During May and June of 2006, ‘Lapins’ sweet cherry (Prunus avium) trees were observed with white fungal growth on blossoms and young fruit in two commercial orchards in central Oregon (Wasco County). Entire blossom clusters and 30% of fruit clusters were affected. Rot on the fruit was firm, light brown, and covered the entire fruit, extending halfway down the pedicel. Affected blossoms were light brown and stuck to adjacent fruit and blossoms. Six disease samples from fruit, four samples from pedicels, and four samples from blossoms were surface sterilized and plated on potato dextrose agar acidified with lactic acid ([APDA] 1.5%). A white fungus producing sclerotia measuring 4 to 8 mm in diameter was recovered from all the samples after 10 days on APDA at 25°C. Koch's postulates were satisfied by inoculating green, pea-sized ‘Lapins’ cherry fruit with mycelial plugs colonized by the white fungus. Symptoms and signs similar to those seen in the orchard were produced. The same sclerotium-producing fungus was recovered from all inoculated fruit. The pathogen was identified as Sclerotinia sclerotiorum (Lib.) de Bary on the basis of the size of sclerotia and nested PCR using fungal universal primer pair ITS4/ITS5 and S. sclerotiorum-specific primer pair SSFWD/SSREV (2). A negative control, devoid of DNA templates in the reaction mix, was included in the PCR assay. S. sclerotiorum is endemic on wheat in Wasco County. The affected orchards were surrounded by wheat fields in which snow mold disease that is caused by S. sclerotiorum was particularly serious in the spring of 2006 compared with previous years. Rot on cherries was first observed and very severe in May of 2005, with symptoms and signs mainly on leaves of all trees across 45 ha. New symptoms continued to appear on cherry until mid-June. During 2006, most symptoms and signs were on fruit with some leaf spotting. Both years had unusually wet springs with 12.2 cm of rainfall received from April to June 2005 and 12.4 cm from April to June 2006, which is well above the 6.81 cm average for the previous 5 years. S. sclerotiorum causes green fruit rot of stone fruits, including almond, apricot (3), nectarine, and peach (1). To our knowledge, this is the first report of S. sclerotiorum causing blossom blight and green fruit rot on sweet cherry in Oregon.
References: (1) K. M. Flint. Green fruit rot. Page 15 in: Compendium of Stone Fruit Diseases. The American Phytopathological Society. St. Paul, MN, 1995. (2) J. Freeman et al. Eur. J. Plant Pathol. 108:877, 2002. (3) R. E. Smith. Phytopathology 31:407, 1931.
