Diseases

Blackleg and soft rot (Pectobacterium and Dickeya)

Blackleg turns the stem base black and slimy, and the same bacteria rot tubers in the field and in store. They are carried in seed, there is no chemical cure, and seed certification is the main control, which is why blackleg tolerances appear in every certified class.

Last updated: 7 November 2026

At a glance
Cause	Pectobacterium atrosepticum, P. carotovorum and relatives; Dickeya species
Main source	Latently infected seed tubers
Chemical cure	None effective
Quarantine pest	No (but certified for in seed)

Cause

The soft rot Pectobacteriaceae are a group of bacteria that produce enzymes which break down plant cell walls. Pectobacterium atrosepticum is the classic blackleg organism of cool climates. Dickeya species, including D. solani, emerged as a serious seed-borne problem in Europe from the 2000s and are more at home in warmer conditions.

Symptoms

Blackleg: an inky black, slimy stem base, with stunted, upright, yellowing plants that may wilt. Plants can look fine and then collapse after rain or heat.
Soft rot: wet, cream to tan, foul-smelling decay of the tuber that spreads in store when tubers are wet or short of oxygen. The decayed tissue is separated cleanly from healthy tissue and the skin often stays intact.

How it spreads

Seed is the main source. Tubers can carry the bacteria without symptoms, and the infection passes to the stem and to daughter tubers.

Machinery, wash water, wet harvest conditions and wounds spread it between lots. A wet tuber in a poorly ventilated store is where it multiplies fastest.

Where the risk is highest

Seed and ware production in Europe, North America and warmer irrigated regions. Dickeya and P. brasiliense are more prominent where seasons are warm. The Dutch NAK inspection protocol, for example, lists blackleg (Pectobacterium and Dickeya) among the diseases inspectors check in every seed field.

Management: cultural

Start with certified seed from a low-blackleg class, and keep the seed generation count short.
Avoid wounding and bruising at planting, harvest and grading. Do not harvest or store tubers wet. Let the skin set before harvest.
Dry and cure tubers, ventilate the store and keep it cool, so condensation cannot form on the tuber surface.
Clean and disinfect machinery between seed lots, and rotate to non-host crops.

Management: resistant varieties

No commercial variety is immune. Varieties differ in susceptibility and this is rated in national variety lists, but good seed health and storage conditions matter more than the rating.

Management: chemical and regulatory notes

There is no effective chemical cure for blackleg or soft rot. Management is preventive: clean seed, good handling and good storage.

Seed certification

Blackleg is counted in field inspections, and every certified class has a maximum percentage. In Scotland's SPCS, for example, the maxima run from 0 percent in Pre-Basic 1 and 1.0 percent in Super Elite to 2.0 percent in Elite and 4.0 percent in Certified 1 (SASA technical notes, as summarised in the Potatopedia corpus).

Dutch NAK inspectors make at least three inspections of each certified seed lot, and laboratories test for Pectobacterium and Dickeya.

Seed certification schemes set maximum tolerances for disease by class, from Pre-Basic to Certified, and regulated bacteria such as ring rot and brown rot are held at zero in every class (EU Directive 2002/56/EC framework; Scottish SPCS and Dutch NAK schemes as summarised in the Potatopedia corpus).