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First Report of Alternaria alternata Causing Postharvest Black Spot of Fresh Date Palm Fruit in Spain

Commercial production of date palm fruit (Phoenix dactylifera L.) for fresh consumption has increased in the grove of Elx (Alacant Province, southeast Spain) after the successful development of tissue culture technologies and induced ripening and cold storage protocols. In a survey of losses after harvest, disease symptoms consisting of superficial, small, and firm black spots irregularly distributed throughout the fruit skin were observed in commercially handled and cold-stored fruit. At room temperature, superficial lesions expanded and produced dark mycelium. The potential causal agent was transferred to potato dextrose agar (PDA), incubated at 25°C in darkness, and subcultured on PDA. The identification was performed at the Spanish Type Culture Collection (CECT, University of Valencia, Spain) using colony morphology on PDA and malt extract agar at 26 or 37°C. At 26°C, the fungus rapidly produced cottony white mycelium that turned olivaceous and dark brown to black. Conidiophores were simple, straight or bent, with plain walls. Conidia were brown, obpyriform to ellipsoid (average 22 to 39 × 8 to 15 μm; n = 50), with both transversal and longitudinal septa, often observed in branched chains with more than 5 conidia. Growth occurred at 37°C. The identification of Alternaria alternata (Fr.:Fr.) Keissler was confirmed by the amplification and subsequent sequencing with the primers NL1 and NL4 of the region D1/D2 in the 5′ end of the 28S rRNA gene of the isolate IVIA DAA-4 (GenBank Accession No. JX987100). A BLAST search showed 100% identity with A. alternata strain DAOM 216376 (JN938894). Selected healthy ‘Medjool’ dates were surface disinfected by dipping them for 2 min in a 0.5% sodium hypochlorite solution and thoroughly rinsed with fresh water. To fulfill Koch's postulates, 20 μl of a spore suspension at 1 × 10⁵ spores per ml prepared from 7-day-old colonies grown on PDA were placed in fresh skin wounds made in disinfected fruit using a sterile stainless steel rod with a probe tip 1 mm wide and 2 mm in length (one wound per fruit; three humid chambers with nine fruits each). Wounded but not inoculated fruit were used as controls (one humid chamber with nine fruit). While disease symptoms were observed on all fruit inoculated with A. alternata (average black spots of 3, 6, and 12 mm after 4, 7, and 10 days of incubation at 20°C), no decay was observed on any of the control fruit. Reisolation of the fungus was performed from 10 infected dates and it was positive in all cases. A. alternata has been reported to cause date palm fruit disease in Israel (1) and Egypt (2), whereas Alternaria spp. have been cited in California (3) and Iran (4). To our knowledge, this is the first report of A. alternata causing date palm fruit rot in Spain.

References: (1) R. Barkai-Golan et al. Hassadeh 69:1446, 1989. (2) H. M. El-Deeb et al. Acta Hort. 736:421, 2007. (3) H. S. Fawcett and L. J. Klotz. University of California Bulletin 522, 1932. (4) F. Karampourland and H. Pejman. Acta Hort. 736:431, 2007.