Description
Barley mild mosaic virus is one of two agents (the other is barley yellow mosaic virus) responsible for yellow mosaic disease of barley. Both viruses are transmitted in soil by the plasmodiophorid fungus Polymyxa graminis. The viruses occur, either separately or together, in autumn-sown barley and cause similar symptoms. Affected fields show yellow patches or appear entirely yellow, depending on the distribution of infested soil. The initial symptoms are irregularly distributed chlorotic streaks of varying size on the youngest emerging leaves, often associated with upward rolling of leaf margins. The streaks are most distinct on the youngest leaves, and develop into a mosaic pattern. The mosaic symptoms are occasionally associated with necrotic patches, yellow discoloration, and/or rapid death of the older leaves. The symptoms on leaves appear in early spring but tend to disappear as the weather becomes warmer. At temperatures above 20°C, new leaves are free from symptoms. The growth of infected plants is greatly diminished by temperatures in the range 5-10°C, especially when this happens under high soil moisture conditions. When such weather lasts until April, infected plants remain stunted and, although the dwarfed plants respond quickly as spring temperatures increase, yield can be lessened by up to 80%