Botryotinia fuckeliana (de Bary) Whetzel (syn. Botrytis cinerea Pers.:Fr.) causes gray mold on the foliage of a large range of horticultural and agricultural crops, including potato (Solanum tuberosum L.). This weak pathogen may also produce pit rots on potato tubers (1). In April 1999, two lots of seed potatoes produced in Scotland were found to contain a significant number of tubers with soft rots. The cultivars were Maritiema and Charlotte, with 2.2 and 0.5% of rotted tubers, respectively. The rots on tubers of cv. Maritiema were all soft, wet, and extensive, with a distinct edge, but the proportion of this type of rot was much lower (approximately 30%) on the cv. Charlotte tubers. Gray sporangiophores developed around tuber eyes. When the tubers were cut, the affected tissue was peach to pink-gray in color but darkened on exposure to air, and was soft and water-soaked in appearance with a pale brown or, occasionally, yellow margin. A faint vinegary odor could be detected occasionally. B. fuckeliana was isolated on potato dextrose agar (PDA) from all 15 tubers of cv. Maritiema and from the three (out of 13) tubers of cv. Charlotte that had large, soft rots. Tubers produced at Scottish Agricultural Science Agency's Gogarbank farm near Edinburgh in 1998 were used for confirmatory pathogenicity tests conducted in late April and May 1999. Using a cork borer, a wound 5 mm in diameter and 5 mm deep was made in each tuber and 5-mm-diameter agar plug from either the edge of a colony of B. fuckeliana or of PDA was inserted into the wound. Nine tubers of cvs. Maritiema and Charlotte were inoculated for each treatment and tubers incubated at 5°C in boxes lined with moist filter paper. Rots, similar to those on the commercial seed tubers, developed after 28 days at wound sites inoculated with B. fuckeliana. The fungus was isolated by placing a small piece of rotted tissue from each rot on PDA. B. fuckeliana was recovered from all rots. The mean width of rots was 51 mm for cv. Maritiema compared with 40 mm for cv. Charlotte. Depth of rots was similar for both varieties. Lesions did not develop at wound sites inoculated with PDA agar only. In a second experiment, tubers of cv. Maritiema were inoculated with B. fuckeliana as described above and incubated at 5°C or room temperature (15 to 18°C). There were nine tubers for each temperature. After 21 days, no lesions had developed on tubers incubated at room temperature, but large, soft rots were present on those incubated at 5°C. The lesions produced by B. fuckeliana in these experiments were relatively large compared with the 5-m-deep rots reported previously (1) after 60 days of incubation. The appearance of these rots is similar to that for other diseases, e.g., pink rot and watery wound rot, and infections by this fungus may have been incorrectly diagnosed in the past. Moreover, the development of such rots may be favored by the recent increase in the use of low temperature storage for seed potatoes.
Reference: (1) H. W. Platt. Can. J. Plant. Pathol. 16:341, 1994.