This disease can cause loses by the wilt and decline of plants. This article describes the symptoms and management of two similar diseases of potato both known as black leg caused by different bacterial pathogens.
Both black leg diseases have similar symptoms. Often the first symptom one might observe is reduction in plant emergence. After emergence, one might notice stunting or wilting of the plant. Eventually, the stem may appear black and rotten. These symptoms usually occur from the ground up. This is because the infection often starts in the seed potato and travels up the stem. Severely affected plants may collapse.
Figure 1. Black leg of potato may cause a wilt of potato plants.
Figure 2. Black leg of potato often causes the stem to become black and rotten.
Black leg symptoms can also be observed on upper portions of the stem; this is known as aerial black rot. Such symptoms can be caused when bacteria are blown onto the upper stem from affected areas on the lower stem. Wounds on the stem are often where black rot symptoms start. Rain splash is usually necessary to cause the spread of aerial black rot.
The bacteria that cause black leg can also cause tuber rot symptoms in storage or transit. Small, off-white lesions may start on the surface of the potato and expand into the potato. Tubers may become slimy and mushy.
Black leg of potato usually starts by introduction of infected potato seed pieces. However, the bacteria may survive in soils for approximately 2 years or longer in plant residue. The bacteria may also be introduced from cull plies or volunteer potatoes from previous years.
Until about 2007, black rot was known to be caused by bacteria identified as Pectobacterium spp. (older literature may identify this bacterium as Erwinia spp.). More recently, bacterium in the Genus Dickeya have been identified in the US. The Dickeya bacteria are known to be generally more aggressive than Pectobacterium. Black leg of potato caused by Dickeya has been confirmed in Illinois, but not in Indiana. Black leg caused by Pectobacterium is widespread throughout the US.
The first step in successful management of black leg of potato is to accurately identify the disease. It is important to know which species of bacteria may be causing black leg in your operation. Plus, other diseases and maladies may be mistaken for black leg. Since the species of bacterium that cause black leg cannot be differentiated on site, a laboratory diagnosis is recommended. Contact the Purdue Plant and Pest Diagnostic Laboratory if you suspect black leg of potato or Dan Egel (contact above).
Seed source is an important factor in avoiding black leg of potato. Use seed certified as black leg free. Avoid planting potatoes in a potato field known to recently have been confirmed with black leg. Remember that cull plies may also spread the pathogen. During the season, excessive nitrogen and irrigation may increase severity of black leg. Copper products may decrease the spread of aerial black leg. Inspect tubers carefully for symptoms of black leg and maintain storage conditions to avoid excessive temperatures. Warm seed potatoes to at least 50 F to cut and handle. Store seed at 40-42 F and 85-90 percent relative humidity. Harvest when soil temperatures are 50-65 F. Tuber temperatures should be at least 50 F at harvest. Avoid injury to tubers.