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Ravensdown agronomist, Chris Lowe, has been looking for the cause and found it in the aphids that transport the beet western yellow virus (BWYV).
"I have lost count of the number of enquires I have had recently about these symptoms; most people put it down to nutrient deficiencies," says Lowe.
A common response, he says, is to apply a large amount of nitrogen and potassium fertiliser.
"Fact is the beet below is not suffering from a nutrient deficiency at all. We tissue test many beet paddocks for nutrients through our research laboratory and most come back fine."
Lowe says that since seeing signs of infection last year Ravensdown has asked AgResearch for help.
The cause of many yellowing paddocks is now confirmed as BWYV. This is transmitted by aphids, the biggest culprit being the green peach aphid (Myzus persicae).
Green peach aphid are often found on the underside of the leaves where they lays eggs in the winter. In spring they hatch and produce stem mothers that give birth to female aphids. These then fly to the summer hosts, like fodder beet, where they suck the plant sap and transmit the virus.
"The virus persists in the aphids for more than 50 days, while the aphid only needs to feed from the plant for five minutes to transmit the disease to the plant."
BWYV is hard to control because of its wide host range and persistent aphid transmission, the minimum inoculation feeding period being 10 minutes.
Lowe says foliar insecticides can help but they have to be timed right.
"This year I was finding aphids underneath leaves about the middle of November. A good indication of whether your beet crop has aphids is the presence of ladybirds as they are a natural predator and where possible I would advise using an insecticide that does not kill your friends."
Globally the resistance to insecticides is huge, Ravensdown says. The best defence against it is correct crop rotation and sowing insecticide treated seed.
Fodder beet seed is generally coated to allow precision sowing.
The company's coating Combi Coat contains a fungicide and insecticide to reduce risks of virus infection.
Lowe urges taking all practicable means to reduce the risk of aphids infecting the fodder beet plants with BYDV.
"Starting with the correct paddock choice, with no-host species prior to fodder beet, and using insecticide treated seed will put the odds in your favour."
Best defences
Insecticide seed treatments e.g. imidacloprid or thiamethoxam + tefluthrin
Later drilling to avoid aphid flights
Avoiding host plant cover crops e.g. beet, radish, peas, potatoes and wild/weed crucifers like mustard, between drillings
Adequate rotations (best defence).
