Soft rot diseases of potatoes

Introduction

Soft rot of potatoes has been caused by a range of bacteria around the world such as Pectobacterium carotovorum subspecies carotovorum, Pectobacterium atrosepticum and Dickeya species. Previously, these bacteria belonged to the genus Erwinia.

P. atrosepticum is not widely reported on potatoes in Western Australia.

Dickeya dianthicola was detected in Australia for the first time in June 2017 in a Western Australian potato crop.

Symptoms

Water-soaked areas of soft tissue are typical of a soft rot infection. Initially, the healthy part of a tuber is clearly distinguishable from the macerated, creamy infected part but eventually the whole tuber becomes infected. There may be a foul smelling odour as the potato is broken down by the bacteria and when secondary invaders occur.

External view of a tuber showing a water-soaked lesion with bacterial ooze

Cross-section of infected tuber showing internal rot

Non-emergence of plants, wilting, browning of tissues, haulm desiccation and plant death have all been linked to infection by soft rot bacteria. These symptoms are favoured by cool, wet soils (10-15°C) at planting and temperatures above 20°C after emergence. These conditions can result in black leg where the bacteria invade the internal vascular system of the plant and cause wilt.

Black leg internal symptoms where the two plants on the left are infected by soft rot bacteria, causing blackened internal symptoms and crop wilt, compared with a healthy plant on the right

Spread

The bacteria that cause soft rot can remain in both potato plants and tubers without any obvious symptoms — latent infection. The symptoms of the soft rot bacteria only become obvious when the potato’s natural resistance is damaged.

The main cause of spread is by wounds or damage to the potato. These usually occur during harvesting and grading, allowing the bacteria to invade the tuber. When this is combined with water on the surface of the tuber, the bacteria can defeat the tuber’s natural defences and start the tuber rot.

Mechanical damage to the tuber in the form of cracks and splits provides ideal surfaces for bacterial infection to occur

Soft rot can occur from as low as 16°C to above 35°C. High temperatures create ideal conditions as oxygen in the tuber is rapidly replaced by high levels of carbon dioxide, causing stress on the tuber. This will lead to soft rot infection. Exposure to sunlight also aids soft rot development by killing tuber cells. The presence of other diseases will lead to increased infection.

Soft rot can be spread by infected seed, with infection in the next generation of tubers being higher as the percent infected of seed potatoes increases. Soft rot should be controlled throughout the life of a crop to reduce its impact on future generations of potatoes.