In August 1996, a commercial retailer of high quality cherries from the Okanagan Valley of British Columbia, Canada, reported severe losses in at least 150 boxes (11.3 kg per box) of packed cherries cv. Stella. Upon examination of a representative box of these cherries, it was found that approximately 80% of the fruit was affected by what appeared to be Mucor spp. The decayed cherries were concentrated below the surface layer and small pockets of cherries had prominent salt-and-pepper-colored whiskers emanating from individual cherries. Counting the exact number of rotten cherries was difficult because they disintegrated on handling. Several infected cherries were surface sterilized by immersing in a 0.5% NaClO solution for 1 min and rinsing with sterile, distilled water. Pieces of the surface-sterilized cherries were plated onto dishes containing potato dextrose agar and incubated at 6°C for 10 days. Ten typical isolates derived from single spores from these dishes were all identified as Mucor piriformis A. Fischer (1). Samples of 200 g of cherries cv. Sweetheart (20 ± 1 cherries) were sprayed with a water suspension containing 1 × 106 sporangiospores per ml with one of the isolates identified as M. piriformis and incubated at 1, 6, 10, 15, and 20°C for up to 25 days. This same procedure was repeated for two other isolates of M. piriformis isolated from infected cherries. As a control, cherries were misted with sterile water and incubated at 20°C for 6 days. At 6°C, a temperature at which cherries are often stored, the three isolates decayed, respectively, 100, 60, and 60% of the cherries in 18 days. In general, all three isolates caused greater than 50% decay at 20, 15, 10, and 1°C in 6, 6, 11, and 25 days, respectively. Decay did not occur in cherries misted only with sterile water. Careful visual examination of decayed cherries revealed similar whiskerlike growth of Mucor as seen in the commercial cherries. Microscopic examination of sporangiophores infecting the fruit found them to be the same as the original M. piriformis isolate, successfully completing Koch's postulates. Previous to this report, M. piriformis, M. hiemalis, and M. plumbeus were isolated from Lambert cherries stored for 4 weeks at 4.5°C in Oregon (2); however, it is unclear if all or any of these isolates were pathogenic because pathogenicity tests were not conducted.
References: (1) T. J. Michailides and R. A. Spotts. Plant Dis. 74:537, 1990. (2) P. G. Sanderson and R. A. Spotts. Fungic. Nematic. Tests 47:43, 1992.
