Diquat and pencycuron are among the key actives the potato sector has had to wave goodbye to in the last year and now the future of mancozeb also hangs in the balance.
Following an EU decision against reapproval last autumn, the fungicide has gained a temporary reprieve here as a result of post-Brexit legislation, which rules that active substances due to expire in the EU within three years of December 31, 2020, will be granted a three-year extension under the new GB regime.
Mancozeb’s multisite activity has been essential in ensuring growers are able to implement a robust spray programme containing alternating modes of action and without it building a cost-effective disease control programme is going to be a problem many growers face going forward, believes seed potato grower Tom Cargill, who farms in north east Scotland.
Mr Cargill’s farm, which sits between Aberdeen and Dundee, exports around 70% of its seed potatoes to countries outside the EU, including Iraq, Egypt and Thailand.
And while alternaria, which is controlled by mancozeb, is not much of a problem in Scotland, some varieties suited to export are susceptible, he told the BCPC Disease Review.
“We grow Atlantic, which is susceptible [to alternaria], for the Thai market, so it will be interesting to see if we can still grow it without mancozeb.
As well as facing challenges with aggressive blight strains and access to only two aphicides to protect potatoes against virus, burndown periods are also causing problems with the crop’s marketability.
“Countries we are exporting to generally want the product earlier and earlier, so rapid burndown is important.
The products available now are not as effective in comparison to diquat, so we’re using a flail and then using a PPO inhibitor like Spotlight, which requires sunlight.
During August we can get very dull days which reduces the [product’s] efficacy and the crop stays greener for longer, which means more problems with aphids and virus.
The flail is also problematic in a wet season and [delays getting on the field mean] potatoes get bigger and become unmarketable.” At the end of December 2020, the Emergency Authorisation (EA) for nematicide, Vydate (oxamyl) which has been rolled over on an annual basis in recent years, expired and has not been renewed.
As things stand, growers will have until February 28, 2021, to store and dispose of stocks unless the new EA applications submitted by AHDB are approved.
Vydate has been an important tool for growers of potatoes, sugar beet, onions, carrots and other field vegetables to control pests, including potato cyst nematode (PCN), but some are now exploring alternative approaches – including cultural control.
Ten years ago, Wantisden Hall Farm, Suffolk, was experiencing very high PCN counts, but since introducing pigs to the rotation, the pest population has fallen to virtually nothing.
Around 55 hectares of the farm, which grows potatoes, sugar beet, veg and cereals, are now let out to a local pig producer.
Speaking during a Cereals Live Q&A webinar, farm manager Tim Pratt said the biggest benefit associated with the pigs was clearing up volunteer potatoes.
“PCN is probably one of the biggest problems in potatoes and there are resistant varieties out there, but realistically we need to get them to market.
“We had some quite high [PCN] levels and going for the second year with pigs in the field, we could basically reduce PCN levels to nothing.
“It works very well for us. Working behind farrowing fields is nice. They are quite level and haven’t made a mess of the field.
Dry sow fields have high [stock] volumes so can make a bit of a mess.
We try to move dry sucklers every year to limit damage done to the field.
“[In 2020] we had a wet autumn like a lot of people and they do make a bit of a mess, but a couple of days after all the pig huts and fences have gone we can level it off, subsoil and get a bit of structure back and create a decent crop after it.” As well as clearing up groundkeepers, there are also fertility benefits, Mr Pratt added, with soil P and K levels typically increasing by an index of 1 after pigs.
“We’re getting paid to put nutrients back in the soil, but site selection is very important.
You’ve got to have nice light, level and sheltered land.” There are some issues with pigs, he added, including erosion and fertility hotspots.
“We also have to be careful what we follow in the next crop.
We usually follow with maize and sugar beet because they’re good scavengers of fertility and we spent quite a lot of time levelling hotspots off where the huts had been.
“We’ve got to a position where we don’t see those hotspots now,” said Mr Pratt.