Dickeya dianthicola in Potatoes, Update for 2016

Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu

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, 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. Tubers were macerated and had a tapioca-like appearance, but did not have the typical, pungent smell associated with Pectobacterium-derived black leg. 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, and 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 blackleg, the main route of entry for the disease is contaminated seed pieces. Under wet conditions the bacterium can rot the mother tuber, resulting in poor emergence. In some instances the mother tuber may not rot completely, but may be colonized by the bacterium, which then moves into the stem. When this occurs you may observe black lesions developing from the soil line, hollow, dry stems, and wilted plants. Very wet conditions can cause the bacterium to spread in the soil to new tubers, which may rot under favorable conditions.

Data from other countries indicates that the bacterium will not likely persist in the absence of a host. Crops such as brassicas and onions may serve as alternate hosts. Other species of Dickeya can colonize corn, but there is no published data indicating that is the case for this particular species. Currently, all samples taken from symptomatic potatoes have been identified as D. dianthicola. There are plans to conduct surveys to better assess the population of Dickeya species 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 be aware of the potential to see D. dianthicola in their fields this season. The following best management practices should be followed:

  1. Save your seed certificate. This will be useful in tracking down infested lots should this be an issue in the future.
  2. Avoid over irrigation, flooding
  3. Plant to maximize airflow
  4. Use a balanced fertilization program
  5. Ensure adequate calcium levels in soil
  6. Scout fields regularly. Initial symptoms will be areas of poor emergence. Symptoms later in the season include rapid wilting and death of plants, especially following very hot conditions. You may observe rotted tubers underneath symptomatic plants. If you see symptoms, have your county Agricultural Extension Agent take a look or contact me at 302-300-6962. We can help confirm if Dickeya is the issue.

If you have symptomatic fields and D. dianthicola is suspected/confirmed:

  1. Harvest these fields last
  2. Disinfest equipment with quaternary ammonium. Typical sanitation products such as bleach will not work against Dickeya spp.
  3. If potatoes are to be stored, ensure rooms are adequately ventilated and are maintaining cool temperatures
  4. Avoid including brassicas or onions in rotations
  5. Manage volunteer potatoes
  6. Avoid placing cull piles near fields or production areas
  7. Check your seed certificate