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‘A better tuber rotter’: Pectobacterium parmentieri

Dr Eugenia Banks, potato specialist with the Ontario Potato Board in Canada, says in a recent email newsletter that Dr Steve Johnson (University of Maine) will give the first presentation of the upcoming Ontario Potato Webinars on March 4th at 11 a.m. His presentation will be titled “Expect Soft Rot and Blackleg to Increase!” Dr Johnson will focus on a very aggressive bacterium that cause soft rot and blackleg – Pectobacterium parmentieri.  

As an introduction, Dr Banks wrote a brief article about this bacterium, saying that Dr Johnson’s presentation will be much more comprehensive, and there will be time for participants to ask questions during the webinar in March.

Potato News Today is happy to publish Dr Banks’ article below with her permission. She can be reached at [email protected] for further information.

Last spring, I was wondering whether the incidence of Dickeya dianthicola was expected to be high in 2020, so I asked Dr Steve Johnson (University of Maine). To my surprise, he said that he was more concerned about Pectobacterium parmentieri than Dickeya. According to him, other members of the Pectobacterium genus are present and continue to cause disease, but not to the catastrophic level as has been associated with field and storage losses in Maine.

P. parmentieri is not a new pathogen; it was found in Europe in the 1960s but reported under a different name. Molecular techniques have made it possible to classify pathogens based on genetic characteristics, and there have been a lot of name changes. P. parmentieri was first reported in Maine by Steve in 2016. The Canadian Plant Disease Survey has also reported it in Canada. It is not a quarantinable pathogen.

Symptoms

Infected stem base

Parmentieri can cause tuber soft rot and blackleg. Affected plants are wilted and stunted with black soft rot extending upwards from the mother tuber. Contamination of foliage can result in aerial stem rot. The symptoms are very similar to those caused by the common soft rot bacterium, common blackleg and Dickeya.

At the recent virtual Idaho Potato Conference, Ken Frost, a plant pathologist with Oregon State University, said that Parmentieri is a lot more aggressive that the common tuber soft-rot pathogen; he called it a better tuber rotter.

Source of inoculum

The bacterium is found in the seed, usually at the stem end of the tubers or in and around the lenticels. Lenticels open when the soil is too wet providing an entry point for bacterial pathogens. Asymptomatic tubers may harbor Parmentieri in a dormant state like Dickeya. Steve Johnson mentioned that there is no evidence that Parmentieri survives in the soil in Maine.

Disease development

Infected seed can break down in the field reducing stands. Stems that do emerge are often weak with wilted foliage. These plants then spread the disease to healthy plants nearby. Parmentieri develops most rapidly during warm, wet summers and often kills the entire plant.

Disease spread

Infected tubers

Parmentieri can spread in the field below-ground, when water moves in soil saturated by rain or irrigation. Infected tubers contaminate harvesting and handling equipment. The infested equipment can transfer bacteria to tubers dug from healthy fields. The infected asymptomatic tubers may show no soft rot and may not decay in storage because of the low temperatures. However, the bacteria are likely to cause and spread disease if such tubers are planted.

I saw Parmentieri during the 2020 season. The incidence of blackleg was high and spread rapidly in a field where there was good soil moisture for the whole growing season.

Parmentieri was confirmed from a sample of infected tubers I sent to a CFIA accredited lab. The hot spots were flagged and not dug; the crop was marketed from the field. This is an excellent strategy to follow when dealing with aggressive pathogens.

Management

Integrating a disease management strategy throughout the entire potato planting, growing, and harvesting cycle is the key to mitigating losses from Parmentieri:
Plant certified seed
Grade seed carefully and grade out suspicious tubers
Disinfest seed cutters often
Avoid bruising and damage during planting and harvest, as well as during the loading of storage bins and shipping trucks, to reduce the risk of disease
Avoid harvesting wet fields or tubers with pulp temperatures higher than 65F, and promote rapid drying of harvested tubers.
Disinfest the potato handling equipment between lots. Potato handling equipment is anything that handles the potatoes, harvesters, windrowers, truck bodies, bin pilers, seed racks, and the like are included.

Note: This information is based on Bulletin #2448 (University of Maine Cooperative Extension) authored by Steven Johnson.

Source: Dr Eugenia Banks
Email: [email protected]
Related: Ontario Potato Webinars

Lukie Pieterse, Editor and Publisher of Potato News Today

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