12 September 1997

Knock-back techniques keep pallida nematodes contained

GROWERS need to know how to exploit potato varieties with partial resistance to the pallida nematode to get the best results.

Resistance rating, rotation and nematicide use all need considering, says Jack Dunnett of Caithness Potatoes, who bred the pallida resistant types Valor, Nadine and Kestrel.

In a trial at Southery in the Norfolk fens, Valor grown on peat containing a high level of pallida nematodes reduced the population without needing the help of a nematicide. There were 83 fewer cysts/100g of soil in a sample taken at the end of the season than one taken at the start. Where nematicide was also used the population reduction was even greater, leaving 283 fewer cysts/100g.

"It is simply a matter of knocking a population down to a low level from which, even at the maximum rate of multiplication or bounce-back, the original level is not restored to the pre-treatment level on a partially resistant type such as Valor. The pallida population is then on the way out, if the field is treated and Valor is planted the next time potatoes come round in the rotation."

This was one of Dr Dunnetts aims when deciding to collaborate with Dr Patrick Haydock of Harper Adams College in Salop.

Another objective is to identify the optimum safe interval between crops of a partially resistant variety. For a susceptible type this is seven years, but no one knows what it is for a partial resistor, says Dr Dunnett. "The maximum pallida multiplication is probably about 25% of what it is on Maris Piper," he says.

That could be exploited by growing two consecutive crops of nematicide-treated Valor to knock the nematode population to a low level. The partially resistant variety could then be grown without chemical at the shorter than normal safe interval.n

Potato varieties with partial resistance to both forms of nematode can be used to wind down infestations, says Caithness breeder Jack Dunnett.