8 March 2002

Red alert for virus yellows

SUGAR beet is forecast to come under the worst virus yellows attack for 28 years this summer.

Yields of unprotected crops could be slashed and even beet from seed treated with Gaucho (imidacloprid) may need an aphicide to extend protection, warns IACR-Brooms Barn.

"You will need to look to see if aphids are colonising crops," says senior entomologist Alan Dewar.

In the east, the region most at risk to the aphid-borne disease, the forecast is that crops sown at the end of March will suffer 78% infection if untreated. "If you are in this area and you have not got Gaucho-treated seed then I would try to get some," he advises.

Figures for the north are even higher but for both regions Dr Dewar feels the forecast may overstate the risk as severe frosts in early January probably killed most over-wintering aphids.

But numbers will have started to rebuild in the exceptionally mild weather (see table) and an early migration is likely.

"The average temperatures are two degrees higher than last year. That is positively tropical in meteorological terms. It is almost as mild as in 1974, which was the worst virus yellows year ever."

Then, without imidacloprid seed treatment, eastern crops suffered nearly 90% infection and the national average was 67% infection, cutting crop output by 40%. "It nearly bankrupted both the British sugar industry and Brooms Barn."

This year, seed orders show 72% of seed has been Gaucho-treated, down from 74% last year, and 3% is treated with Force (tefluthrin). Growers tempted to try to make their treated seed go further by mixing with untreated lots are warned not to do so.

Trials at Brooms Barn last year showed such mixes suffered similar virus losses to untreated plots. "Using a 50/50 Gaucho and untreated mix is not a good idea. You are going to get virus yellows so do not do it," says Dr Dewar.

Growers who rely on the aphicidal effect of carbamate nematicides may need to change products and increase rates to extend protection, he says.

Temik (aldicarb) offers more protection than Vydate (oxamyl), Posse (carbosulfan) or Oncol (benfuracarb), and growers should consider a 7.6kg/ha rate rather than the 4.7kg/ha that is widely used to combat nematodes. "It should protect crops for a week to ten days longer."

Whatever protection is used, growers will need to keep a close watch for aphids colonising crops as treatments run out of steam. Up to 12 leaves, the threshold for an aphicide spray is one green, wingless aphid every four plants.

"Even Gaucho growers are going to have virus and for some of these crops infection levels will be high enough to justify a spray," he concludes. &#42

Resistance worry

Imidacloprids extension to use across both cereals and oilseed rape is a concern considering sugar beet growers reliance on it to combat the main virus-carrying aphid, Myzus persicae, says Dr Dewar. "This year for the first time we have a lot of oilseed rape treated with Chinook. It will be interesting to see if there is any change in the resistance status of the aphids."

Mean air temperatures (C)

Jan1-Feb14

2001 2002

Region

East 3.8 6.2

North 3.2 5.9

West 4.4 6.1

VIRUSTHREAT

&#8226 High virus forecast.

&#8226 Worst since 1974.

&#8226 Be ready to spray even after seed treatment.

&#8226 New Year frosts may have moderated risk.