Improved seed potato management to minimise losses due to blackleg and Pectobacterium species

Aims

1. To identify the sources, including the main source, of contamination of high grade seed stocks;
2. To monitor the seed production process for points within the system that may lead to an increase or decrease in bacterial contamination;
3. To test the validity of novel storage control methods.

Background

Bacteria such as Pectobacterium atrosepticum (Pba) are able to thrive in wet conditions with low oxygen levels and this poses a challenge when management of the issues they cause (blackleg, soft rots) is dependent on minimising the number of bacteria on tubers. In the field, wet weather and waterlogging can lead to high levels of disease developing from seed that initially had few bacteria. Conversely, heavily contaminated seed can give rise to little or no disease when conditions are dry. Given that field conditions can be erratic, a more reliable period for management of bacterial numbers is during storage, when more of the key environmental conditions are under a grower’s control.

 This project followed on from earlier work on Pba looking at how the bacteria first get into the seed multiplication chain. It had a focus on seed storage and the researchers also reviewed the published information on the factors which contribute to variation in bacterial numbers on tubers, including humidity, curing, storage conditions, and they also summarised the efficacy of physical and chemical control measures aimed at reducing bacterial loading.

Results

The information from the review (11120031 Review_Final) and the results from the project have informed the summary below. A more detailed explanation, and rationale, for each practice is provided in Table 30 of the Final Report.

Best storage practice for seed potato crops, to minimise contamination by bacteria causing soft rots and blackleg

Harvesting

1. Pick off rots, mother tubers, clods and stones on the harvester
2. Ensure skin set and minimise damage on the harvester
3. Minimise soil in boxes
4. Do not overfill boxes
5. Note tuber temperature

Action from early storage onwards

1. Ensure air movement in store is optimal by correct layout of boxes and reducing shortcuts in air movement
2. Ventilate with air +/- 4^oC of the temperature of tubers in store (preferably below)
3. Dry tubers as quickly as possible
4. Remove field heat and heat of respiration & CO₂
5. Cure wounds
6. Reduce temperature at the earliest possible opprotunity 
7. Monitor stocks regularly
8. Avoid condensation at any stage of storage.  Pay particular attention when temperatures rise in early spring and when stores are being unloaded
9. Limit condensation after sprouting occurs

Grading

1. Clean the grader thoroughly before the grading season starts
2. Where tubers are warmed prior to grading, ensure the warming process doesn’t cause condensation
3. Minimise damage by cushioning where tubers drop.  Step or jump graders may create damage or expressed moisture on tubers
4. Minimise application of moisture onto tubers: to reduce dust where the box tipplers empty onto the grader and where tubers are sprayed with fungicide
5. 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

Post-grading handling and seed transport

1. Seed retained on farm in boxes should be dried and returned to a cold store as soon after grading as possible
2. 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
3. Seed retained in boxes for transport should be retained in ambient conditions in a ventilated area until uplifting
4. Ensure that seed is transported to the purchaser as soon after grading as possible
5. Ensure that the purchaser understands how to handle seed on arrival

Pre-planting

1. Minimise condensation when removing seed from storage
2. Minimise de-sprouting at planting
3. Control Rhizoctonia solani on seed prior to planting using seed tuber fungicides

Within the project the new work to identify the major routes of initial contamination of high-grade tubers has shown that none of the ~30,000 minitubers tested were contaminated with Pba. Therefore, it is extremely unlikely that minitubers are the source of Pba infection in high grade seed potatoes.

High grade seed crops grown in geographic isolation from other potato crops (on Orkney) still became contaminated with Pba.  The nearest crops with blackleg were within a 30km radius of the isolated crops and these were considered to be the potential source of Pba although the possible pathway of such transmission remains unknown. Spore traps placed at 10 mainland Scottish locations were sampled twice a week for 2-3 months during the potato growing season but no Pba was detected. A new project will look at potential routes of infection in more detail

Preliminary trials carried out on AHDB Spot Farms had shown some promising results using a NaCl product as a desiccant and therefore, the toxicity of salt to Pectobacterium species was studied in lab conditions. None of the bacteria tested were able to grow on plates containing 8% salt. Glasshouse/field experiments would now need to be carried out to determine what concentrations of NaCl are required to both desiccate the plant and kill any Pectobacterium present.  

In the trials to evaluate potential control options in storage, none of the treatments tested (UV or bacteriophage) caused a reduction in blackleg symptoms in crops grown from treated seed tubers.