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Dickeya solani biosecurity case study – USA 2015

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In 2014 and 2015, growers in many parts of the eastern United States region started to notice blackleg symptoms in fields. However, careful inspection of plants suggested that this was not typical blackleg, which is a seed issue resulting from contamination with Pectobacterium followed by excessively wet growing conditions. In this case, growers noticed significant plant loss after planting, and rapid wilting of plants during the season, particularly after very hot weather. Blackleg caused by Pectobacterium differs from Dickeya in that it starts on the outside of stem tissue, infects through wounds, and then moves downward as well as upward causing stem rot that is dark brown. Infected stems were not mushy, as typically observed with black leg, but were dry, black, and hollow. Tubers had a tapioca-like appearance, but did not have the typical, pungent smell associated with Pectobacterium black leg. Plant tissue affected by Dickeya typically has an earthy smell; occasionally it has an offensive smell indicating soft rot bacteria are also present.

Samples from affected fields were sent for DNA identification, and in all cases, the bacterial pathogen associated with plants was Dickeya dianthicola.

Dickeya dianthicola has been present in the United States for many years, but it was only recently observed causing issues in potatoes at a significant level. Dickeya is an aggressive pathogen that can travel long distances and cause significant yield loss. Since first emerging, the pathogen has spread to potato crops in the mid-Atlantic region, and has been detected in Pennsylvania, Maryland, Virginia, North Carolina, West Virginia and Florida. Plant pathologists, agronomists and researchers are scrambling to control the disease as there is a lack of good chemicals.

Dickeya has the potential to cause more severe losses than species of Pectobacterium (aka Erwinia). Infection and growth of the bacterium can also occur at higher temperatures than what is considered optimal for Pectobacterium, consequently the greatest losses have been in the southern portion of the northeast and further south where total crop loss has occurred. Dickeya can degrade potato tubers much more rapidly than Pectobacterium, and at much lower levels of infestation. Very wet conditions can cause the bacterium to spread in the soil to new tubers, which may rot.

This bacterium is not known to be able to survive in soil more than about two months, which is not long enough to be a source of inoculum for the following growing season. As symptom development is limited by cool temperatures, typical in seed producing areas, the pathogen can be present in a plant but cause no distinctive symptoms. Infected seed can appear healthy. Dickeya developed in 2016 crops with seed that tested negative with the dormant tuber test.

Potato seed that is free of Dickeya is the best management practice for this disease so it is recommended that growers plant clean seed and as the bacteria can’t survive long in the soil, a one-year crop rotation is also recommended. At this stage, there are no known resistant varieties. The bacteria are also spread during handling, cutting, loading and harvest, which can contaminate other potatoes so careful handling is recommended.