Dickeya Blackleg Updates And Considerations For 2016 Potato Crop

 

In 2014 and 2015, growers in many parts of the region started to notice black leg symptoms popping up in fields.  However, careful inspection of plants suggested that this was not your typical black leg, which is a seed piece issue resulting from contamination with Pectobacterium followed by excessively wet growing conditions.  In the case of the atypical black leg, growers noticed significant blanking after planting (Figure 1), and rapid wilting of plants during the season, particularly after very hot weather.  Infected stems were not mushy, as typically observed with black leg, but were dry, black, and hollow (Figure 2).

 

 

 

 

 

 

 

Tubers were macerated and had a tapioca-like appearance, but did not have the typical, pungent smell associated with Pectobacterium-derived black leg (Figure 3).  Samples from affected fields from several states were sent for assessment via DNA based techniques, and in all cases, the bacterial pathogen associated with plants was Dickeya dianthicola.  Only special, DNA-based techniques can accurately identify D. dianthicola.

 

 

 

 

D. dianthicola is an organism that has been present in the United States for many years, but only recently have we observed it causing issues in potatoes at a significant level.  The organism can degrade potato tubers much more rapidly than Pectobacterium at much lower levels of infestation.  Infection and growth of the bacterium can also occur at temperatures above what is considered optimal for Pectobacterium.  Like typical black leg, the main route of entry for the disease is contaminated seed pieces.  Under wet conditions the bacterium will either rot the mother tuber, resulting in poor emergence, or colonize the mother tuber and infect stems, resulting in above ground symptoms.  Stem symptoms originate from the bottom of the plant and move upwards.  Under appropriate conditions, bacteria can move from infected mother tubers to surrounding tubers.

 

Data from other countries indicates that the bacterium does not persist in the absence of a host.  Crops such as brassicas may serve as alternate hosts, as well as onions, although we do not know what role, if any, these crops may have in Dickeya outbreaks in potato growing regions of the USA.  Other species of Dickeya can colonize corn, but there is no research indicating that this is the case for this particular species (Figure 4).  Currently, all infected potato samples from the USA that have been tested have been identified as D. dianthicola.  There are plans to conduct surveys to better assess the population of Dickeya spp. associated with potatoes in the region and other areas in the United States where potatoes are grown.

 

Now that many growers are planting their potatoes they should consider the potential to see D. dianthicola in their fields this season.