During the 2006 growing season, grape berries in several Red Globe vineyards in Kern County, California were found exhibiting black fungal sporulation and typical sour rot symptoms. Symptoms included berry cracking and leakage along with a pungent vinegar smell. In California, sour rot (also known as summer bunch rot) has been attributed to a complex of microorganisms that invade ripe berries following injury. Fungi typically associated with sour rot include Aspergillus niger, Alternaria tenuis, Cladosporium herbarum, Rhizopus arrhizus, and Penicillium spp. Various yeasts and bacteria have also been associated with the disease, especially Acetobacter bacteria, which give the grapes their pungent acetic acid smell (2). In June 2006, the two fungi most commonly isolated from affected berries (postveraison) were A. niger and A. carbonarius. Identification of the two species was confirmed by colony and spore morphology (1) and PCR analysis of the internal transcribed spacer (ITS) region of rDNA. Although A. niger was more common, A. carbonarius has not previously been reported as a pathogen of grape in California. Conidia of A. carbonarius were globose, dark with very rough walls, and could be distinguished from A. niger by their wall structure and larger size ([5.1] 6.0 to 7.6 [8.8] μm in diameter). Sequence analysis of the ITS region of isolates morphologically identified as A. carbonarius showed 100% similarity to known isolates of this species. To confirm pathogenicity, postveraison Red Globe berries on standing grapevines were wounded with a needle and inoculated by dipping entire clusters into A. carbonarius spore suspensions (106 conidia/ml) for 30 s. Sterile water was used as a control. Twenty berries on each cluster were wounded and 10 clusters were used for each treatment. The experiment was repeated in two different vineyards in Kern County in 2006 and 2007. After 48 h, water-soaked lesions could be seen on the wounding site of grapes inoculated with A. carbonarius. After 1 week, 100% of the inoculated grapes exhibited dark, black sporulation, and after 3 weeks, the clusters were almost completely rotted. Koch's postulates were completed by isolating A. carbonarius from the affected berries. A few (less than 5%) of the wounded control berries also exhibited black sporulation and rot after 3 weeks. These infections were probably the result of natural inoculum at the sites because spore traps placed in the vineyards also contained A. carbonarius and A. niger propagules. Furthermore, soil surveys in the same vineyards showed that both A. niger and A. carbonarius were present on plant debris on the vineyard floor and in the soil. To our knowledge, this is the first report of A. carbonarius causing sour rot of grapes in California.
References: (1) M. A. Klich. Identification of Common Aspergillus Species. Centraalbureau voor Schimmelcultures, Urecht, The Netherlands, 2002. (2) J. J. Marois et al. Bunch rots: Miscellaneous secondary invaders and sour rot. Page 69 in: Grape Pest Management. 2nd ed. The Regents of the University of California, Division of Agriculture and Natural Resources, Oakland, 1992.
