Barley yellow dwarf (BYD) disease occurs in most grain growing regions of the U.S. where barley, maize, oats, rye, and wheat are cultivated. This disease is the most widely distributed and economically most important virus disease of wheat. Yield losses up to 20% have been reported. The name, barley yellow dwarf, describes the typical symptoms of an infected barley plant. However, since the BYD viruses can infect wheat, oats and over 100 annual and perennial grasses causing the same disease, the same name is used to describe the disease regardless of the host affected. On oats "red leaf" or "grey spot" are sometimes used to describe the symptoms of BYD. The disease is caused by at least eight viral species that belongs to the Luteoviridae virus family, and it is transmitted by aphids.
Symptoms of BYD
Early symptoms of BYD are frequently overlooked because infected plants resemble those with mineral deficiencies, low temperature damage, root rots, herbicide injury, or other problems. Grain growers often fail to associate early aphid infestations with the incidence of BYD because symptoms usually do not appear until after aphids are gone.
Symptoms may develop two or three weeks after aphid feeding. In Ohio, it is rare to see entire fields affected, but most frequently BYD occurs on scattered plants or in circular patches within fields (Fig. 1). Distribution of diseased plants depends on the flight of winged aphids into the field.
|Fig. 1. Wheat field infected with barley yellow dwarf virus. Characteristic yellowing patches are visible in the middle.|
|Fig. 2. Yellowing and reddening of wheat leaf tips due to barley yellow dwarf.|
Leaf symptom development is dependent on the time of infection. Seedling infections slow plant growth, but rarely kill the plant. Symptoms generally appear first on the older leaves of young plants as faint yellowish-green blotches near the leaf tip. The blotches enlarge rapidly and merge, changing to shades of red to purple in oats, yellow to red in wheat and bright yellow in barley (Fig. 2). The leaves may lose their flexuous appearance and become erect or stiff. The leaf margin near the tip may roll inward and become necrotic forming a stiff point. Plants infected early (in the fall) are usually dwarfed and yield much less than plants infected at a later stage of development. Early fall infection frequently predisposes plants to winter killing even if they are symptomless. Older plants may develop symptoms only on the leaves actively growing at the time of infections. Infections that occur in the spring may result in discoloration of the flag leaf. In barley, the most characteristic symptoms are dwarfing and the brilliant yellow coloring of the leaves.
Severe dwarfing and yellowing are less common in wheat. Symptom development is enhanced by bright sunlight and cool temperatures between 60 and 68°F (16 to 20°C).
Reddish-purple or yellowish discoloration of the flag leaf is the most characteristic symptoms of BYD, but visual symptoms alone are not sufficient to properly identify the disease. For an accurate identification, samples of symptomatic leaves should be tested in a laboratory.
|Fig. 3. Severe aphid infestation on wheat plant.|
Aphid Vectors of BYDV
The occurrence of BYD in a field is totally dependent on the activity of its aphid vectors. Over 20 species of aphids are known to transmit BYDV, of which only four are common: the oat bird-cherry aphid (Rhopalosiphum padi), the corn leaf aphid (Rhopalosiphum maidis), the English grain aphid (Macrosiphum avenae), and the greenbug (Schizaphis graminum). Most of the viruses are transmitted by only a few aphids; in other words, there is considerable vector specificity. BYD is no exception, an outbreak is heavily dependent on which species of aphids are present in the field, the source and strain of the virus, efficiency of the aphid at transmitting the virus, aphid mobility, aphid feeding habits, the plant age and susceptibility level when infected, and various climatic factors. For example, the aphid species responsible for the disease outbreak may not necessarily be the most abundant one. One very active aphid feeding for short periods on many plants is a much more important vector than 100 stationary aphids.
Some aphids, such as the green bug (Fig. 3), cause damage to plants by injecting phytotoxic secretions into the plant during feeding. These toxic substances produce a pattern of tiny spots on the leaves or stems where the aphids have fed. The very small spots become brown to black and adjacent tissues turn tan-brown after a period of yellowing. Before an aphid is capable of inoculating a healthy plant, it must acquire the virus by feeding on infected plants for a period of 12 to 30 hours (sometimes as short as 30 minutes). Once the aphid acquires the virus, it is capable of transmitting it for the rest of its life. It generally takes 4 hours or more of feeding on a plant for the virus to be transmitted.
The barley yellow dwarf virus survives from one crop to the next in volunteer wheat, oats, barley, perennial and annual grasses, and in its aphid vectors. In the fall, emerging seedlings can be inoculated by aphids that acquired the virus from infected volunteer cereal or grass hosts. Early spring infections are caused by aphids overwintering as adults on grasses or winter cereals. During the growing season each adult aphid may produce from 10 to 20 young each day. These young aphids must first acquire the virus from infected plants. These aphids, normally wingless, produce more aphids and move only short distances by crawling from plant to plant or by being blown in the wind. Barley yellow dwarf epidemics occur when the weather conditions favor the multiplication of the aphid vectors.
Cool (50 to 65 degrees F or 10 to 18 degrees C) and moist weather is most favorable. Aphid movement can be local, from one field to another, or when assisted by winds, aphids can be carried hundreds of miles. Generally, aphids migrate from the southern states to the northern states in spring and from the north to the south in fall. BYD infections can occur throughout the growing season, but are most damaging in spring in areas where aphids overwinter. The earlier the plant is infected, the greater the yield loss. Inoculated plants become systemically infected and develop symptoms in two weeks at 68 degrees F, in four weeks at 77 degrees F, but no symptoms develop at 86 degrees F (30 degrees C) or above. Barley yellow dwarf virus is not transmitted by seed, soil or by rubbing infected leaves onto healthy leaves.
Barley yellow dwarf cannot be adequately controlled in the field at present. The following practices should greatly reduce the incidence and damage to these cereal crops.
- A few BYD resistant varieties are available, but these are resistant to only a few of the BYD virus species or isolates within a species. It is therefore necessary to know which species of the virus are predominant and the species to which each variety is resistant in order to make the correct variety selection for each area. Consult with your seed dealer for varieties with resistance to BYD.
- Plant winter wheat after the Hessian fly-free date and barley as late as practical to avoid early fall infections. Fresh green leaves of early autumn plantings attract aphids and may lead to severe infestations. Later planting helps winter cereals escape aphid population buildup in the fall. Plant spring oats as early as possible. Vigorously growing plants are more tolerant of BYD than are weaker ones. Large populations of aphids usually do not appear in Ohio until later in the spring.
- Proper fertilization is necessary for good crop growth. Plants with nutritional stress are more susceptible and yield less.
- Control volunteer wheat, barley and oats. Volunteer plants can be a problem in double-cropping systems. These plants can serve as important reservoirs of the virus for the next crop.
- Contact insecticides act directly on aphids, but have short residual effect and may not last long enough to protect the field from a subsequent influx of virus-carrying aphids. Some systemic insecticides have been used successfully in some states where severe BYD epidemics occur. In these areas, aphids typically fly in from virus infected grain crops. Systemic insecticides require that the aphids feed on the plant and feeding may be long enough to inoculate the plant with virus, however it reduces the chances of spreading the disease by killing the insect before they can inoculate other plants.
- Cereal Disease Encyclopedia, http://cereals.ahdb.org.uk/cereal-disease-encyclopedia.aspx
- Barley Yellow Dwarf Virus, South Dakota State University, http://www.sdstate.edu/ps/research/plant-pathology/virology/bydv/
- Barley Yellow Dwarf Virus, Oklahoma State University, http://www.youtube.com/watch?v=J8EUD-B9VfU