Potato tuber rots

A number of fungi and bacteria are capable of causing decay in potato tubers. Symptoms are often present at lifting, but some of the problems will also spread through stored tubers, and a few will only develop after prolonged storage.

Black leg (Pectobacterium atrosepticum) on potato 'Charlotte'. Credit: RHS/Pathology.

Quick facts

Common name Potato tuber rots
Scientific name Pectobacterium atrosepticum (blackleg/soft rot), Pectobacterium carotovorum (soft rot), Phytophthora infestans (blight), Phytophthora erythroseptica (pink rot), Fusarium species (dry rot), Phoma exigua var. foveata (gangrene)
Plants affected Potatoes
Main symptoms Soft or firm rots of the tuber
Caused by Bacteria, fungi and fungus-like organisms
Timing Year round, particularly during storage

What are potato tuber rots?

Potato tuber rots are a frequent cause of losses prior to, or after, lifting. Significant problems often follow a wet growing season, particularly if the tubers are then lifted from wet soil. Some of the pathogens causing tuber rots also produce symptoms on the aerial parts of the plant.

Symptoms

You may see the following symptoms:

Blackleg/bacterial soft rot (Pectobacterium atrosepticum/P. carotovorum)

  • Tubers develop a soft and often foul-smelling rot
  • Symptoms may affect any part of the tuber, but blackleg frequently invades through the stolon
  • The blackleg bacterium also causes a soft, black rot at the base of the stem, leading to yellowing and wilting of the foliage

Blight (Phytophthora infestans)

  • Affected tubers have discoloured patches on the skin
  • Cutting the tuber open reveals a reddish-brown, granular rot, often just below the skin
  • Affected tubers often develop secondary bacterial soft rots
  • Blight also attacks the foliage, causing brown lesions on leaves and stems

Pink rot (Phytophthora erythroseptica)

  • Affected tubers have soil sticking to them at lifting, and discoloured patches on the skin
  • Internally, the flesh is quite rubbery & watery, smells of vinegar, and turns pink after a few seconds exposure to the air

Dry rot (Fusarium species)

  • Symptoms develop in storage
  • Brown patches on the skin correspond to internal cavities lined with white, pink or bluish fungal growth
  • Tubers may shrivel to form ‘mummies’

Gangrene (Boeremia foveata)

  • Symptoms develop in storage
  • Sunken ‘thumb-mark’ lesions develop on the surface
  • These correspond to extensive internal cavities, often much larger than suggested by the external lesion
  • White fungal growth and pinhead-sized black fruiting bodies may be present within the lesion

Control

Non-chemical control

  • Use good quality, certified seed tubers. If using your own seed, do not save seed tubers from a disease-affected crop
  • Do not plant any seed tubers that are unduly soft or have obvious patches of decay
  • Do not lift tubers from wet soil, or if the soil is very dry and hard. Leave tubers on the soil surface for two to three hours after lifting so that the skins dry
  • Lift and handle the tubers carefully to avoid any damage
  • If rots are present dispose of affected tubers and use the rest as soon as possible – do not attempt long-term storage
  • Store tubers in dry, cool conditions and check them regularly for rots
  • Earth up well around the stems of the growing crop to protect the tubers from blight spores. If cracks appear in the ridge, earth up over them
  • Lift as soon as possible if blight appears in the crop and tubers are of usable size
  • Choose resistant cultivars. The British Potato Council has produced the British Potato Variety Database which lists cultivars with resistance to some of the tuber rot diseases

Chemical control

  • Foliage can be protected against blight with regular fungicide applications
  • No products are available for the direct control of tuber rots

Biology

Blackleg develops when bacteria spread along the stolons from decaying stems and enter the daughter tubers.
Contaminated seed tubers are the most important source of the blackleg bacterium (Pectobacterium atrosepticum). Both this and the soil-borne soft rot bacterium (P. carotovorum) can also infect tubers via damage, or act as secondary colonisers following other diseases. Wet soil during growth and lifting of the crop make bacterial problems much more likely.

Blight affects the tubers when spores are washed down into the soil from lesions on the foliage. Blight does not spread to any extent during storage, but secondary bacterial rots do, and can cause extensive losses. The fungus-like blight pathogen also affects tomato crops.

Pink rot, dry rot and gangrene are caused by soil-borne fungi. Any damage suffered by the tubers at lifting will make them much more prone to infection. It may take weeks or even months of storage for symptoms of dry rot or gangrene to develop.

In addition to the diseases mentioned above, there are several other bacterial and fungal diseases causing decay of tubers, e.g. rubbery rot, violet root rot, watery wound rot. The bacterial diseases brown rot and ring rot, whilst unlikely to develop in garden crops, are caused by notifiable organisms. Where significant problems develop it may be worth having a sample examined by experts to determine the cause.

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