Potato growers battling potato cyst nematode (PCN) problems are set to benefit from significant breeding efforts into producing fresh varieties resistant to the key nematode species Globodera pallida.

Some promising varieties with resistance to this key pest are moving to the end of breeders’ pipelines with the potential to crack a fresh market dominated by a handful of susceptible favourites.

While speculation continues about when nematicides such as soil-fumigant metam sodium and granular products oxamyl and fosthiazate might be lost, there is an increasing call for more non-chemical tools to be used on farm.

The most important is varietal resistance, which can help growers reduce PCN populations and, if combined with tolerance, do so without compromising yield and subsequent profit (see ‘How do varieties behave in the presence of PCN’, below).

See also: How to sample soil for potato cyst nematodes

Currently, many of the most widely grown potato varieties have strong resistance to Globodera rostochiensis (single H1 resistance gene), but not for one or more of the three major G pallida pathotypes (see ‘Resistance status of the 12 most widely grown ware potato varieties across Britain in 2017’, below).

Resistance status of the 12 most widely grown ware potato varieties across Britain in 2017
Potato variety	GB planted area (ha)	Resistance to Globodera pallida* (1 to 9 with 9 most resistant)	Resistance to Globodera rostochiensis* (1 to 9 with 9 most resistant)
Maris Piper	16,310	2	9
Markies	6,030	2	9
Maris Peer	5,000	2	2
Melody	4,300	2	9
Lady Rosetta	3,460	2	9
Estima	2,990	2	2
Taurus	2,770	3	8
Pentland Dell	2,750	2	2
Marfona	2,400	2	2
Innovator	2,470	8/9	Not resistant
Sagitta	2,440	Not resistant	Resistant
Royal	2,390	3	9
*Resistance scores to Globodera pallida pathotypes Pa2/3,1 and G rostochiensis pathotype Ro1 on a scale of 1-9, with 9 being most resistant.

The situation is changing quickly in the processing sector, which has some good pallida-resistant options making a dent in the British potato area, particularly Innovator and partially resistant Royal and Taurus.

Breeding efforts

However, the fresh sector is still searching for a commercial breakthrough that has robust resistance to G pallida, but this is certainly not from lack of effort from breeders, according to E Park & Sons agronomist Peter Blaylock.

The Doncaster-based potato packer and producer is a stakeholder in the GPS (Grampian Parks Skea) breeding programme, which includes Scottish commercial partners Grampian Growers and Skea Organics.

Using commercially successful varieties with known G pallida resistance and crossing with genetic material from the James Hutton Institute, the group is searching for fresh varieties with resistance to G pallida or ideally, dual resistance to both G pallida and G rostochiensis.

GPS has many clones in trials across Britain and for the first time this year, the Netherlands. They also have larger plots of more advanced selections and varieties, tested on different sites.

In 2018, these included an encouraging salad variety with dual resistance and several other potential table varieties that could help fresh growers manage PCN.

“I think breeders have really woken up and responded well to the pallida problem, but it has to be remembered that using traditional breeding methods, it takes about 12 years for a variety to come to fruition, so it’s no easy task.

“I think there will soon be wholesale opportunities for fresh growers to use dual resistance, which will be a big step forward,” explains Mr Blaylock.

Difficult task

This is a sentiment echoed by breeder IPM Potato Group’s Graeme Prentice, who says G pallida resistance has been a focus of the company’s breeding programme – carried out in conjunction with Ireland’s agricultural research institute, Teagasc – for the past 15 years.

From making initial crosses in the early noughties, some are now approaching the end of the crucial field based trialing period, which typically takes a minimum of 12 years.

“We are now just starting to see some promising seedlings come through,” adds Mr Prentice.

Unforeseen issues

Even if a variety does show promise, once out in the commercial world it can soon succumb to an unforeseen issue.

Mr Prentice was tight-lipped on naming specific seedlings, but tells Farmers Weekly that several new IPM seedlings have dual resistance and are achieving high resistance scores for G pallida in its own assessments.

Two of those have also received some encouraging feedback from packers on suitability for the fresh market, with good taste and skin finish.

He says while not everyone has issues with G pallida on their land, it is promising fresh growers may soon have a handful of varieties with resistance and a significant end market for their potatoes.

“These can then be targeted on the land where the problems are, potentially helping to bring land back into fresh production where it hasn’t been possible due to PCN,” he adds.

Selection pressure

With indications suggesting it is a matter of when, not if, a highly dual resistant variety is going to arrive for fresh potato growers to join those in use in the processing sector, it does bring in to question how such varieties should be used on farm.

A single H1 resistance gene is enough to give a variety strong resistance to the Ro1 pathotype of G rostochiensis found in the UK. However, with G pallida the situation is much more complex, with multiple pathotypes present in the UK – Pa 1 and Pa 2/3.

Populations of G pallida pathotype Pa 2/3 are known to have significant genetic diversity, according to Harper Adams University nematologist Matthew Back, and potentially makes variety choice and use complex.

Resistance to G pallida is typically polygenic (made up of multiple genes) or quantitative (quantitative trait loci) if the resistance derived from the wild solanum species, Solanum vernei. The resistance conferred is typically partial.

There is some concern that with insufficient information on the resistance a variety has, or on the PCN population present in the soil, certain varieties may be used in the wrong situation.

“Ideally, it would be useful to characterise pallida populations for given fields and then select varieties with corresponding resistance to the pathotype present.

“At the very least growers should routinely test infested land for the species present,” explains Dr Back.

Integrated management

AHDB Potatoes knowledge transfer manager Anne Stone says this has been demonstrated in starch production in the Netherlands, where growers have planted G pallida resistant varieties on tight rotations and seen genetic variants emerge.

She doesn’t believe British potato producers will shorten rotations as more resistant varieties become available, but says the selection risk must be acknowledged and negated through integrated pest management.

This includes maintaining long rotations, practicing good cross-rotational volunteer control, using biofumigation or trap crops and appropriate use of chemical nematicides.

Dr Stone also notes a good information flow between breeders and growers and agronomists will be required to make sure varieties are used in the appropriate situations and in a responsible way.

Rotate resistance

To do this, growers will need to speak to breeders about the pedigree of a variety and its sources of resistance, then try not to grow those with similar traits in consecutive rotations, where possible.

“Initially, it’s likely that a grower won’t necessarily be able to grow a resistant variety in every rotation [for marketing reasons], so alternating resistant and non-resistant varieties in combination with other control methods will be vital.

“Another requirement is growers willing to trial new varieties and feed information back to breeders about how it responds on farm,” she adds.

“This will help monitor for any potential break down of resistance or shift in populations,” adds Dr Stone.

How does your variety behave in the presence of PCN?

To use varietal resistance to potato cyst nematode effectively, it is vital to understand how a variety will behave when planted into infested soil.

What is PCN resistance?

To screen for PCN resistance, candidate varieties are inoculated with pure populations of PCN of known pathotype.

The multiplication of the PCN pathotypes on the candidate variety is compared with the multiplication on the susceptible genotype (typically Desiree). The degree of multiplication (relative susceptibility) is used to assign a resistance score on a 1-9 scale to each pathotype.

For example, a score of 3 would mean that PCN multiplication would be 50% of the susceptible control, while a score of 4 would have 25% of the multiplication seen on the susceptible control.

A score of 1 is highly susceptible and will aid significant multiplication of PCN in the soil, a 4 has some useful resistance and can reduce pest numbers, while a 9 is highly resistant and can dramatically reduce populations.

What about tolerance?

There is considerable variation between potato varieties’ tolerance to PCN, which can be defined as its ability to produce a similar yield compared with that in a PCN-free soil under otherwise similar soil conditions.

Independent data on varietal tolerance is lacking limited, so the industry relies on breeder and anecdotal evidence to predict how a variety might perform in the presence of PCN.

PCN resistance and tolerance are not mutually exclusive, so a variety can have resistance and tolerance, one or the other, or neither.

The table below shows the resistance and tolerance status of popular varieties, along with how they are likely to impact on PCN populations in the field.

There are differences between varieties for resistance and tolerance to potato cyst nematodes
Variety	Tolerance status	Resistance to Globodera pallida* (1 to 9 with 9 most resistant)	Resistance to globodera rostochiensis* (1 to 9 with 9 most resistant)
Camel	Very tolerant	9**	9**
Cara	Very tolerant	2	9
Performer	Very tolerant	8	4
Royal	Very tolerant	3	9
Arsenal	Tolerant	9	6
Brook	Tolerant	2	9
Eurostar	Tolerant	9**	9**
Lanorma	Tolerant	5	9
Markies	Tolerant	2	9
Marvel	Tolerant	5	8
Cabaret	Moderate	2	9
Divaa	Moderate	5	3
Harmony	Moderate	4	4
Maris Piper	Moderate	2	9
Rock	Moderate	9**	9**
Rooster	Moderate	2	2
Estima	Intolerant	2	2
Innovator	Intolerant	8	2
Maris Peer	Intolerant	2	2
Nadine	Intolerant	3	9
Panther	Intolerant	8	2
Pentland Dell	Intolerant	2	2
Ramos	Intolerant	4	8
Sante	Intolerant	4	9
** Based on breeder data only – treat with caution Resistance rating source: AHDB