Potato Review

PEST CONTROL Cultural control and nematicides Protecting yields while preserving land for the long-term is key to securing the potato sector’s future and winning the battle against PCN, say specialists. A SK a potato specialist how you best tackle potato cyst nematode (PCN) populations and they will likely tell you that without varieties considered to combine good resistance and tolerance, you’re in for a tough battle. While resistant varieties are accepted to be the sustainable solution, especially over the long term, other measures, such as rotation length and the use of specialist cover crops combined with the diligent use of nematicides, are just as important. e frustration of all concerned is that there simply aren’t enough resistant and tolerant varieties with broad market acceptance to make rapid progress. In 2017, the last year for which data is available, 12 varieties accounted for 53,310 hectares, roughly half of the crop area in Great Britain. e majority of these varieties o er strong resistance to Globodera rostochiensis, but poor resistance to G. pallida. is partly explains why over the past 20 years, G. pallida has become the dominant species, accounting for 89% of the PCN detected on infested land, according to a 2016 survey. Of the 12 most popular varieties (see table) grown in 2017, only Innovator has good resistance to G. pallida. at growers continue to plant varieties with such poor resistance to G. pallida is hardly their fault, said Eric Anderson, Senior Agronomist with Scottish Agronomy. “ e commercial pressures facing growers are often far more immediate than the concerns caused by PCN, but it is that threat above all others that has the capacity to decimate our industry,” he said. At the heart of the issue is grower reliance on granular nematicides and the concern that Nemathorin 10G ( fosthiazate), might not receive re-authorisation when its current approval expires in October 2024. “Even if the varieties with good resistance and tolerance to G. pallida, and to a lesser extent G. rostochiensis, had broad market acceptance – which they don’t – it would still take ve to six years to multiply seed stocks needed to meet grower demand. I fear that we could lose a big part of our industry before anyone accepts the need for change,” said Eric. Attitudes are, however, changing. A large processor has begun the transition to a variety with good resistance and tolerance to PCN while others have trials in place, but the lack of movement among the multiple retailers is considered the biggest impediment to progress. “ e processors are moving in the right direction, but the real hurdle to progress is the multiple retailers which continue to show a preference for varieties that exacerbate the situation. ey’re simply not invested in developing sustainable solutions,” Eric said. While PCN may dominate discussions, they are only one part of the nematode threat. Free-living nematodes, speci cally stubby-root nematodes (Trichodorus and Paratrichodorus species) are of increasing concern. Stubby-root nematodes are vectors for Tobacco Rattle Virus (TRV) which manifests as necrotic rings in the esh of tubers in a condition called ‘spraing’. More than 5% internal defects ( from TRV) and the batch is likely to be rejected from packing. Root damage in icted as Stubby-root nematodes feed has also been found to lead to a higher incidence of internal rust spot (IRS) through reduced calcium uptake. Integrated measures Until varieties that combine good resistance and tolerance to PCN and to a lesser extent TRV, gain market acceptance, growers are encouraged to adopt cultural practices that will improve population management. is includes taking a more objective approach to site selection through to the use of certi ed seed and the use of nematicides wherever soil sampling indicates that viable eggs and juveniles are present. Beyond this, the use of a suitable trap crop such as sticky nightshade (Solanum sisymbriifolium) or a biofumigant such as oil radish (Raphanus sativus) in conjunction with a resistant variety where possible, are also encouraged. Grower success with cover crops has been highly variable and is often dependant on the conditions during the season, so perseverance is key. It may be that more than one cover crop is needed between potato crops to have a meaningful impact on PCN populations. Soil conditions at the time of sowing have been 38 POTATO REVIEW MAY/JUNE 2023