Barley Yellow Dwarf Virus in Cereals
Significance and Control
Authors: M.A.W Stufkens & D.A.J Teulon
Crop & Food Research
Source: Crop & Food Research, Lincoln (March 1998)
 

Barley yellow dwarf virus (BYDV) can cause losses in susceptible cereal cultivars. In the early 1960's it was realised that BYDV was the most important virus disease affecting cereals in NZ, and in some years losses of up to a fifth of the autumn-sown wheat crop occurred. Since then new wheat cultivars have become available. It is possible that some have been selected with some degree of resistance to this virus.
Recent outbreaks were recorded in 1985, 1987 and 1990 soon after the initial discovery of the rose grain aphid in New Zealand.  

The Virus
BYDV affects cereals and grasses, with about 100 species known to be susceptible. Many variants or strains of the virus exist that differ in potency, severity of symptoms, and the species of aphid which can spread the strains. The virus cannot be transmitted through the seed and is spread only by aphids. BYDV is widespread throughout NZ.  

Losses caused by this disease can be in the range 5-20% loss of yield. The losses will depend on the cultivar used, on the time of sowing, and hence on the number of plants likely to be infected.  

Symptoms
These vary with the crop cultivar, age of plant at time of infection, strain of virus, the number of aphids present, and environmental conditions. In general, this virus can cause stunting of plants, inhibition of root formation, delay or prevention of heading and reduction in yield. BYDV often induces colour changes in the leaves of cereal plants. Symptoms are expressed in crops as follows:  

Oats: BYDV causes leaves to turn reddish or purple, sometimes with yellowing and stiffening of the whole plant. Seed heads are often badly blasted. Symptoms are common in oats sown in early autumn for greenfeed.  

Wheat and Ryecorn: BYDV causes leaf yellowing, sometimes a little reddening, and often the leaf edge has a cut symptom. In infected areas of wheat crops the ears stay erect, become black and discoloured during ripening.  

Barley: BYDV causes a bright-yellow discolouration beginning at the leaf tip and margins which move rapidly down the whole leaf. The yellowing may be confused with the chlorosis due to nitrogen deficiency, but with BYDV, infected plants may be scattered through the crops, whereas nitrogen deficiency is generally more uniform.  

BYDV may also be present in cereals without external BYDV symptoms.

Aphid Carriers
There are three main aphids; the cereal aphid CA (Rhopalosiphum padi), the rose grain aphid RGA (Metopalophium dirhodum), and the grain aphid BCA (Sitobion sp.). Two less common carriers are the apple-grain aphid AGA (Rhopalosiphum insertum) and the corn leaf aphid CLA (Rhopalosiphum maidis). The virus is carried in a persistent (circulative) manner which means that once the aphid has acquired the virus it will remain infective for the rest of its life. Field infection is only by aphid transmission by winged or wingless aphids.  

CA, BCA and AGA live on grasses as well, where it has been shown that between 6-55% of the pasture can contain BYDV. CA and AGA live low down in the crop both on the leaves and stems, CLA lives in the newly opening leaves, RGA is found on all parts of the plant except the heads and BCA is found on the leaves and heads.  

The winged aphids carry this virus from pasture grasses and volunteer cereals into cereal crops, and also spread it within the crop as they move from plant to plant. In NZ there are two main flight periods of the cereal aphid. These are in the spring (September-November) and the autumn (March-mid June). Cereals are sown in autumn or spring according to district, crop species and cultivar. Unfortunately for cereal growers there is a risk that early autumn sown crops and any sown in spring may emerge during aphid flights and become infected because of primary virus spread.

[View Figure 1] In NZ, wingless aphids are present all the year around on grasses (especially those in waste places), and these aphids are reproducing continuously (though more slowly in winter). During spring and autumn, winged aphids develop and fly into crops, where they initiate not only wingless aphid populations in cereal crops but also spread in BYDV.

With autumn sown cereals there is a period of primary spread when aphids carrying virus fly into a crop in the autumn (Fig 2). A few aphids survive the winter and as temperatures rise in the spring the number of wingless aphids increase and spread BYDV through the crop (secondary spread).  

If a cereal grower can postpone sowing till late May or early June the crop will emerge after autumn aphid flights are over. A crop sown at this time is still in danger of infection with BYDV in the spring, but because older plants are less severely affected by BYDV the losses will be minimised. However, there are some areas in NZ where the cereal growers may have to sow in early autumn, because of the possibility of wet winters, and therefore they have to be prepared to treat in early August to destroy the over-wintering wingless populations of the cereal aphid and so reduce the secondary spread of BYDV.

[View Figure 2] This graph shows that flights of winged cereal aphids can cause primary spread of virus into early autumn-sown wheat crops. Virus spread within crops occurs as a result of an increase in numbers of wingless aphids in wheat during August- September. Control strategies are either to sow late to avoid aphids and virus or to use insecticides on early sown crops, applied either at sowing or in early August to prevent wingless aphids increasing in numbers, and spreading BYDV.

Control
The two general methods for control of the cereal aphid and BYDV are to sow late in the autumn to avoid aphid flights, and treating to kill the aphids. More detailed alternative control strategies for reducing the effects of BYDV in autumn sown wheat are as follows:  

• For April-May sowings, apply either: Systemic insecticide granules at sowing to protect the crop until the aphid flights have ceased. This single treatment will remain effective for 3 weeks after plant emergence. or: systemic insecticide sprays in early August to prevent the number of wingless aphids increasing, thus stopping the secondary virus spread. or: systemic insecticide granules may also be topdressed in August and this would have the same result.  

• Wheat sown in early June or later will emerge after the aphid flights have ceased, and avoid BYDV.  

• Resistant varieties are being developed, but total resistance is not available.  

• Insecticide seed treatment has shown some yield increase. For BYDV control in spring sown cereals, some trials have shown that systemic insecticide granules applied at sowing, followed by the topdressing of further granules or spraying after 5-6 weeks gave adequate protection. It is thus not possible to make firm recommendations for control in spring cereals, though in these crops BYDV appears to be much less of a problem.