Course: Cereal diseases | Last Updates: 27th April 2017
What is barley yellow dwarf virus (BYDV)?
Barley yellow dwarf virus is a viral disease, transmitted by aphids, which infects all cereal crops, including barley and wheat, and many grasses.
The term collectively describes several different yellow dwarf luteoviruses, three of which are important in the UK (see table). Grain and bird-cherry aphids spread BYDV.
The viruses pass through the aphid’s gut into its body. The virus particles then accumulate in the salivary gland ready to be injected into the next plants the aphid feeds on. This process takes several hours.
Crop-to-crop transmission is caused by winged aphids migrating from infected source plants. Perennial grasses maintain a reservoir of infection. These winged aphids cause initial foci of infection and may deposit wingless aphids on infected plants.
Within-crop transmission then follows as the wingless aphids reproduce and their progeny spread out to adjoining plants, causing a patch of infection.
The earlier a plant is infected, the more severe the symptoms that develop. Plants infested at the seedling stage may die. Those infected before the start of stem extension are likely to be stunted with discoloured leaves, yellow in barley, yellow to red in wheat and dark red in oats.
Where infection occurs after the start of stem extension, only those leaves produced after the plant is infected become discoloured.
Control of autumn infection can result in yield increases of around 2.5t/ha and control of infections over winter of around 1t/ha (see graphic). Yield losses in barley are generally more severe than in wheat.
|MAIN BYDV STRAINS IN UK|
|Virus||Aphid vector||Main problem In:|
|BYDV-PAV||Bird-cherry aphid||Warm autumn after mild winter (SW)|
|BYDV-MAV||Grain aphid||Warm autumn and/or mild winter|
|CYDV-RPV||Bird-cherry aphid||Green bridge|
Factors affecting control
Green bridge infection may occur when aphids transfer to new crop from ploughed-down grasses and volunteer cereals. In such cases symptoms may be very severe and difficult to control as the aphids can feed on the roots of the new crop below the effective zone of seed treatment and sprays.
A gap of five weeks between ploughing and sowing or the use of a herbicide to kill the previous crop residues is the best way to prevent infection through this route.
The earlier a crop is drilled, the greater the risk, because crops will be exposed to autumn aphid migration for longer and it allows more time for aphids to reproduce on the crop in warmer autumn weather.
The number of aphids flying is related to autumn grass growth – the engine producing winged aphids –and the suitability of the weather for flight. The number of bird-cherry aphids settling on a cereal crop is also related to the mildness of the previous winter.
Cold winters favour strains that return to bird-cherry trees to over winter as eggs. Temperature governs aphids’ reproduction speed and growth. It takes several weeks for the first generation of wingless aphids on the crop to mature and start to reproduce.
It is at this stage some movement between plants and within crop spread of BYDV starts, a process that accelerates rapidly when the second generation starts to reproduce.
The amount of virus spread and yield loss is then related to the thermal time between the start of this rapid transfer stage and the crop reaching GS31. BYDV infection after the start of stem extension has little impact on yield.
Typical control strategies
The two available strategies for control are to use a seed treatment to protect vulnerable seedlings or wait and use timed insecticide sprays. The most effective seed treatments will control aphid vectors for 6-8 weeks. This takes the crop through the most vulnerable seedling stages.
A follow-up spray may occasionally be required in exceptional cases, such as early-drilled crops in coastal areas, where there could still be some aphid movement in late autumn and a slow build-up of aphids through winter.
A T-Sum system has been developed to decide when to treat crops. Temperatures are accumulated from either crop emergence, the end of seed treatment protection, or the date of a previous spray.
Mean temperatures (minus 3C) from each day are added up. A T-Sum of 170 coincides with the potential start of the second generation of wingless aphids. If this stage is reached in the autumn an insecticide should be applied with the next herbicide application to the crop.
If no such opportunity has been found by the time the T-Sum reaches 340 a separate application will be an urgent priority as a rapid increase in BYDV spread can be expected after this point. Bird-cherry aphids are more vulnerable to frost than grain aphids.
However, grain aphids are likely to occur everywhere so in fairly mild winters they will survive and will continue to spread BYDV. Sprays may then still be worthwhile up to the start of stem extension.
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