Gangrene
Phoma exigua var. exigua and var. foveata
Causative agent and transmission
Gangrene is a late blight of the tuber caused by the fungus Phoma exigua var. foveata (more aggressive) as well as P. exigua var. exigua.
The fungus remains alive in the soil as saprophytes. The contaminated soil adhering to the tuber is used for the propagation of the fungus, which penetrates through wounds made in the tuber during harvesting and conditioning.
The storage premises and the equipment can also represent sources of contamination.
Low temperatures immediately after harvesting or conditioning encourage the development of Phoma from wounds in the tubers, insofar as they slow the healing of these wounds.
Description of symptoms on crops
Attacks on the crops often remain unnoticed, causing only premature yellowing of the foliage. Black pycnidium may appear at the base of the stems at the end of the crop or after haulm defoliation/desiccation.
Description of symptoms on crops
(clic on photos to enlarge)
Dark depressed areas can be seen on the tubers giving rise to "thumbprint" patches which are generally located on the wounds, eyes, lenticles and stem-bases.
The cut surface of the tuber shows a dark-brown coloured late blight associated with the development of pycnidium in existing cavities.
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Control
The Phoma is a wound parasite, and the methods for control include the following measures:
- Harvest within 3-4 weeks after haulm defoliation;
- Limit wounds on the tubers during handling (harvesting, conditioning, etc.);
- Eliminate infected tubers;
- Ventilate the batches of potatoes when harvesting to encourage healing of wounds;
- Avoid (sudden) low temperatures immediately after harvesting and conditioning;
- Treat shortly after harvesting with an effective fungicide (imazalil + thiabendazole).
