Bacterial Ring Rot of Potato - Some Key Questions

What Causes Ring Rot?

Ring rot is caused by a bacterium:

• its properties differ significantly from fungi and viruses.
• many fungi can be controlled by chemical sprays, bacteria generally cannot.
• many viruses can be controlled by controlling their vectors, bacteria cannot.

• A great many ring rot bacterial cells are present in even a very small infection.
• Ring rot bacteria can be spread about readily.
• The extent that ring rot has spread by the time it is detected is difficult to know.

• Under the right conditions ring rot bacteria persist for many years.
• Ring rot bacteria survive in infected potato plant and tuber tissue - wherever they may be.
• Ring rot bacteria survive particularly well under dry or cold conditions.

• It came from an off-farm source.
• It persists at a low level on the farm site.

• Infected seed potatoes bought from another farm.
• Infected culls brought on-site as cattle feed.
• Un-cleaned commercial trucks or other contaminated equipment brought on farm.
• By man or animals from neighbouring, infected tablestock or processing crops.

• Insufficient clean-up after ring rot occurrence - gradually builds up again - spread by equipment.
• Maintenance of a low level of infection (latent) on farm - short generation time - flush through but back infection - spread by equipment.

• TOTAL clean up.
• Remove ALL potatoes from operation.
• "Flush-through system" do not keep seed on farm for several years subsequent to BRR.
• Clean all potentially contaminated surfaces.
• Disinfect completely all cleaned surfaces.
• Get rid of, or sterilize, all potentially contaminated disposables eg. gloves, knives, overalls, etc.

• Visual inspection - limited sensitivity due to latent infections, success varies with variety and growing conditions.
• Lab tests - better sensitivity than visual, but still limited sensitivity due to sample size and test factors.

• Ring rot is randomly distributed in a lot; each plant is equally likely to be infected.
• Is the assumption more true for small lots than it is for big lots ?????

• One lot planted with seed from different sources.
• Different equipment was used for harvesting, planting, etc. of a portion of the lot
• Seed stored in different places.
• One lot planted in different fields.
• Seed lot grown adjacent to non-seed field.
• Low incidence of BRR, seed cutters inoculates adjacent seed -NOT RANDOM.

• Testing CANNOT prove that a potato lot is free from ring rot.
• Testing is an extension of visual inspection.
• Testing sensitivity is limited by sample size and the laboratory test.
• Positive test result indicates, with a high level of certainty, that ring rot is present.
• Negative test result indicates that the level of infection is below detection level.

• ELISA - tests for presence of slime produced by the bacteria.
• IMF - tests for the presence of cells of the ring rot bacterium.
• PCR - tests for the presence of DNA of the ring rot bacterium.

• Excellent screening test.
• Subject to false positives.
• Sensitivity limited by testing in composite samples.

• Eliminates false positives of ELISA and confirms true positives.
• Discerns typical bacterium cell structure.
• Indicates level of infection in the plant or tuber.

• Extremely sensitive.
• Risk of contaminating sample from infected neighbouring lot.
• It’s a tricky test prone to test problems.
• Costly do do.
• Patent-protected method.

• Sensitivity is decreased by testing in composites.
• Sensitivity is decreased by using a positive/negative threshold to avoid false positive results.
• In addition to the nature of the test, sensitivity is affected by sample concentration, dilution, and volume of sample extract actual used in the test.

Positive test result:

• Err on the side of caution.
• A positive is only called when there is no doubt.
• This approach prevents a false BRR diagnosis.
• This approach favors the seller over the buyer.
• Negative test result:
• Can never be certain that there is no BRR infection below test sensitivity or outside of sampled units.