Barley yellow mosaic virus and Barley mild mosaic virus are pathogenic viruses which cause barley yellow mosaic disease of winter barley. Symptoms include yellow and mosaic flecking of young leaves often appearing in winter and early spring, where plant leaves start to show pale green and yellow colours. Over time the changes become more severe as the virus spreads throughout the plant. These changes can take the form of curled leaves and chocolate-brown necrosis associated with the resulting lesions. Barley yellow mosaic disease can heavily impact yield with up to 50% crop losses.

These viruses can be contracted soon after sowing, through Polymyxa graminis within soil infecting the plant root. This can cause this virus to be systemic and far reaching, as well as being difficult to control and prevent.

The barley yellow mosaic disease can cause a significant impact on the expected yield of your crop as affected plants usually have fewer tillers and irregular grain size. However, symptoms are not always obvious and sometimes the crop can undergo an apparent recovery. However this symptom recovery doesn't necessarily lead to a recovery in yield.

The symptoms can often be mistaken for nutrient deficiencies so understanding what is happening in the crop can lead to more informed decision making.