By Professor Fiona Burnett (SRUC) for the Farm Advisory Service

Temperatures this October are predicted to get above 20°C in some regions of the UK – pleasant for summer, but unseasonably warm for the second half of October.

While saving on heating bills and jumpers, it does add to the pest and disease risk in Scottish crops. True to form, the end of September saw a large increase in aphid flights into winter cereals, bringing with it a risk of barley yellow dwarf virus (BYDV) transmission.

Levels of BYDV in 2017 harvested crops were high and so there is a high chance of transfer to winter crops.

This virus is transmitted by several species of aphids, commonly the bird-cherry aphid and the grain aphid. Fortunately, the grain aphid – sitobion avenae – which has resistance to pyrethroid insecticides is not one of the aphids that is currently flying in big numbers.

Crops that had a seed treatment like chlothianidin (as in Deter) will have early protection against the harmful aphids which reduces the need for early aphid sprays.

It is sensible to walk crops and look for aphids on emerged crops and treatment should only be used where colonies are a found. It is worth noting that this is the last year for chlothianidin, so from next autumn spray applications might be the only option on early drilled crops. Delayed drilling will reduce the risk, but isn’t often feasible without incurring a yield penalty or, worse, missing the available drilling window.

BYDV can infect all winter cereals so wheat, barley and oats are commonly infected, as are rye and triticale. Early sown ones are more at risk, which is one reason why the disease is more commonly seen at severe levels in barley and oat crops.

The earlier the infection gets into crops, the greater the damage. Small patches of infection will not have a dramatic impact on yield over a whole field but the impact on those individual plants is high and so, where infection is common in a field, losses can be significant.

As with so many other crop diseases, any additional stress will exacerbate the symptoms and make yield losses higher – the wheat pictured shows BYDV in a light dry field and about 50% of the yield potential was lost.

Although we classically think of BYDV as patches in a field where individual aphids have got into the crop and colonies have dispersed from that central point, infection can also appear evenly over a field where aphids have arrived together in high numbers.

That was the affect seen this year in spring barley crops where individual yellow leaves were scattered throughout crops. The symptoms start with yellowing and develop into the stunted symptoms with bright yellow or red leaves.

Symptoms can be confused with other causes of leaf yellowing, such as low nitrogen, but for BYDV the fact that it is often upper leaves affected and the fact that yellow leaf tissue sits right alongside completely green tissue is indicative.

For nutritional problems, there is often a more graduated appearance to the yellow tipping and, for nitrogen, older leaves will be more affected.