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Potatoes - phoma or gangrene | AG0308 |
Roger Osborn, Knoxfield
June, 1995 |  |
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Cause
Gangrene is caused by a microscopic fungus called Phoma exigua var. foveata.
Symptoms
Gangrene is an important storage disease of potatoes. Symptoms on tubers first appear as small dark depressions associated with wounds, eyes or lenticels. The depression can enlarge to form "thumb mark" or larger irregularly shaped areas. Gangrene also causes an internal rot beneath the superficial depressions, which is characterised by extensive cavities with walls which are discoloured and lined by dark grey or pinkish-purple strands of the fungus. The extent of the internal rots is not related to the size of the diseased area on the surface, and it is common to find relatively minor surface damage masking extensive internal damage.
Economic importance
Gangrene is a problem in many potato growing areas of the world and has been reported in all the main districts of Victoria. Severe losses can occur in stored potatoes, particularly where the crop has been harvested in cold weather.
Disease cycle
Stems growing from diseased tubers may be affected by gangrene, but disease is not obvious until the tops begin to die back. At this time small black fruiting bodies of the fungus called pycnidia may be seen on haulms, especially near the nodes. Pycnidia contain many spores and, under wet conditions, spores are washed to the base of plants, where a proportion may cause disease on daughter tubers.
Spores are also released from the seed piece and most gangrene which occurs on daughter tubers in soil is thought to be derived from the seed piece.
Infection of tubers is predominantly through wounds made during digging, grading and handling, although it is possible that some infection occurs through lenticels and eyes in wet soil.
Figure 1. Potato tuber affected by Phoma (gangrene), showing surface lesions with thumbnail cracks.
Figure 2. Tuber affected by Phoma, showing internal breakdown below lesion© Stat
e of Victoria, Department of Natural Resources and Environment 2002 Page 2
Factors that favour spread and development of gangrene
- Wounding is the single most important factor favouring development of gangrene. The type of wound can also influence severity; more severe gangrene develops from crush wounds than from shallow cuts.
- Spread of the gangrene fungus to unaffected tubers can occur not only naturally in soil, but also by mechanical transmission of spores on digging and grading equipment.
- Severity of gangrene is increased when tubers are left in soil for extended periods after haulm destruction.
- Cold wet soils and cold conditions during handling also favour the development of gangrene.
Control measures
- Plant clean seed to reduce the risk of transfer of gangrene from the seed piece to daughter tubers.
- Prior to, and periodically during harvest and grading operations, disinfect and wash equipment to restrict contamination of tubers by spores of Phoma. These are known to be present in soil and debris adhering to diggers, graders and bins.
- Take care to minimise damage to tubers during digging, during the transfer of tubers to bins and during grading. Most gangrene develops from infections through wounds, so fewer wounds mean less disease.
- Avoid leaving tubers in the ground for extended periods after defoliation, especially in cold soils.
- Wherever possible cure tubers at 16 to 20°C for one to two weeks after harvest. The curing of tubers allows the potato tuber to heal wounds and this is most effective between 16 and 20°C. The healing process is slower under cold conditions and allows time for infection by the gangrene fungus.
- Treat tubers with the fungicide thiabendazole as soon as possible after harvest and within 14 days of each subsequent handling operation.
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The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.
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