Symptoms: Symptoms range from mild-green to prominent-yellow leaf mosaics. The unfolding new leaves show mottles and streaks. Stunting can vary from moderate to severe and certain strains of the virus can cause rosetting of plants. Generally, as the young leaves unfold, they appear mottled and develop parallel streaks. The symptoms, which are more pronounced in low-lying areas because these areas favor the fungal vector, are most prominent in early spring growth and rarely appear in the autumn.
Development: As temperatures rise in the spring, disease development slows and eventually stops. When this happens, symptoms will often be confined to the lower leaves. Temperatures of 15°C (range 10-20°C) promote SBWMV. At temperatures above 20°C, the disease's progression is stopped.
Vectors/Hosts/Distribution: SBWMV can be sap-transmitted, but the natural vector is Polymyxa graminis, a soilborne fungus. Normally this virus is only a problem in autumn-sown wheats; however, it may also infect rye, barley, and hairy bromegrass (Bromus spp.). Spring wheats are susceptible, but very seldom show symptoms of SBWMV infection in nature. SBWMV has been recorded in the eastern and central USA, Japan, Egypt, Italy, Argentina, and Brazil.
Importance: The virus is most prevalent in the United States, where entire fields or areas of a field may be so badly infected they do not warrant harvesting. In some years, the damage this virus causes probably rivals that caused by barley yellow dwarf virus (BYDV).
