Trials provide answer to potato variety’s yield decline

Free-living nematodes might be causing more harm to certain potato varieties than growers realise, trials conducted for North Norfolk Potato Growers suggest.

The 10-strong growers’ group supplies 50,000t of crisping potates – mainly Saturna, Lady Rosetta and Hermes – on a long-term storage plan.

Saturna is vital to the group because of its ability to store through to June, NNPG managing director Dan Hewitt says. “It is a banker in terms of quality.”

Tuber sugar levels remain stable throughout storage meaning that the variety still gives the desired crisp fry colour after long-term storage, he explains. “Until we find a variety to replace it we have to grow Saturna.”

But NNPG found that the second or third time it was grown within the rotation yields dropped off significantly from 50-57t/ha (20-23t/acre) to around 42t/ha (17t/acre).

It was independent agronomist John Purslow, brought in to advise NNPG in early 2008, who thought he knew what might be the problem – free-living nematodes.

Saturna is a poor vigour and rooting variety, he explains. Anything that could reduce rooting vigour could have a negative impact on yield, so he surmised that feeding damage from free-living nematodes could be the cause.

“I’d seen something similar elsewhere,” he says.

He also thinks handling of soil testing samples might mean free-living nematodes are underestimated. “You take the sample, bung it in the back of the car, let it bounce about for a bit, before you send it off about a week later when you have a batch to send.”

That’s fine for potato cyst nematode samples which are protected by their cyst casing, but free-living nematodes are much more sensitive, he says.

“You have to be a lot gentler. I’ve been trying to put them in paper bags rather than plastic, and then sending them to CSL in 24 hours.”

Using that system should protect, in particular, smaller nematodes, he believes. “You get a different result.”

But it’s not just the numbers of free-living nematodes, it is also the species that matters. Typically it is the stubby root nematodes (Trichodorus and Paratrichodorus species) that transmit tobacco rattle virus, which causes spraing in potato tubers that growers have sought to control.

But soil testing at NNPG found large numbers of root-lesion nematodes (Pratylenchus species) were present. These nematodes feed and multiply within the roots of host plants, according to Andy Evans of SAC, and have been implicated in allowing bacterial and fungal infections to exacerbate the damage.

Mr Purslow also thought that different potato varieties showed differing levels of damage from nematode feeding.

To test those theories, with the help of his employee, Lucy Fawcett, a specialist in field trials, he set up a large replicated trial with 18 potato varieties. Each was left untreated, or treated with 30kg/ha of the nematicide Nemathorin, 3.0 litres/ha of the fungicide Amistar (azoxystrobin), or both.

The fungicide was included to check whether it could help minimise effects from secondary infections, particularly of rhizoctonia.

All varieties were planted, harvested and graded by hand. Observations during the growing season suggested that varieties did differ in their response to the treatments.

In particular, emergence on Saturna, Satellite, Hermes and Lady Claire was significantly improved where Nemathorin was used compared with the untreated 32 days after planting. In other varieties, particularly the fast-emerging Markies, there were no significant differences.

Most varieties showed an improvement in percent groundcover 60 days after planting, but the difference was most marked for Sassy, Orchestra and the coded variety FL2026. Improvements of 20-25% were seen in those varieties over the untreated for both the Nemathorin and Amistar treated plots, with typically a slightly better response to the nematicide. But there was little additional improvement from applying both products.

There was also a significant increase in stem numbers per plant for Sassy, Saturna and Markies, in particular. Both Sassy and Markies responded to treatment from either Nemathorin or Amistar, putting on an extra stem and a half in the case of Sassy, and an extra half a stem on average for Markies.

Saturna, in contrast, responded much more to the Amistar treatment than Nemathorin, increasing by one extra stem per plant for the former.

In all three varieties applying both products had an even greater effect, with the highest stem numbers per plant being observed on the plots receiving both treatments.

“In crisping varieties an increase in stem numbers should lead to more tubers and therefore give increases in yield,” Mr Purslow says.

That certainly followed with Saturna where yields increased by 20t/ha following treatment with Nemathorin compared with the untreated. Amistar also increased yields over the untreated by around 10t/ha. Similar increases in marketable yields commercially would be worth over £2000 and £1000/ha respectively.

However, using both products did not increase yields over that of Nemathorin alone, despite the increased stem numbers.

Markies responded equally well to both fungicide and nematicide – both increasing yields by over 20t/ha, but again the combination of both inputs did not further increase yields.

Most varieties, however, gave a positive yield response to Nemathorin, Mr Purslow says. “The only one that didn’t was Maris Piper, while Estima only gave a small response.”

In general most of the varieties responded better to the nematicide than the fungicide. Tests on haulm and soil by CSL from various plots did not find any particular fungal issue, leaving it unclear whether the Amistar was having an effect in controlling rhizoctonia or Verticillium wilt.

But the results do suggest growers might be able to tailor input use to particular varieties depending on levels of free-living nematodes and, possibly, even which species are present, Mr Purslow believes. “It could be that different varieties select different species.”

The results were an eye-opener for Mr Hewitt. “It’s only one year’s result but they speak for themselves.”

Nematicide use for free-living nematode was being completely overlooked for the most part by growers in the group, he admits. But that is likely to change this season.

Already 50% of the farms in the group have undertaken free-living nematode soil tests, and based on those results Mr Purslow will design a control programme, Mr Hewitt explains.

“If we can get 21-22t/acre of Saturna on those fields second and third time around it make a huge difference to the business and help us support the processing company with a variety they want.”

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