21 February 1997

Low level PCN worth control

POTATO cyst nematode treatment thresholds may need to be lowered to offset hidden quality losses as market standards tighten.

Although total yield losses caused by poor rooting, reduced canopy development and early senescence are well recorded, marketable yield losses are not, Simon Bowen, Anglian Produces field services director, told a recent Rhône-Poulenc conference at Newmarket.

Dr Bowen suspects even low PCN populations are costing farmers money, particularly through reduced tuber size.

"Market specifications are becoming tighter and tighter. For packing potatoes, we have now potentially moved to a 50mm bottom riddle, making tuber size very important." This year sees a similar trend for processing contracts. On some, there will be no payment for tubers smaller than 45mm.

Other quality traits can also suffer. "Poor crop growth delays maturity, which depresses dry matter. Last year, dry matters were down 1.3%, generally, compared with 1995." A lack of late-season sunshine compounded by the diluting effect of rain were largely to blame. But Dr Bowen believes the added stress of even low levels of PCN could have tipped the balance in many crops.

PCN damage is exacerbated by other pests and diseases, causing greater losses than expected, he said. "I think this interaction is a much underestimated cause of crops failing to meet market specifications."

PCN can interact with verticillium wilt, and rhizoctonia. "I would suggest this combination occurs very regularly, especially on sandy light soils in close rotations."

Early senescence

Drought stress also increases damage. "A lot of crops died off last year, especially on fields at low risk of PCN. We saw a lot of early senescence, certainly in varieties such as Estima.

"For tight specification markets, evidence suggests we must try to implement control measures at lower PCN populations than are traditionally advised."

&#8226 To protect export markets which demand PCN-free produce, growers should strive to preserve clean land, said Dr Bowen. Good hygiene – avoiding dumping of soil into clean fields, care when moving between clean and infested fields and effective groundkeeper control – is vital to keep the pest at bay.n

Low populations of potato cyst nematode could be worth controlling, says Simon Bowen of Anglian Produce. But use nematicides with care.


&#8226 Reduces tuber size.

&#8226 Lowers dry matter.

&#8226 Damage exacerbated by drought, pest and disease stress.

&#8226 Light soils more vulnerable.

&#8226 Protect clean land.