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Source: British Soc. Plant Pathol., New Disease Reports, Vol. 8 [edited]

Deep sunken lesions: an atypical symptom on potato tubers caused by Colletotrichum coccodes during storage
I. Glais, Groupement National Interprofessionnel des Semences, 44 rue du Louvre, 75001 Paris; I. Glais <iglais@rennes.inra.fr>, INRA-Centre de Rennes, UMR BiO3P, Domaine de la Motte, BP35327, F-35653 Le Rheu Cedex, France; and D. Andrivon (as for Glais, INRA) . Accepted for publication 20 Oct 2003

Colletotrichum coccodes [Cc] causes a potato tuber-blemishing disease, commonly known as black dot. Typical black dot symptoms are silvery-to-brown patches on the tuber surface, bearing black microsclerotia (Dillard, 1992). Severe infection can cause tuber shrivelling (Hunger & McIntyre, 1979).

Furthermore, Mooi (1959) attributed deep lesions observed on infected tubers stored at -1 deg C to Cc. Similar symptoms have also been occasionally observed on tubers kept in commercial cold stores under irregular temperature regimes (Gaucher, 1998).

To our knowledge, these symptoms have never been reproduced in control inoculation experiments, so the implication of Cc as the sole causal agent remains unclear.

Deep sunken lesions, similar to those reported by Mooi (1959) and Gaucher (1998), were obtained on potato tubers from commercial stores, that had been kept for several weeks at 5-15 deg C, after an artificial inoculation with Cc. The inoculation was performed by depositing a 10 microliter drop of a conidial suspension (calibrated at 3 million/ml) of Cc (isolate 91.22g from the INRA culture collection) on each end of healthy mini-tubers of potato cv. Charlotte. Water droplets without inoculum were used on control tubers.

Inoculated tubers were incubated in the dark at 5, 10, and 15 deg C and 100 percent relative humidity (rh). After approximately 10 weeks of incubation, dark brown, irregular shaped lesions with clear contours were observed. Five months later, symptoms had extended over the whole tuber surface . Cc was the only pathogen isolated from these lesions.

The proportion of tubers showing deep lesions was higher at 10 deg C (~ 20 percent), than at 5 or 15 deg C (~ 5 percent). These lesions were never observed on tubers inoculated in the same way but kept at 20-25 deg C and 100 percent rh. On these tubers only typical black dot symptoms developed. No lesions or black dot symptoms were observed on control tubers.

These observations demonstrate that Cc alone can cause deep tuber lesions on potato tubers stored for extended periods at 5-15 deg C.

References
Dillard HR, 1992. Colletotrichum coccodes: the pathogen and its hosts. In: Bailey J.A., Jeger M.J., eds. Colletotrichum: Biology, pathology and control. Wallingford, UK: CAB International, 225-236.
Gaucher D, 1998. La dartrose. In: Maladies de la pomme de terre. Paris, France: ITCF-ITPT, 33.
Hunger RM, McIntyre GA, 1979. Occurrence, development, and losses associated with silver scurf and black dot on Colorado potatoes. American Potato Journal 56, 289-306.
Mooi JC, 1959. A skin necrosis occurring on potato tubers affected by black dot (C. coccodes) after exposure to low temperatures. European Potato Journal 2, 58-68.

[Cc is a relatively weak pathogen of potato. The pathogen is soil-borne, residing in tubers and soil. Disease management is difficult because there are few available chemical fungicides and little or no resistance in current potato cultivars. Cultural procedures may offer some degree of disease control. -Mod.DH]
An additional reference: <http://www.ext.nodak.edu/extnews/newsrelease/2003/041003/08ndsupl.htm>