Deep sunken lesions - an atypical symptom on potato tubers caused by Colletotrichum coccodes during storage

Colletotrichum coccodes 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°C to C. coccodes. 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 C. coccodes 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°C, after an artificial inoculation with C. coccodes. The inoculation was performed by depositing a 10 µl drop of a conidial suspension (calibrated at 3x10⁶ spores per ml) of C. coccodes (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°C and 100 % relative humidity (rh). After approximately 10 weeks of incubation, dark brown, irregular shaped lesions with clear contours were observed. After five months incubation, symptoms had extended over the whole tuber surface (Fig. 1). C. coccodes was the only pathogen isolated from these lesions.

The proportion of tubers showing deep lesions was higher at 10°C (~ 20%), than at 5 or 15°C (~ 5%). These lesions were never observed on tubers inoculated in the same way but kept at 20-25°C and 100% rh. On these tubers only typical black dot symptoms developed. No lesions or black dot symptoms were observed on control tubers. These observations therefore demonstrate that C. coccodes alone can cause deep tuber lesions on potato tubers stored for extended periods at 5-15°C.