Numbers of the potential spraing-causing Trichodorid nematode in soil have increased by 300% over the past decade, according to the Scottish Agricultural College (SAC).

Of the 456 soil samples that had nematodes extracted from them in 2004/05, just under one third (29.6%) tested positive for the presence of tobacco rattle virus (TRV), said the SAC’s Andy Evans.

“This is pretty significant, in that last season, 51% of fields tested had populations of nematodes that could be damaging potatoes directly through feeding, never mind the transmission of TRV.”

The problem may not be isolated to potatoes, as feeding damage has been witnessed in other crops, including cereals, he said. Dr Evans warns growers that the virus can spread between fields in weed seeds and infected potato seed and some varieties could be carrying TRV without exhibiting visible signs of spraing.

“Sharp, very hard frosts could kill those [nematodes] near the soil surface, but if cold conditions are gradual, the nematodes simply go deeper into the soil profile. Drought can take out a lot of nematodes, but irrigation plays into their hands,” he added.

Growers should get a TRV test and nematode count carried out and use a nematicide if numbers are potentially damaging, he said.

In order to get the best from nematicides, careful soil incorporation is needed to get the right concentration and persistence for as long as possible, added Pat Haydock from Harper Adams University College.

Additionally, there may be a case for rotating products from different chemical groups (e.g. carbamates, fosthiazate) to achieve greater persistence, he said.

“Growers who have historically used aldicarb or oxamyl, which have very similar chemical structures, may now achieve greater persistence with fosthiazate (Nemathorin).”