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Is water a source of blackleg contamination?

Publication Date: 
4 December 2018

In Britain we produce some of the most sought-after seed potatoes in the world, due to the quality and ‘high-health’ status of our crop.

Maintaining this position as a global leader requires a combination of exacting on-farm standards and a willingness from those in the industry to confront issues and make improvements on a continual basis.

AHDB, SASA, SRUC Fera, SBCSR, JHI and a number of industry partners have combined on a three year research project that looks at the causes of blackleg in seed potatoes – specifically instances caused by Pectobacterium species. Blackleg in Britain is normally caused by the bacterium Pectobacterium atrosepticum (Pba).

Blackleg – how, when and where?

At the Seed Industry Event, held in St Andrews in November, the project’s leader Prof Ian Toth of the James Hutton Institute and Scotland’s Plant Health Centre revealed some of the findings of the study.

“Irrigation could be an important factor in blackleg development” he said.

“We took clean, tested and verifiably Pectobacterium free mini-tubers for this trial and grew them with and without irrigation through a ‘first generation’ growing season.

“The unirrigated crop came out of the ground 100% free of blackleg – but in the irrigated tubers we saw that up to 7% showed blackleg.”

It is not normal practice to irrigate first generation seed, however by doing so in this study using mini-tubers known to be Pba free, the researchers are able to isolate water as a potential source of blackleg infection.

This is early in the research cycle, and there are further replications of the trial to come, but it could be an important step in finding one of the sources of the disease and, ultimately, providing breeders and growers with information that will help keep crops blackleg free.

“We now know blackleg can develop directly from bacteria in the environment” Prof. Toth continued.

“In future trials, we need to learn more about the role that irrigation is playing in the infection process and, ultimately, in blackleg development.”

This work will continue as the team build a better picture of environmental factors that affect the various strains of Pectobacterium.


Some free living nematodes (FLN) feed on the roots of plants stunting growth, and may affect yields. Prof. Toth and the team are investigating their links to the spread of blackleg.

“Irrigation can take these mixed FLN populations into the top soil, and in a pot trial we found ten times as much bacterial inside the plants when nematodes were present in the soil.”

While not the case this season, previous AHDB funded studies, including one conducted by Dr Mark Stalham at NIAB CUF, have shown that even some of the most effective growers will err on the side of over-irrigation. A problem exacerbated by the unpredictable nature of rainfall in this country.

“Reducing irrigation in ware crops, especially in the presence of FLN, may help to reduce blackleg caused by direct environmental infection” stated Prof Toth, although he acknowledged that there is still much to be done to verify the findings”.

In store

Stuart Wale from SRUC has been examining the role of storage in blackleg control, as part of the project. This is an area that has received AHDB research funding in the past.

Prof. Toth said that recent advances in technology and new novel uses for them in crop storage meant that there was call for a revisit in this area.

“An important factor in achieving good quality ware potatoes is in the quality of the seed potatoes so we need to know the optimal time when refrigeration should be switched on, and whether seed potatoes benefit from being warmed up before grading and planting.”

This new project includes backing from several private firms from industry and aims to uncover practical solutions by mixing new technologies with established best practice.

Burn-down without Diquat?

Combining data for the project with a separate trial conducted by Jim Reid from Milton of Mathers Farm and colleagues, it was found that blackleg-causing pathogens tend to build up around the stem base of potato plants around the time of desiccation, or ‘burn-down’, before harvest.

The loss of diquat will be damaging to the sector for many reasons, and it will also challenge our ability to control the spread of blackleg.

“The use of the chemical results in a faster death, and it killed off the bacteria as well” continued Prof. Toth, who has found from Jim’s trials that a combination of alternative methods (Spotlight plus and Gozai) may offer the best replacement solution.

“There are higher blackleg levels when carfentrazone is used but, on the other hand, there is less damage.

“The trick to success appears to be to wait from the first spray for five to seven days before applying the flail and to follow this with a second spray that targets the stems” he concluded.

The future: partnership and collaboration

Claire Hodge, AHDB Senior Knowledge Exchange Manager for Scotland, said:

“In recent years the seed industry has made a concerted effort to engage with the research community and tackle blackleg head-on.

“The involvement of growers such as Jim Reid and James Thorburn is a great example of how, when people are prepared to open their farm gates, it can stimulate discussion that we can all benefit from.”

James Thorburn is a former Seed Manager at Greenvale in Scotland, who has become one of Britain’s (the world’s?) most Northerly seed potato producers by launching Farewell Farms in Orkney.

He presented during the blackleg workshop at The Seed Industry Event on his reasons for choosing an isolated base for early generation (PB 1 & 2) seed production and believes that when it comes to Blackleg, working together is vital.

“We’ve been talking about blackleg for years, so it is great to see things moving forward. What do our competitor countries do, both in Europe and further afield? They work hard on knowledge exchange and it benefits everyone. That is why I was pleased to be involved in this workshop.”

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