Testing weed roots for the presence of tobacco rattle virus, which causes spraing, is as reliable as the traditional soil bait test and is much cheaper and quicker.

But trials suggest neither can be used reliably to target nematicide treatments within fields, says SCRI’s David Robinson.

The weed test takes advantage of TRV using more than 500 common weeds as a host.

“Testing weed roots takes only 48 hours and, because you don’t need expensive glasshouse space for the test, it is much cheaper.”

CSL’s accelerated bait test, in which bait plants are grown in the sampled soil, comparison takes three weeks, whereas the traditional bait test takes over a month.

Expensive tests have stopped growers from being able to map fields in sufficient detail to target at-risk areas within fields, he says.

“Current practice is for growers to treat the whole field if TRV is identified [despite the patchy distribution of the virus within fields].”

But trials testing the feasibility of using the new weed test for detailed mapping have not proved its value so far, Mr Robinson adds.

In the trials, three sites about to go into potatoes were sampled at 45 points on a 25m grid over 2ha (5 acres) using both the CSL accelerated bait test and the test on weed roots.

Potatoes from each grid point were tested for spraing at harvest.

At two sites, although the weed test indicated whether spraing would be a problem in the field as reliably as the bait test, it didn’t give a good enough correlation to allow targeting of nematicide to specific areas, he reports.

At a third site, spraing symptoms were closer to the incidence suggested by the weed test.

“But it wasn’t perfect,” says Mr Robinson.

“So far the value of a detailed map for a field is not proven.”