Managing barley yellow dwarf virus and cereal yellow dwarf virus in cereals

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Barley yellow dwarf virus (BYDV) and cereal yellow dwarf virus (CYDV) are found damaging cereal crops worldwide. They infect wheat, barley, oats and grasses and are transmitted by aphids. These viruses are not seed-borne and persist from one growing season to the next in oversummering grasses. BYDV and CYDV infection decreases grain yield and also causes shrivelled grain. These viruses can cause serious problems in cereal crops in the southern and south-west agricultural areas (more than 500mm of rain). BYDV is often more prevalent than CYDV.

BYDV and CYDV requires living plant tissue to survive and cannot survive in stubble or in the soil. Infection in the plant is restricted to the conducting tissue (phloem). However, these viruses differ in their ease of transmission by different vector aphid species and to some extent in the severity of the damage they cause in different cereal varieties.

Virus spread

BYDV and CYDV survives the hot summer months mainly in perennial grasses that persist in roadside ditches or at the edges of creeks or rivers. However, grasses surviving in soaks or swamps and in irrigated pastures and gardens are also hosts. The four most important grass species acting as reservoirs for YDV are kikuyu grass, paspalum grass, couch grass and african lovegrass.

Perennial grasses also harbour the aphid vectors over summer. During the subsequent growing season flights of winged aphids from infected grasses then spread YDV to cereal crops. Aphids migrate into cereals mainly in autumn and spring, the extent of the autumn flight being particularly important because early infection can affect crop yields severely. Once an aphid acquires the virus after feeding from the phloem of an infected plant it will continue to transmit the virus to any plant it feeds from for its entire life.

Virus is first introduced to the crop by winged aphids flying in from the outside. These aphids create primary foci of infection. Wingless progeny of the original winged aphids spread the disease further outwards from the primary infection foci. As a result patches of virus infection develop which gradually expand as the season progresses. Patches tend to first develop on the edge of the crop and are often seen in waterlogged areas. When the level of infection is high, patches merge, resulting in infection of the entire crop.

Epidemics

The widespread epidemics are often a result of wet summers followed by mild winters. Wet summer conditions encouraged the summer growth of grasses, and this increased the carry-over of YDV and of aphids. Aphids building up on infected grasses in autumn spread the virus extensively to cereals. Also the mild winters favoured a big increase in cereal aphid populations within crops.

Aphid vectors

In Western Australia, the virus is spread mainly by the oat (Rhopalosiphum padi) and the corn leaf (Rhopalosiphum maidis) aphids. The oat aphid is the most important; it feeds on wheat, barley, oats and grasses. The corn leaf aphid usually only feeds on barley and some grasses. Other minor aphid vectors include the grain aphid (Sitobion sp.) and two cereal root aphids (Rhopalosiphum insertum and Rhopalosiphum rufiabdominalis); whereas Russian wheat aphid (Diuraphis noxia) is a poor vector.

Oat aphid vector of Barley yellow dwarf virus
Oat aphid vector of Barley yellow dwarf virus

Symptoms

Symptoms of BYDV and CYDV can be confused with those caused by nutrient deficiencies, waterlogging or other plant stresses that cause yellowing, reddening and striping of leaves. Leaf symptoms differ between wheat, oats and barley. The severity depends on the age of the plant at infection, environmental conditions, the virus present and the cereal variety involved.

Wheat

Infected wheat plants develop a slight to severe yellowing or pale striping between veins (interveinal chlorosis) in young leaves. Leaf tips can also die (necrosis). Reddening of the leaf tips (particularly of the flag leaf) can often be seen and is the most characteristic symptom of virus infection in wheat. 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.

Barley yellow dwarf virus infection of wheat
Figure 1.  Barley yellow dwarf virus infection of wheat

Barley

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. In some varieties, reddening of leaf tips also develops. 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.

Barley yellow dwarf virus infection of barley
Barley yellow dwarf virus infection of barley causes yellowing of older leaves and stunting of the plant

Oats

In oats, the symptoms of BYDV infection are very striking. Most varieties develop reddening (crimson-pink) of the leaves from the tips down, which sometimes begins as blotching especially on older leaves. Young leaves often have yellow stripes. However, some varieties only develop a yellow/orange coloration. 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 virus is greatest in early-infected plants.

Reddening of oat leaves from BYDV infection
Reddening of oat leaves from BYDV infection

Grasses

There are no obvious symptoms of virus infection in many grasses (for example, kikuyu grass). However, some grasses (for example, annual and perennial ryegrasses) may develop reddening or purpling of leaf tips while others (for example, Phalaris) may develop yellowing of older leaves.

Yield losses

BYDV and CYDV infection is most serious in cereal crops when plants become infected before tillering. In susceptible cereal varieties, where the entire crop is infected soon after sowing, yields of wheat, barley and oats can fall by up to 80%. Moreover, the amount of shrivelled grain increases, and this can affect marketable yield. If the crop is infected late, yield may be reduced by only 10-20%. Trials with wheat in WA have shown that yield losses caused by BYDV infection vary greatly between varieties depending on their tolerance or susceptibility to the virus. Yields of sensitive varieties were reduced by up to 67%. Similar losses develop in barley and oats.

Control

The best way to control BYDV and CYDV is to use appropriate management strategies.

Sowing of resistant varieties

Sowing resistant varieties is the most effective method of reducing losses. See crop variety guides for susceptibility ratings.

Insecticides

Strategic applications of insecticides can be used against the aphids that carry BYDV or CYDV to reduce its spread. It is important to protect the crop during the first 10 weeks after emergence. First pyrethroid spray: three weeks after emergence (or 2-leaf stage if aphids easily found). Second pyrethroid spray: seven weeks after emergence. In risk situations, seed dressings containing imidacloprid applied to seed before sowing are recommended for good early season control but requires a follow-up pyrethroid spray.

Delayed sowing

Delayed sowing avoids the autumn peak of cereal aphid activity and reduces the incidence of BYDV. However, delaying sowing generally reduces yields, and this loss must be balanced against the benefit of lower virus incidences.

Contact information

Brenda Coutts
+61 (0)8 9368 3266

Author

Brenda Coutts