By Noah Rosenzweig
Aerial stem rot, blackleg and tuber soft rot was recently detected in Michigan potato fields. Like the past two growing seasons, potato aerial stem rot, blackleg and tuber soft rot disease continues to be a problem.
Posted on June 12, 2017 by Noah Rosenzweig, Saltanat Mambetova, Paula Somohano and Chris Long; Michigan State University Extension, Department of Plant, Soil and Microbial Sciences
Aerial stem rot, blackleg and tuber soft rot of potato have been found in fields in MIchigan. Tuber soft rot results in rotting tissue that is mushy, slimy and water-soaked (Photo 1). Soft rot of the potato seed piece can occur following planting and results in poor emergence or create foliar symptoms) such as weak plants with curled and drooping leaves that often resemble wilt disease or water deficiency (Photo 2).
Photo 1. Tuber infected with Dickeya dianthicola. Rotting tissue is mushy, slimy and water soaked; infected areas often turn brown or black around the rotting area upon exposure to air.
Photo 2. Symptoms of tuber soft rot infection and blackleg (arrows) can result in a field with poor emergence.
Aerial stem rot symptoms appear as black lesions that are produced on infected stems (Photos 3-5). Blackleg symptoms appear as black lesions at the base of the stem (Photos 3 and 5) and can quickly girdle the stem as they expand upward (Photo 3 and 6). Unlike tuber soft rot, infected plant stems have a black appearance (Photos 1-6).
These diseases were not a major issue in the U.S. until the 2015 growing season. During 2015, infected seed and a wet June were contributing factors in a high incidence of aerial stem rot in parts of Michigan.
Blackleg, aerial stem rot and tuber soft rot are caused by either the bacteria Dickeya spp. or Pectobacterium spp. Identification by DNA analysis is under way by Michigan State University’s Potato and Sugarbeet Pathology Lab to identify the species of bacteria (Dickeya spp. or Pectobacterium spp.) from the submitted samples. The confirmed infected variety of potato was Reba.
Photo 3. Aerial stem rot symptoms appear as black lesions that are produced on infected stems (A). Blackleg symptoms appear as black lesions at the base of the stem (B), and can quickly girdle the stem as they expand upward (C).
Photo 4. Aerial stem rot infected plant stems are inky black and can extend from the decaying seed piece up the entire length of the stem or appear only on upper parts of the stem (arrows).
Photo 5. Aerial stem rot infected plant vascular system, or pith, may be decayed and discolored (gray) above the severely decayed portion of the stem. In time, leaves turn yellow and leaflet margins tend to roll upwards.
Photo 6. Blackleg symptoms appear as black lesions at the base of the stem.
The first step in effective disease management of potato is accurate pathogen identification and diagnosis. Visually, Dickeya and Pectobacterium cause similar symptoms, and culture and DNA based detection methods are now available to distinguish between these two pathogens.
Because of similarities between the pathogens and symptoms, growers should send a sample of a plant with a suspected pathogen to a diagnostics lab, such as MSU Diagnostic Services, for accurate identification to determine effective management actions.