Storage practices to minimise the risk of soft rot and blackleg in seed potatoes

Guidelines on the storage practices for seed potato crops have been developed from the results of a research project completed in 2020 and from the experience of the project scientific partners. The tables highlight the key practices and provide a description of why they are important.  

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The tables below are taken from the final report for project 11120031. There is also a review of the effect of storage conditions on the bacterial loading of seed potatoes available from the same project.  

Harvesting

Pick off rots, mother tubers, clods and stones on the harvester

Rotting tubers entering store will spread bacteria throughout the stock through surface contact and act as foci for further soft rotting, generating heat which will cause condensation and moisture on adjacent tubers.  If possible, identify and record the type of rot.  Where rotting is severe and difficult to pick off during harvesting, keep the stock separate and provide prolonged ventilation. Clods and stones can result in increased damage. 

Ensure skin set and minimise damage on the harvester

Skinning and wounding will occur where the skins of tubers are not set prior to harvesting.  Damage from skinning and wounds create niches where soft rotting bacteria can infect and multiply.

Minimise soil in boxes

A high dirt tare in boxes reduces effective air movement (passive or positive) and drying – allowing bacteria to multiply.

Do not overfill boxes

Over-filled boxes will restrict airflow after loading into store.

Note tuber temperature

The tuber temperature provides a guide to the temperature of air required for ventilation and indicates how quickly wound healing will take place.

Early storage onwards

Ensure air movement in store is optimal by correct layout of boxes and reducing shortcuts in air movement

Optimising air movement improves drying and cooling, ensures efficient removal of heat generated by the crop and saves money.

Ventilate with air +/- 4oC of the temperature of tubers in store (preferably below)

Frequently, outside air is used for ventilation but fluctuating outside air temperature can result in condensation on the crop.  Mixing inside and outside air can minimise condensation risk.  There is no risk of condensation where air temperature used for ventilation is lower than the stored crop.  During the early period of ventilation, moist air is expelled to the outside.  Where a fridge unit is used for ventilation, moisture is removed by the action of the fridge.

Dry tubers as quickly as possible

Positive ventilation (air moved through boxes of tubers) is more efficient at drying than passive air ventilation (air moving along pallet apertures).  Initially, ventilation will be continuous but once tubers are dry and cool, ventilation will be intermittent and sufficient to sustain a uniform temperature.

Remove field heat and heat of respiration & CO2

When tubers are harvested, they may have been lifted from warm soils and thus have residual heat.  At the same time, when tubers are harvested, their rate of respiration will be high.  Respiration results in heat and CO2 generation Where heat is not removed it may result in condensation and create conditions for bacterial multiplication.

Cure wounds

With seed crops, which are usually harvested earlier, tuber temperatures are frequently greater than 10oC.  At these temperatures, primary suberisation occurs in a few days and a specific wound healing period is not required.  Where tuber temperatures are less than 10oC, longer periods are required for curing.

Reduce temperature at the earliest possible opportunity

Once seed reaches 4oC or lower, bacterial multiplication is minimal.  Reducing temperature in store can begin before the store loading is complete provided the temperature of the crop entering store is close to that of the crop already in store.  Once a store is loaded, it is good practice  to begin to reduce temperatures to the holding temperature as soon as possible, usually by 0.25 to 0.5oC per day. 

Monitor stocks regularly

After store loading, stocks of seed should be monitored every few days initially and at least weekly once final storage temperatures are reached.  Besides looking for evidence of free moisture, rotting and sprouting, tuber temperature should be checked using a hand-held thermometer and compared to within box ventilation system thermometers.

Avoid condensation at any stage of storage.  Pay particular attention when temperatures rise in early spring and when stores are being unloaded

Condensation is avoided by maintaining as even a temperature in store as possible, preferably with less than 1oC difference across a store.  Intervals for re-circulation of air within a store should be adjusted to minimise differences in temperature. Identify localised areas in the store that are prone to condensation e.g. due to restricted air circulation or temperature fluctuations.

Limit condensation after sprouting occurs

Maintaining a uniform store temperature reduces early sprouting but it may be inevitable in seed stores with multiple stocks. Once sprouting begins, tuber respiration and heat generation increases, with an enhanced risk of condensation.  Ventilation duration or intervals between ventilation may need to be adjusted to prevent condensation.

Grading

Clean the grader thoroughly before the grading season starts

Removing built up soil on all elements of a grader will reduce contamination by pathogen spores left from the previous season’s grading

Where tubers are warmed prior to grading, ensure the warming process doesn’t cause condensation

In practice, this means warming  slowly or where more rapid warming is used, positive ventilation is applied to remove condensation soon after formation.

Minimise damage by cushioning where tubers drop.  Step or jump graders may create damage or expressed moisture on tubers

Where expressed moisture is present, aim to ventilate the box or bag the seed is graded into - if possible

Minimise application of moisture onto tubers: to reduce dust where the box tipplers empty onto the grader and where tubers are sprayed with fungicide

Where excess moisture is applied, aim to ventilate the box or bag the seed is graded into - if possible.

Use of pressurised water atomisers rather than water applied through spray nozzles will reduce dust with minimal wetting of tubers.

Where possible, avoid grading high grade stocks immediately after low grade stocks. Ideally, use a separate grading line for the highest grade seed stocks or grade them first

Bacteria will spread through surface contact within and between seed stocks during handling and grading. 

Post-grading handling and seed transport

Seed retained on farm in boxes should be dried and returned to a cold store as soon after grading as possible

The action of the fridge and ventilation in the cold store should remove any surface moisture from the tubers.

Seed in polyprop (jumbo) bags should be retained in well ventilated ambient conditions, preferably placed upon a pallet and with gaps between bags until uplifting

Air movement through polyprop bags is restricted and there is potential for bacterial multiplication where surface moisture persists, especially under ambient temperatures. 

Seed retained in boxes for transport should be retained in ambient conditions in a ventilated area until uplifting

There is greater opportunity for surface moisture to be removed from tubers stored in boxes than polyprop bags.

Ensure that seed is transported to the purchaser as soon after grading as possible

Particularly with polyprop bags, delays in transport can result in bacterial multiplication.

Ensure that the purchaser understands how to handle seed on arrival

If not for immediate planting, seed transported in boxes should be placed in a cool ventilated store on arrival.  Seed transported in polyprop bags should be decanted into boxes and the boxes placed in a cool ventilated store.

Pre-planting

Minimise condensation when removing seed from storage

Where seed is removed from cold storage it can be difficult to prevent condensation.  Some growers plant straight from the cold store, others remove boxes into ambient storage several days before planting to allow acclimatisation.  Condensation can still occur by this latter method and where possible boxes should be ventilated.

Minimise de-sprouting at planting

Broken sprout tissue provides an ideal medium for Pectobacterium atrosepticum (Pba) multiplication and an entry point into tuber tissue.

Control Rhizoctonia solani on seed prior to planting using seed tuber fungicides

Stem canker on developing sprout tissue can provide an ideal entry point for Pba and initiation of blackleg.

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