Despite issues such as silver scurf, dry rot, gangrene and skin spot – which can develop and cause problems in store – often being season specific, SAC Consulting seed production expert Innes Jessiman says risk from all four is always lurking in the background.
As a result, growers should remain vigilant year on year, be aware of the conditions that favour each pathogen and address any potential sources of infection for fresh tubers heading into store.
An integrated strategy to minimise the spread of inoculum and development of disease symptoms can then be planned for the season ahead.
The main area where Mr Jessiman believes the seed industry can up its game is store hygiene, which makes a considerable difference to the final disease levels of a seed crop when loaded out of store.
The window between old crop going out and a new crop coming in should be utilised to thoroughly clean stores and grading areas, removing any soil and dust with a vacuum cleaner.
One of the main sources of infection in stores is dust carrying fungal spores, which is kicked up by forklifts and other machinery, eventually settling on clean tubers coming in from the field or grading shed.
Mr Jessiman adds that where possible, buildings should also be power washed, preferably with a disinfectant such as peroxyacetic acid (for example, Jet 5) to kill any remaining spores.
Fogging stores with peroxyacetic acid, typically carried out by a contractor, is also an option and although more expensive, is highly cost effective, having been shown to kill up to 90% of fungal spores after treatment.
Nick Green is director of Lincolnshire-based Stored Crop Conservation and chairman of the National Association of Agricultural Contractors (NAAC) CIPC application group, whose members also specialise in store hygiene.
He says fogging costs about 40p/tonne and has become popular amongst top seed and ware producers.
For maximum efficacy, pre-cleaning of stores must be thorough, with all dirt and dust vacuumed and washed from floors and building fabric, enabling fog to hit every surface.
With the vast majority of seed producers harvesting straight into boxes, these can also provide a source of inoculum for the new crop and can be stacked in store during fogging for a “free” disinfection.
If not fogged, the dirtiest boxes should also be power washed, while cleaner boxes can be left outside for UV light to kill any resting fungal spores.
Mr Green stresses that peroxyacetic acid is contact-acting and has residual activity, so fogging should be carried out as close to store loading as possible.
“Hygiene and cleanliness around stores and in adjacent buildings such as grading sheds is also important, as forklifts or wind can pick up dust and recontaminate clean buildings,” he explains.
Harvest and grading machinery are also areas of concern for Mr Green and he advises the use of a knapsack sprayer to disinfect all components that come into contact with tubers as regularly as possible and between each field or seed lot.
“It doesn’t have to take long, as it can be done from a static position while machines are running,” he adds.
Harvest is another area where practices can have a big impact on final tuber disease levels.
Seed should be lifted at the earliest opportunity to avoid the crop sitting in the ground for extended periods or wet conditions at the back end of the season, as both exacerbate all major tuber diseases except dry rot.
With damage offering diseases such as dry rot and gangrene an easy entry into tubers, Yorkshire-based SPUD Agronomy potato specialist John Sarup says completing haulm destruction in good time is also paramount.
“Preventing damage at harvest is a particular challenge on stony soils in the Yorkshire Wolds, so focussing on achieving a good skin set will help. It is the first line of defence,” he notes.
Correct harvester set up also plays a key role in reducing tuber damage and Mr Jessiman says experienced operators should know how to maximise harvester efficiency while remaining sympathetic to tubers.
However, where relatively inexperienced staff are used, adequate training is vital to help limit any nicks and scrapes to skins or internal bruising.
Tubers should be adequately cushioned by soil as it comes up the web, any hard falls into boxes broken, aggressive separation avoided, and damage levels monitored on a regular basis to identify problems early. Adjustments can then be made to the machine.
“The crop is in the store for a long period, so if you can get it in as efficiently and as gently as possible, you save a lot of hassles with disease problems later on.”
Further to hygiene and reducing damage, broad-spectrum seed treatments such as imazalil (Gavel) can provide a good insurance policy against infection and development of tuber diseases.
Mr Jessiman says the decision to apply a seed treatment lies with the grower and whether any problems have been identified, but he would advocate a seed treatment in most years if the crop is clean and dry at the point of application.
Work carried out by Scotland’s Rural College (SRUC) over a number of years have shown treatments are best applied within 48 hours of harvest, either on the harvester or over a roller table if split grading into store.
With the vast majority of Scottish growers harvesting into boxes, most will opt to avoid tipping and handling seed a second time, so will apply using a liquid applicator situated on the harvester’s final drop or elevator.
This should be situated where spray coverage is least likely to be affected by the wind.
While efficient, Nick Badger of Gavel approval holder Certis says one issue when applying tuber treatments on the harvester is that it covers all tubers, including those too large for the seed fraction.
Some growers opt to sell these “tops” into ware markets and because neither imazalil or alternative thiabendazole is permitted for use in ware crops, growers can run the risk of rejection or maximum residue limit (MRL) exceedances.
“With so many crop protection products already under the regulatory spotlight, it is important to ensure these products don’t inadvertently enter the food chain.
“Where growers are intending to sell tops into the ware market, it is best to gear up to split grade into store and only apply product to the size fractions suitable for seed,” he notes.
Whichever option is used, Mr Jessiman says regular maintenance and calibration of application equipment is important in achieving optimum product coverage.
“For roller tables, you need to ensure you have a constant throughput and the right flow from the nozzles. Also make sure you fill the table and get the tubers rolling 3-4 times during the spraying process. You don’t want them sliding along the table,” he adds.
Finally, once boxed and loaded into store, crops should be dried and cured with positive ventilation, ensuring there is not a significant gap between air and tuber temperatures. This can cause condensation and increase the risk of disease spreading, particularly silver scurf.
Store temperature should then be brought down to about 4C, either with ambient air if cool enough, or using a fridge, and the crop monitored regularly during the first 2-3 weeks of storage.
Monitoring methods include washing and visually assessing seed lots, sending samples for laboratory testing or placing tubers in a hotbox to encourage expression of any disease present.
Mr Jessiman adds that where possible post-storage, seed should be loaded into 1.25t bulk bags just in time for haulage to customers where possible to avoid heat and condensation developing in the centre.
Where storage in bags is enforced, avoid stacking them too close together and place on pallets to encourage air flow around the bags.
“Once delivered, encourage customers to empty seed straight into boxes and ventilate as soon as possible.”
Summary – minimising fungal diseases in potato seed
For more information visit the tubercare website.