By Beth K. Gugino
In May 2016, an aggressive form of black leg caused by the bacterial pathogen Dickeya dianthicola was confirmed in a potato seed lot being grown in New Jersey. This is the same pathogen that caused widespread yield losses across the region in 2015 when it was first confirmed in the U.S. It subsequently has been detected and confirmed in DE, PA, MD and VA this season. Pectobacterium atrosepticum which has long been associated with blackleg has also been detected.
Blackleg symptoms on potato.
Symptoms of blackleg caused by Dickeya infection are very similar to those caused by Pectobacterium atrosepticum. It can manifest itself as poor seed emergence as well as a water soaking and blackening of the stem pith and vascular system under wet conditions. The leaves then become yellow and curl upward. It also caused a soft rot of potato tubers since the bacteria are able to degrade or feed on the pectate found in the cell walls causing them to break open and leak their cell contents. The disease is favored by cool temperatures below 65°F and moist conditions.
Unfortunately, the pathogen is seedborne and once symptoms develop in the field, management tools are severely limited and primarily preventative. Since the pathogen is primarily seedborne, it tends to be associated with specific seed lots. In one case this season, a seed lot planted on two different farms in two neighboring states both developed blackleg which was confirmed to be caused by Dickeya dianthicola. Although this problem has primarily been associated with some seed lots originating from Maine, it has also been confirmed in other seed production regions of the U.S. and Canada.
Efforts are being made to address the situation however despite potential losses of up to 100%, APHIS recently announced that the pathogen has been classified as a non-reportable/non-actionable pathogen which indicates that no national level regulatory action will be taken. A link to the USDA/APHIS website for information on Dickeya dianthicola detection and control can be found on the USDA APHIS website.
This season it is important to identify problematic seed lots and work to eliminate them from the production system. If you suspect black leg in your field, please contact by email at bkgugino@psu.edu or by phone at 814-865-7328 or the Penn State Plant Disease Clinic. We are interested in collecting samples for confirmation of blackleg as well as determining whether it is caused by Pectobacterium or Dickeya.