First Report of Alternaria alternata Causing Postharvest Brown Spot of Plums (Prunus domestica) in Pakistan

Plum (Prunus domestica) is the most famous and commercially important stone fruit in Pakistan. Pakistan produces 50,465 metric tons of plum annually (http://www.fao.org/faostat/en/). In June 2017, plum fruits (n = 20) showing visible symptoms of brown spot were collected from storage. The disease incidence of brown spot was 20% on the observed fruit. The fruit skin showed irregular, necrotic brown lesions. The lesions gradually enlarged in size and coalesced with each other causing fruit rot. It decreased the market value of fruit. The diseased tissues (2 mm²) were excised from symptomatic fruit, surface sterilized with 1% NaClO for 10 s, and washed with sterilized distilled water. The diseased segments were plated on potato dextrose agar (PDA) medium and incubated at 25 ± 1°C for 6 days. The fungal isolates were purified through single-spore culture. On PDA, the colonies first appeared white that later turned dark brown to black with whitish edges and abundant aerial mycelia. Conidia (n = 50) were brown, ellipsoidal to ovoid, measuring 11.6 to 38.9 × 7.6 to 22.8 µm with a 7.2-µm-long conical beak, having one to five transverse and one to three longitudinal septations per spore. Based on morphological characteristics, the fungus was tentatively identified as Alternaria alternata (Fries Keisslr) (Tziros 2013; Zhang 2003). Genomic DNA was isolated from isolate COL-31 using PrepMan Ultra in a thermal cycler at 100°C for 10 min. The internal transcribed spacer region ITS1F/ITS4 (White et al. 1990) and endopolygalacturonase gene PG3/PG2b were amplified from genomic DNA and sequenced. Nucleotide BLAST search analysis of the sequences of isolate COL-31 showed 99% similarity with A. alternata (GenBank accessions KY609180 and MG773212.1), respectively. The sequences were deposited to GenBank (accession nos. MH607611 and MK034745). Pathogenicity of isolate COL-31 was tested on 20 mature, healthy, asymptomatic plum fruit. The fruit were inoculated by spraying 10⁶ spores/ml on the fruit. Control fruit were sprayed with sterilized distilled water only. The fruit were incubated for 7 days at 25 ± 1°C. After 3 to 4 days of inoculation, 80% of fruit started to develop the symptoms of brown spot. Control fruit did not develop any symptoms. Reisolation was only from infected fruit showing symptoms of brown spot, satisfying Koch’s postulates. A. alternata has been reported to cause disease of plum in Latvia (Grantina-Levina 2015). To our knowledge, this is the first report of Alternaria brown spot of plum in Pakistan. Because Alternaria species are pathogenic to a wide variety of crops, it might therefore have a potential impact on plum production in Pakistan.