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In potato plants with milder symptoms of Dickeya, leaves are curled. (Photo: Amy O. Charkowski)

Guarding Against Dickeya

With clean seed recognized as the best defence against this new blackleg-causing pathogen, the demand for Dickeya testing is growing fast.

After triggering major crop losses in the United States, Dickeya has been dominating the potato meetings circuit in North America for the past year or two. In Canada, some of the discussions revolve around the consequences for growers should the new disease makes its way into this country — but some industry experts maintain Dickeya is likely already here.

“I don’t know what the extent is [in Canada], but we found Dickeya in samples that were sent to us from Ontario and from New Brunswick,” says Gary Secor, professor of plant pathology at North Dakota State University (NDSU). “I think that certainly the potential is there for Dickeya to be present and I think the Canadian farmer should be aware of that and take it seriously.”

Researchers in the Plant Pathology department at NDSU, along with six labs in the U.S. and Canada, has been conducting research into Dickeya testing as part of an initiative to develop better and more standardized methods for diagnosing the disease.

Led by Amy Charkowski, head of Colorado State University’s Bioagricultural Sciences and Pest Management department, the collection of researchers also includes Mathuresh Singh, director of Potato New Brunswick’s Agricultural Certification Services Inc. (ACS) lab in Fredericton, N.B. Both Charkowski and Singh spoke to Canadian growers about Dickeya at Manitoba Potato Production Days in Brandon, Man., in January.

Because Dickeya spreads primarily through infected seed, testing potato seed lots for the pathogen (or adding Dickeya to wider disease screening) is becoming an increasingly popular choice for potato seed growers and buyers in the U.S.

Dickeya testing is also on the rise in Canada. A&L Canada Laboratories in London, Ont. has been offering Dickeya testing since 2015, and company officials say more and more customers are requesting the service as a way to ensure no Dickeya-infested seed is planted in their potato fields.

The ACS lab in New Brunswick started Dickeya testing in mid-2016. Singh says it’s because most of New Brunswick’s seed potatoes are sold in Maine and other states along the Eastern Seaboard of the U.S., and some of those buyers are now asking for Dickeya screening.

“Our lab is owned by the potato growers of New Brunswick and we offer any services they need, so that’s why we got involved in the Dickeya testing — not because we had a problem here but just because there’s a demand for this,” Singh says.

Origins of Outbreak

Dickeya is one of a number of bacterial pathogens that can cause blackleg in potatoes. Capable of spreading over long distances through infected plant material that can include other vegetables and some ornamental plants, Dickeya species have been affecting potatoes in Europe since the 1970s. Three different genotypes of the disease have been found in the U.S. but most of it is Dickeya dianthicola.

An example of a stem lesion caused by Dickeya. The browning of the vascular system above the lesion is a typical Dickeya symptom. (Photo: Amy O. Charkowski)

Experts agree that prior to the D. dianthicola outbreak in the U.S. in 2015, the pathogen was probably present in seed potatoes and farms in affected states for several years. It is believed rains in 2013 and 2014 likely helped spread the disease, but symptoms were held in check by cooler weather; Dickeya symptoms typically develop when temperatures exceed 25 C.

Warmer temperatures in 2015 resulted in a major outbreak on commercial farms in Maine, which then spread to other states along the Eastern Seaboard and in the Midwest. According to Secor and Charkowski, Dickeya has now been confirmed in 20 U.S. states.

Charkowski says the Dickeya situation wasn’t as bad last year, likely due to fewer infected seed lots and less favourable weather conditions for symptom development, as well as increased awareness of the disease among farmers. However, she adds, some growers in New York, New Jersey, Rhode Island, Pennsylvania and Florida still experienced serious crop losses in 2016.

Secor says the Dickeya found in the Ontario sample that his lab tested in 2015 came from seed potatoes imported from the northeastern U.S. Due to confidentiality agreements, he couldn’t provide any details about the Dickeya finding in the New Brunswick sample other than it happened in 2016.

Charkowski says that given the nature of the potato seed trade, “I’d be very surprised if [Dickeya] wasn’t already in Canada.” She adds the disease could be in a latent condition and wouldn’t express itself in potato fields in an area until weather conditions were right.

If that were to happen in Canada, Charkowski believes Dickeya symptoms likely wouldn’t be as dramatic as in areas further south because of the cooler Canadian climate. “In places like Manitoba, I’d expect there would be fewer problems than we’ve seen in the southern U.S. [Dickeya] seems to be a problem that’s exacerbated by heat and high humidity,” she says.

Singh thinks what Charkowski describes is a possibility, but adds “there is no indication right now that [Dickeya] could become a huge issue in Canada.” One reason why is that the Canadian Food Inspection Agency’s (CFIA) seed potato inspection program has strict tolerance for blackleg infection, which helps ensure fields are free from pathogens that cause the disease.

In the U.S., blackleg has not been part of the industry’s seed certification process. “One of the issues is the blackleg that’s produced by Dickeya is not as visible as the blackleg produced by Pectobacterium. It’s more difficult to see Dickeya and we don’t yet have a good idea of what the tolerance should be,” says Secor.

“I think what generally is happening is that the certification industry is waiting for science to catch up with more information about epidemiology of Dickeya before they make any changes in the rules and regulations,” he adds, noting that the state of Maine introduced a tolerance for blackleg in its seed certification program in the fall of 2016.

Current testing methods for Dickeya involve a DNA analysis technique called polymerase chain reaction (PCR) assays. After analyzing various PCR techniques to determine which ones are the most successful, Charkowski’s group provided recommendations for PCR testing for the potato industry.

This protocol includes using one test to detect the Dickeya genus and a different test to identify which species is present, but Charkowski says researchers are hoping to develop an all-in-one method that’s less time-consuming and expensive.

In an effort to devise more tools for fighting the disease, including developing Dickeya-resistant potato varieties, Charkowski, Secor and Jay Hao, a plant pathology professor at the University of Maine, submitted a major grant request to the U.S. Department of Agriculture in March 2017.

“If we get funded, one of the objectives is to develop a faster and more robust testing procedure for Dickeya,” says Secor.

He adds another key objective within the grant proposal is finding out how Dickeya arrived in the U.S. in the first place. “What we need to do is find the source because we don’t know how Dickeya is getting into potato seed lots; if it’s coming from water or through horticultural plants or in landscape material or flower bulbs that are being brought in,” he says.

“If we can identify where its coming from then we eliminate that source, we can flush out all the infected seed lots [with testing], and eventually we’ll get back to having clean seed. That’s really going to be the secret, I think — getting the clean seed.”

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