn bright yellow. Plants infected in later stages of growth will have only the flag leaves discolored (especially noticeable in oats), and some cultivars are stunted but show no discoloration.

The Vector

More than 20 species of aphids can vector this virus. After a single feeding on an infected plant, the aphid is able to transmit the virus for several weeks through successive molts. However, the virus is not transmitted to the aphid's offspring. Disease symptoms begin to develop one to four weeks after infection.

Disease Cycle

All host plants, including many grassy weeds, serve as reservoirs of BYDV where the virus persists during periods of low crop and aphid activity. As aphids become active in fall and spring, they transmit the virus. Therefore, spring grains are severely affected only in years when high populations of aphids are able to overwinter. In Utah, autumn infection of winter grain is more prevalent and is likely to produce significant disease. Since the aphids may be borne long distances by wind, the disease can spread rapidly. BYDV epidemics are most prevalent in cool (50 - 65F), moist weather which favors both crop growth and aphid activity.

New infections commonly occur in autumn, even though spring grains have been harvested, because volunteer grain, weeds, and nearby corn fields harbor the virus until aphid activity peaks again in fall. These autumn infections of winter grain are most damaging.

Disease Control

None of the cultivars of wheat, barley, or oats that are well-adapted to Utah conditions are resistant to BYDV. The best available control is to time the planting of winter grains so that the seedling stage does not coincide with high aphid activity. This usually means delaying fall planting as long as possible. Also, plant so that the seedling stage will not overlap with late growth stages of other host crops, especially corn, as infected aphids will migrate to the new crop. Destroying weeds and volunteer grains before planting will eliminate reservoirs of the virus.

Spraying for aphids has limited value and is seldom economical because infected aphids easily migrate from other areas. However, insecticides may be warranted if aphid populations are very high and the planting date cannot be postponed. The use of granular systemic insecticides, such as Di-Syston or Thimet, applied at planting time will reduce aphid numbers. However, yield responses to insecticide applications may vary and insecticide use may not be justified. Thimet or Di-Syston should be applied at planting, using equipment designed for granular formulations. Apply up to 1.6 oz. of either material per 1000 ft. of row, on wheat, or Di-Syston only for barley; do not exceed 1 lb ai/acre for either material.

Symptom severity and yield losses can be reduced by maintaining adequate nitrogen fertility and insuring that the infected crop is not stressed for moisture.