19 April 1996

Dont run the ring rot risk

By Andrew Blake

RING rot is a bigger threat to the UK potato industry than brown rot, according to a leading pathologist.

"The risks from potato ring rot must not be overshadowed by the more immediate concerns of brown rot," warns Dr David Stead of the Central Science Laboratory, Harpenden.

Both diseases are caused by bacteria. But unlike brown rot, which until recent events was seen mainly as a disease of warmer countries, ring rot mostly occurs in cool, temperate climates – like those of England, Scotland and Wales.

A recently introduced EC Directive, if adhered to, should provide a solid defence against UK infection, says Dr Stead. It is aimed specifically at controlling and eventually eradicating ring rot from member states, requiring each to conduct surveys.

"I certainly feel we are doing enough at the moment to keep it out." Unlike other EC states, the UK only rarely imports seed from non-member countries, he notes. "But constant vigilance will be needed. Ring rot is the most important disease of the seed potato industry in the USA where it is known as the curse."

First described in Germany in the early 1900s it was introduced to Canada in the 1930s and soon spread right across North America. In Europe it has also been found in Denmark, Sweden and Finland.

Published data on yield effects in the field and in store from ring rot is scant but serious losses have been recorded. "This lack of information may be a deliberate omission on the part of at least some infected countries," he observes.

Most countries with active certification programmes require a "nil tolerance" of ring rot in seed. Importers that do not yet have the disease usually demand pathogen-free seed and require exporters to guarantee its status, says Dr Stead.

It is ring rots complex biology that makes it so dangerous, he believes. "It has a long latent phase."

In some varieties this may last several generations. Complicating the picture is the fact that some varieties, for example Desiree, rarely or never display wilting or rotting, but continue to allow the bacterium to multiply. Some such symptomless carriers, once thought to resistant but now termed tolerant, have been outlawed in the US, he notes.

"Despite costly testing no country with ring rot has successfully eradicated this insidious pathogen," says Dr Stead. "They have proved in the US that you cant control it by visual inspection. You have to have testing. But it is far harder to spot than brown rot." Serological tests often give false positive results because of the presence of other closely related bacteria. "It can be remarkably difficult to detect and diagnose."

Testing method

The EC approved method of testing using inoculated egg plants can take seven weeks and be unreliable unless glasshouse temperatures are kept below 30C (86F), he adds.

Dr Stead has another concern. Ring rot, which is mainly tuber-borne, does not survive or spread well in soil or water – unlike brown rot. But it can contaminate machinery and stores surviving in a dry form to infect subsequent crops.

"If ring rot ever got into the UK it could be very serious, especially for the British seed industry," says Dr Stead. &#42

&#8226 Caused by bacteria, like brown rot.

&#8226 Favoured by UK type climate.

&#8226 Serious losses recorded.

&#8226 Long latent phase.

&#8226 Hard to detect & diagnose.

&#8226 Tolerant varieties confuse the picture.

&#8226 Survives in dry on machines.


Potato ring rot, which is more suited to UK conditions, poses a bigger threat than brown rot, says a CSL scientist. (CSL Crown Copyright)