Last year’s reports of Dickeya dianthicola from Maine to Michigan are this year’s concern for potato growers. What’s worrisome is that the bacteria is spread through seed potatoes causing blackleg and soft rot.          

These diseases are very similar and the different names indicate where the disease occurs says Tracy Shinners-Carnelley, director of research and quality enhancement, Peak of the Market and one of the organizers of the recent Manitoba Potato Production Days in Brandon. This topic was on the agenda to allay concerns. Many people in the North American potato industry are talking about Dickeya spp. this winter.

Blackleg affects the vines during the growing season. Tuber soft rot affects tubers in the field and during storage and transit.

Both Pectobacterium and Dickeya spp. cause similar wilt and decay symptoms in plants and tubers. The symptoms are often indistinguishable.  Pathogen “behaviour” is different. Dickeya spp can initiate disease with less inoculum, has greater ability to spread through vascular tissue, is more aggressive and needs a higher optimum temperature to flourish. 

Dickeya appears to be less hardy in the soil and outside of plants compared to Pectobacterium. It’s unlikely that Dickeya can overwinter freely in the soil.

There are multiple species of Dickeya. Shinners-Carnelley says the first report in potatoes was in the Netherlands and most early findings were of D. dianthicola. A more aggressive strain emerged in the mid-2000s and was named D. solani.

From 2007-2009, Solke De Boer, senior research scientist with the Canadian Food Inspection Agency (CFIA) in Prince Edward Island, led a survey to identify the pathogens associated with bacterial stem rot in Canada. The team’s results indicated that the predominant pathogen isolated from infected stems and tubers was P. atrosepticum. Two isolates were presumptive positive for Dickeya spp, but further testing confirmed they were not D. solani. 

“I’m not aware of any more recent Canadian blackleg surveys,” says Shinners-Carnelley. “While the incidence of blackleg can fluctuate from year to year, there are no reports of significant occurrence and losses due to blackleg. In Canada, the CFIA’s Seed Certification Program has strict tolerances for blackleg and this may be largely responsible for limiting the incidence of blackleg in the potato industry.”

In recent years, there have been some severe losses in the eastern U.S. resulting from seed decay caused by D. dianthicola. This occurrence has created much discussion and concern about Dickeya and the potential impact that it could have on the industry.  Seed Certification Programs in the U.S. do not have established tolerances for blackleg.

This background is for information purposes only and to make growers aware of the risk and to be proactive to prevent the introduction and establishment of any new disease threats, including Dickeya spp. 

Going forward, Shinners-Carnelley recommends that growers follow best recommended practices for management of all bacterial diseases. This includes using clean seed, cleaning and disinfecting seed cutting and handling equipment, planting seed that is well suberized, and into well drained soil 10 – 13°C. Handle tubers in such a way to minimize bruising and wounding. Store tubers with good air flow, humidity and as cool as possible. 

In conclusion, blackleg and tuber soft rot can be caused by a few bacterial pathogens. A more aggressive pathogen has emerged. These pathogens may “behave” differently and can be very difficult to distinguish by field symptoms. Management strategies remain the same. Be aware of the risk and always follow biosecurity best management practices. 

 I’m not aware of any more recent Canadian blackleg surveys.

~ Tracy Shinners-Carnelley