Tightening legislation is putting a question mark over the continued use of chemical nematicides, but there are some novel alternatives in the pipeline, as James Andrews discovered

Alternative measures for controlling potato cyst nematodes dominated proceedings at a two-day conference at week dedicated to the pest. Breeding PCN-resistant varieties, developing natural nematicides from plant extracts and controlling populations with biofumigant crops had all shown promise.

But most at the Association of Applied Biologists conference at Harper Adams University College agreed that combining a number of alternative control measures would be the most sustainable solution.

Biofumigant crops such as mustard showed promise in a number of trials, but the challenge was fitting them into a commercial rotation, said Agrovista agronomist Andrew Wade.

“I’m convinced biofumigants are a valuable tool for controlling PCN and they do wonders for soil structure. We need to find a way of fitting them into a standard rotation.”

Mustard varieties tested were too late to mature in the spring and had to be macerated before they were ready, he said. “We need the mustard to flower before it is destroyed as this is when the glucosinolates that kill PCN are at their most concentrated.”

Caliente mustard planted in 2008 didn’t fully mature until April and all but one of the five farmers in the trial decided to destroy the crop in March. But the one trial that was completed showed a significant reduction in PCN numbers.

“The number of juveniles found in roots was four times lower where the mustard was planted and macerated post-flowering,” said Mr Wade.

In 2009 trials, Nemat and Caliente varieties were planted together to encourage more vigorous growth, but the cold 2010 spring meant the crop wasn’t ready any earlier.

This autumn Mr Wade was experimenting with a number of brassica mixes, including brown mustard and fodder radish. “We established them in late July following winter barley. They look good at the moment and hopefully some will mature early enough in the spring,” he said.

Developing varieties with inherent resistance to the two PCN species globodera pallida and G rostochiensis was an important step in limiting the effects of PCN, said SCRI plant breeder Finlay Dale.

Varieties with resistance to the G Rostochiensis species, such as Maris Piper, were already available, but there were only two varieties with partial resistance to G pallida, and both of these had limited marketability, he said.

“GM is probably the answer, but Europe is not ready for it yet. In the meantime, there is a lot we can do with marker-assisted conventional breeding,” said Mr Dale. “We can target PCN resistance genes using markers and incorporate these into new varieties.”

But there were 50-60 other characteristics, such as late blight resistance and fry quality that also needed to be considered, he said.

Eventually, GM should allow multiple traits to be established in one potato variety, he noted. 

Garlic, chilli and mustard extracts were showing promise as natural nematicide products, said Harper Adams PhD student Wiseborn Danquah.

“When garlic is crushed it produces a sulphuric compound called allicin, which has nematicidal properties.”

Research in 2009 applying liquid garlic extract to G pallida eggs showed a significant reduction in the number of viable eggs, he said.

A natural nematicide called Dazitol containing chilli and mustard extracts also showed promise. “When Dazitol was applied to G pallida it significantly reduced the nematode’s multiplication rate and tubers tended to be larger on treated plots.”

Tubers were also larger than plots treated with conventional nematicides, although tuber numbers were lower, he added.

“Plant extracts show potential as nematicides for PCN control. But no single extract seems capable of replacing existing nematicides. Integrating a number of plant-based nematicides with complementary modes of action may give a level of control similar to nematicide products, but more research is needed,” said Mr Danquah.

PCN management

Potato growers should be taking a long-term approach to managing potato cyst nematode, said David Nelson, field director for Branston.

This should start with a comprehensive and well-timed sampling strategy, he told conference visitors.

“Sampling immediately after the crop has been harvested is the best way to keep tabs on PCN populations and come up with a suitable management plan.”

If growers left sampling until they were ready to plant, PCN populations would have declined and may not be detected, he said. “Elevation of populations takes place in the growing crop and you get the most accurate representation just after the crop has been harvested.”

If PCN was detected at this time, growers would have time to adjust their strategy. “You can choose to grow on non-infested land, leave infested parts of a field uncropped, or grow varieties that prevent PCN multiplication.”

Early detection was particularly important on what was thought to be clean land, he said. “We need to be much more pro-active. It is much better to stop PCN spreading in the first place, which we can do if we identify it early enough.”

Landowners renting ground to potato growers need to take responsibility for PCN monitoring on their own ground, he said. “Potato growers who take on rented land are unlikely to sample for PCN after the crop is harvested, so the land owner needs to take responsibility for this.”

About 64% potato land in the UK was infested with PCN, which, if unmanaged, could lead to considerable financial loss, he said. “A 60t/ha crop can drop to 40t/ha with PCN infestation and it doubles the number of undersized tubers.”

Limiting PCN spread and preventing build-up in infested land was essential, he warned.