Barley yellow dwarf virus (also called cereal yellow dwarf) is caused by a group of closely related strains of a Luteovirus. It is probably the most widely distributed and most destructive virus disease of cereals. Yield loss is greatest from infection early in the growing season which can be up to 80 percent in severe cases where a susceptible variety has become infected soon after sowing.
Symptoms of BYDV can be confused with those caused by nutrient deficiencies, waterlogging or other plant stresses that cause yellowing and striping of leaves.
Besides changing colour infected plants are stunted or dwarfed. In severe infections, heads may fail to emerge or fewer tillers may develop and sterility is not uncommon. Leaf symptoms differ between wheat, barley and oats.
Infected wheat plants develop a slight to severe yellowing or pale striping between veins (interveinal chlorosis) in young leaves, (Figure 10.17). Leaf tips can also die. If a sensitive variety is infected before tillering the plant is usually stunted, has fewer tillers and more sterile ones. Grain matures early, yield is greatly reduced and grain is shrivelled. Effects are milder with a late infection.
Figure 10.17 Yellow leaf symptoms of BYDV in wheat
In barley, infection causes a characteristic bright yellowing of the leaves (particularly older leaves) and pale yellow stripes between the leaf veins plus chlorotic blotching of young leaves, (Figure 10.18). Early infected barley plants are stunted and produce low grain yields and shrivelled grain. Tillering can be stimulated by infection, but most tillers then develop poorly and produce sterile heads. Plants infected after tillering have milder symptoms and yields are less severely affected.
Figures 10.18 Yellow leaf symptoms of BYDV in barley
In oats, the symptoms of BYDV infection are very striking. Most varieties develop reddening (crimson- pink or purple colour) of the young leaves from the tips down, which sometimes begins as blotching especially on older leaves, (Figure 10.19). Stunting, an increase in sterile tillers or abortion of florets result in low grain yields and shrivelled grain. As in wheat and barley the effect of BYDV is greatest in early-infected plants.
Figure 10.19 Red leaf symptoms of BYDV in oats
The disease occurs on most cereals and numerous grasses but is not known to effect dicotyledonous plants. Infection in the plant is restricted to the conducting tissue (phloem).
BYDV is not transmissible through seed, soil, sap or by insects other than aphids. Aphids migrate into cereal crops in autumn and spring. The extent of any autumn migration is important as early infection of BYDV in crops can affect crop yields severely. Aphids need to feed on an infected plant for at least 5 minutes followed by a latent period of 12 hours, before the virus will transmit to a healthy plant. Aphids remain infected for the rest of their life. Six distinct strains of BYDV have been identified, but in Australia the strain PAV is the most common.
There are no obvious symptoms of BYDV infection in many grasses (e.g. kikuyu grass). However, some grasses (e.g. annual and perennial ryegrasses) may develop reddening or purpling of leaf tips while others (for example phalaris) may develop yellowing of older leaves.
WHEAT STREAK MOSAIC VIRUS (WSMV)