Course: Oilseed rape diseases | Last Updates: 12th October 2015
Turnip yellows virus (TuYV) is a crop pathogen that could be robbing you of up to 30% of your oilseed rape yield.
The damaging virus may be the most important, yet least understood, viral disease of Britain’s number one break crop. Managing the disease requires an understanding of the aphids that carry it, as well as the disease itself.
What is Turnip yellows virus?
TuYV was originally identified in the USA in the late 1950s and at that time it was, rather confusingly, named beet western yellows virus (BWYV). As well as oilseed rape, it infects a wide range of hosts, ensuring its survival and spread throughout the year.
It is carried and transmitted by about 20 aphid species, but by far the most important is the peach-potato aphid (Myzus persicae). Winged aphids can bring it into a crop from emergence until, potentially, Christmas depending on the weather during the autumn. The risk is usually highest during September and October when the largest numbers of aphids are on the move.
HGCA-funded monitoring work carried out by Rothamsted Research has shown a large proportion of peach-potato aphids can carry the virus – up to 72%. A more usual figure for similar crop viruses is 1%. In a mild year where numbers of aphids are high, this means a high primary infection of crops, when winged aphids feed for the first time on plants after migrating into the crop. But there’s a double-whammy, as secondary spread is also high because aphid progeny then spread it to neighbouring plants.
TuYV is a persistent virus that must be passed through the digestive system and then into the salivary glands of the vector before it can infect another plant. This means a delay of between one and four days from the time aphids pick up the virus. But once they have it, they retain the ability to transmit the virus and can infect many plants.
What is its effect on the crop?
For an oilseed rape crop, becoming infected with TuYV is a bit like getting a bad bout of flu – productivity in almost every part of the plant is hampered. It affects crop height and photosynthesis. There is a build-up of starch in leaves and the plant produces fewer pods, although seeds tend to grow larger. It can significantly raise glucosinolate levels of the oil, affecting processing quality.
How do you identify it?
The problem with TuYV is that you don’t usually see the symptoms until spring. Early symptoms of TuYV are expressed by intense purpling of the leaves; later symptoms of interveinal yellowing and reddening of leaf margins are not usually expressed before stem extension and can easily be confused with other stress symptoms and nutritional deficiencies, which is why the effect of the virus is underestimated.
Discolouration makes its way through the canopy and even the pods can show symptoms just prior to crop senescence.
Typical early symptoms of turnip yellows virus on an oilseed rape leaf
Late symptoms of turnip yellows virus on an oilseed rape leaf.
The map below shows the highest-risk areas for TuYV infection across the UK, based on work carried out by farming company Velcourt in the season of 2009-10 that monitored aphid numbers between September and the end of November and looked at the amount of virus carried into crops. Overall, 29% of aphids trapped in water traps on 13 Velcourt farms in autumn 2009 were carrying TuYV.
Distribution of TuYV spring 2010 (based on 41 locations).
Why is it a problem?
It is difficult to know whether TuYV is becoming more of a problem or whether recent research has raised awareness of the virus. But recent autumns have favoured the migration of virus-carrying aphids, and subsequent mild winters encouraged the overwinter survival of them and their progeny, which all increase the risk.
TuYV is likely to become more of an issue for growers since the implementation of a restriction on neonicotinoid insecticide seed treatments, formally the main means of controlling the aphids that spread TuYV. From 1 December 2013, seed treated with clothianidin (such as Modesto), imidacloprid (such as Chinook) or thiamethoxam (such as Cruiser OSR) cannot be planted.
This will have a major impact on growers this autumn. To gain an idea of the impact, in 2012, 70% of OSR was treated with these actives. The treatments were used in winter and spring OSR crops to protect them during the first six to eight weeks of growth from peach–potato aphids, which transmit TuYV, as well as cabbage stem flea beetles.
TuYV affects approximately 60% of the OSR area in the UK. HGCA-funded research has shown the average yield loss in untreated crops is 15%, although yield losses of up to 30% can occur. Without insecticides, the calculated annual tonnage lost from TuYV could be 207,283t, costing the industry approximately £69m/year (9% of the total crop value).
How do you assess risk?
Early sown crops are more vulnerable as they are exposed to potential aphid colonisation for a longer period. A warm autumn will encourage aphid flight, population development and movement within the crop.
AHDB Aphid News is a weekly email alert on regional aphid migration. The monitoring, which is led by Rothamsted Research, combines information on migrating winged aphids collected from regional suction traps alongside aphids caught in yellow water pan traps. Together, this information can help identify peak migrations of aphids flying into crops, identifying the most effective time to apply an insecticide if necessary.
During the summer predators, such as ladybirds, and parasites can often be relied on to keep aphid populations in check. But they are generally slower to respond to changes in the aphid population during the autumn as temperatures decline and some predators start to hibernate for the winter.
How are the aphids best controlled?
Foliar applied insecticide pymetrozine (such as Plenum) is the only viable alternative to which the peach–potato aphid has not developed resistance. Pymetrozine can be applied once in the autumn and provides protection for approximately two weeks. Application timing is therefore crucial for effective control and AHDB Aphid News can help identify when to spray.
Some pyrethroid insecticides are also approved alternatives; however, HGCA-funded research shows that resistance to this chemical group is widespread in UK peach-potato aphid populations, so control from a pyrethroid is highly unlikely to be effective.
There are three different mechanisms of insecticide resistance in peach-potato aphid. The mechanism of resistance to pyrethroids is knockdown-resistance (kdr). Two forms exist: kdr, and a more potent variant super-kdr. Kdr is currently rare; however, super-kdr is common and widespread. In 2013, all the field-sampled peach-potato aphids tested were resistant. The majority of peach-potato aphids in the UK at present carry super-kdr to pyrethroids and modified acetylcholinesterase (MACE) resistance to pirimicarb.
The mechanism of resistance to organophosphate insecticides is via elevated carboxylesterase in the insects; however, resistance has fallen substantially following a decline in the use of organophosphates against aphids.
The mechanism of resistance to carbamates is due to MACE resistance. This is common and widespread throughout the UK and in 2013 all peach–potato aphids tested were resistant.
Neonicotinoid resistance has also been discovered in southern mainland Europe, but as yet, has not been detected in the UK.
Varietal tolerance to TuYV
HGCA-funded research has identified that there are significant differences in the tolerance of TuYV in different varieties. This area of research is undergoing further investigation.
However, new oilseed rape variety Amelie is reported to have TuYV resistance. The variety completed National List trials in 2013 and will be considered for entry into HGCA Recommended List trials in 2014.
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