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Ministry of Agriculture, Food and Rural Affairs

black leg

Stem end lesion Internal symptoms of soft rot Internal symptoms of soft rot Symptoms of infection on potato plant Symptoms of infection on potato plant Rot moving up from the seed piece Stem breakage due to severe blackleg Infected plant wilting and dying Blackleg (left) and ring rot (right)
Click to enlarge.

Scientific Name
Pectobacterium atrosepticum

Identification
Symptoms may develop at any time through the season.
Foliage and Stems

Infected plants are stunted and look stiff and erect, especially early in the season. The foliage becomes chlorotic and leaflets tend to roll upward at the margins.

Eventually plants wilt and die Stems show an inky-black decay that starts from the seed. The decay extends a variable distance up the stem, anywhere from a few centimeters to the tip of the stem. Often infected tissue is invaded by secondary bacteria that produce a fishy odour.

Severe blackleg may cause the stem to break. In wet weather, the decay is soft and slimy. Under dry conditions, infected tissues may become dry and shrivelled.

Tubers

New tubers infected from the mother plant have sunken, circular, black lesions that start at the stem end. Internally, a soft rot develops in the pith, extending into the tuber. The rot extends from the stem end into the pith. The infected flesh first appears cream coloured. With time the rot turns darker. In an advanced stage the infected tissue turns greyish-black and mushy.

Often Confused With
Late blight on stems
Ring rot in tubers

Period of Activity
Throughout the growing and harvest season.

Scouting Notes

  • Scouting should start shortly after emergence.
  • When walking fields, stop at many sites as possible and check plants at random.
  • Look for stunted plants with leaves rolled upward. Pull suspicious plants and examine the base of stem and seed piece. Always check areas of poor drainage

Thresholds
Growers should grade the seed carefully so that infected tubers are discarded before planting. At plant emergence, a few infected plants scattered throughout the field (1 plant/acre) are common. Blackleg also develops after heavy rains on plants growing on rows damaged by farm equipment. Scouts should estimate percentage of blackleg infection so that growers can make management decisions.

Advanced

Scientific Name
Pectobacterium atrosepticum

Identification
Symptoms may develop at any time through the season.
Foliage and Stems

Infected plants are stunted and look stiff and erect, especially early in the season. The foliage becomes chlorotic and leaflets tend to roll upward at the margins.

Eventually plants wilt and die Stems show an inky-black decay that starts from the seed. The decay extends a variable distance up the stem, anywhere from a few centimeters to the tip of the stem. Often infected tissue is invaded by secondary bacteria that produce a fishy odour.

Severe blackleg may cause the stem to break. In wet weather, the decay is soft and slimy. Under dry conditions, infected tissues may become dry and shrivelled.

Tubers

New tubers infected from the mother plant have sunken, circular, black lesions that start at the stem end. Internally, a soft rot develops in the pith, extending into the tuber. The rot extends from the stem end into the pith. The infected flesh first appears cream coloured. With time the rot turns darker. In an advanced stage the infected tissue turns greyish-black and mushy.

Often Confused With
Late blight on stems
Ring rot in tubers

Biology
The blackleg bacterium is primarily seed-borne. It is quickly spread during set cutting operations, especially if conditions for wound healing are not provided. Once infected, seed pieces can decay and sprouts fail to emerge. Wet soils and cool temperatures favour spread of the disease. The bacterium spreads from infected seed to the stems of the developing potato plant causing the characteristic wilting and leaf yellowing symptoms to develop. The disease is especially prevalent in the lower, wetter areas of a field. Tubers are susceptible to attack throughout the growing season and at harvest time. If diseased tubers are stored, they can be the source of inoculum for disease outbreaks in the following season.

Period of Activity
Throughout the growing and harvest season.

Scouting Notes

  • Scouting should start shortly after emergence.
  • When walking fields, stop at many sites as possible and check plants at random.
  • Look for stunted plants with leaves rolled upward. Pull suspicious plants and examine the base of stem and seed piece. Always check areas of poor drainage

Thresholds
Growers should grade the seed carefully so that infected tubers are discarded before planting. At plant emergence, a few infected plants scattered throughout the field (1 plant/acre) are common. Blackleg also develops after heavy rains on plants growing on rows damaged by farm equipment. Scouts should estimate percentage of blackleg infection so that growers can make management decisions.

Management Notes

  • Use healthy seed.
  • Clean and disinfect seed cutters and planters between seed lots.
  • Select well-drained fields.
  • Avoid crop stress:
    • Provide adequate levels of nutrients
    • Manage other pests
    • Match irrigation with crop needs. Do not over-irrigate.
    • Avoid harvesting when conditions are wet.
    • Harvest tubers when the skin is set and avoid mechanical injury.