The withdrawal of CIPC (chlorpropham) sprout suppressant has highlighted the need for other tools to control sprouting and triggered a three-year AHDB research project*.

Glyn Harper, who leads the work which has just completed its second year, says: Using longer dormancy varieties can reduce our reliance on sprout suppressants which is particularly important post-CIPC.

Unfortunately, the availability of independent data on dormancy for commonly grown varieties is both sparse and conflicting.

The project aims to generate independent relative dormancy ranking for growers to use as extra information when choosing varieties. Ranking should also help guide storage decisions, timings for other sprout control treatments and ultimately sales, adds Dr Harper.

Longer dormancy varieties offer the potential of higher tuber quality over long-term storage with reduced need for sprout control. This project is largely concerned with endodormancy , says Dr Harper.

Sprouting

Dormancy break occurs before sprout development, but theres no simple method for determining actual dormancy break. So, for this project, visible sprouting is used as a proxy and is an important quality parameter requiring management.

Thirty-nine varieties, skewed towards those for crisping and chipping markets, have been grown on two SPot Farms demonstration plots in North Lincolnshire and Shropshire, with nine extra varieties trialled at the latter in the second season. The plots received the same crop inputs and protection as the surrounding commercial field crops.

After hand harvesting, daughter tubers were stored at 15degC and assessed for sprouting at Sutton Bridge Crop Storage Research. Dormancy was recorded as days from estimated tuber initiation to 50% sprouting (>3mm) of the 50-tuber sample population.

Many methods have been, and are, used by different organisations and companies to provide data on varietal dormancy.

Unfortunately, in many cases the methodology or description of dormancy rating is somewhat or entirely unclear, says Dr Harper.

Dormancy is generally considered to be initiated at tuber initiation , but in most available reports it is measured from harvest date.

Previous work has shown growing conditions post-TI can have an effect, he adds.

It found unusually cold or hot weather during tuber development in the field often resulted in long or short dormancy respectively.

We found a surprisingly big difference in some varieties dormancy at the two sites. It ranged from nothing in Ivory Russett to as much as 47 days in Mozart.

The reasons unclear as there was no apparent overall site effect, although harvests were a month apart. The seed was randomly drawn from the same stock and sites were planted within a week of each other.

However, the two years of trials have allowed a relatively robust variety dormancy ranking list to be drawn up, Dr Harper says.

The third and final year of the trials should provide additional data to support the ranking. The variation in the length of dormancy of seed from a common sample of the same stock when grown at different sites was unexpected and still unexplained.

However that did not generally affect the relative dormancy when ranked as an average duration, he adds.

This variation will be important in establishing the confidence limits of the ranking for the entire trial.

• Tubers, or more strictly the buds, may pass through three phases of dormancy:
• Endodormancy is a period of deep dormancy in which sprouting does not occur even under favourable conditions due to internal physiological factors
• Ecodormancy in which sprouting could occur under favourable conditions but is inhibited by unfavourable external factors, for example low temperature
• Paradormancy is the physiological control of lateral buds by the apical bud apical dominance

Any sound research to help future sprouting control should be welcomed, according to Norfolk-based potato consultant Simon Alexander.

He says: Where stores are destined to contain more than one variety, the more information we have to help load those with similar habits the more efficiently we should be able to manage sprout control.

However, Mr Alexander believes the results to date should be regarded as broad guidelines only.

I dont believe we should take the figures too literally.

He welcomes measuring dormancy from TI.

It removes any interference from management practices, for example delayed harvesting, he adds.

Its also useful as varieties may be taken down at different stages in their life for different markets; but growers need to know how long varieties will be expected to live to interpret this.

Difference

The difference between Sagitta and Markies isnt going to be as great as the chart may indicate because the latter will most likely be in the ground longer. Im concerned by some of the site and annual variations in some varieties. Does this mean that the figures are too inconsistent to be of use on those particular ones?

Commercial fields are always variable some more than others, he adds.

So it wont be until weve looked at the data in relation to how commercial crops behave in any one year that well see the potential benefit of the information.

Dormancy and propensity to sprout are not the same things, he says.

Although the dormancy is important, vigour of sprout growth may be more so. For example, in a store containing both Fontane and Markies, the former will drive the decision when to treat for sprout control because it will start to regrow sooner. This has nothing to do with dormancy.

• *AHDB project: 11140058 Understanding sprouting dormancy rankings in potatoes
• September 2018- September 2021
• Funding: £75,000 (all from AHDB)
• Project partners: AHDB, SBCSR and AHDB Knowledge Exchange
• The contributions of Graham Tomalin, Vegetable Consultancy Services UK and the many collaborating breeding companies are acknowledged