13 March 1998

Nematodes need nailing before yields threatened

By Andrew Blake

LONG-TERM control of potato cyst nematode, a growing threat to potato production, must come before annual yield considerations.

That was a key message running through several papers at the British Potato Council agronomists conference at Peterborough last week.

Concentrate on keeping low populations of PCN as low as possible, specialists stressed. GPS technology to map infestations also came under fire.

Approaches to date have clearly failed the industry, according to the BPCs Mike Storey. Several speakers offered hope for the future through biological control and especially breeding. But despite nematicides and tolerant varieties, PCN problems are growing, Dr Storey noted. Up to 45% of soils in England and Wales are infested, according to one estimate.

That half the days proceedings were devoted to the subject reflected agronomists concerns, he added.

"We have effectively followed the wrong management strategies for the past 20 years by protecting yield rather than protecting the land."

Mark Philips of the Scottish Crops Research Institute highlighted the ease with which PCN populations recover from treatment and emphasised the value of containing them at manageable levels.

Assuming an initial level of 100 eggs/g of soil, nematicide use might lift an untreated yield of 60% of full potential to an apparently acceptable 94%. But that still leaves a boosted population of 140 eggs/g to threaten future crops, he explained.

Even at the ADAS treatment threshold of 10 eggs/g, the same tactics boost the PCN level, albeit delivering near maximum yield. Only when the initial population is as low as 1-2 eggs/g is there a chance of stopping a rise in numbers during the season through using nematicides, claimed Mr Philip. "Its when populations are low that you can get control and keep control."

Determining that population though is fraught with difficulty, according to colleague David Trudgill who had little to say in favour of PCN mapping.

"Sampling is frighteningly inaccurate." Patches of PCN infestation are easily missed in the field without more thorough grid-based methods. At laboratory level there is a one in three chance that current sub-sampling methods (100g from a 2kg field sample) will fail to pick up an infection of 30m cysts/ha, Dr Trudgill added.

Checking soil from under the grader is a much better way of detecting whether the farm has a PCN problem, he suggested.

Anti-feeding genes from rice could give growers PCN-resistant varieties within four years, adds Dr Peter Urwin of Leeds University. &#42

Peter Urwin with the hairy root material he uses to test the success of inserting anti-feeding genes against nematodes.

Parasitic fungi, here colonising a PCN cyst in an IACR Rothamsted trial, offer promise of biological control. Applied to the seed tuber one in particular has given 82% control of Globodera pallida, says Dr Ken Evans. "It is early days, but we are quite hopeful."

PCN sampling leaves a lot to be desired, says David Trudgill.