Blueberry (Vaccinium virgatum), a member of the Ericaceae family, is an increasingly important crop in China because of its abundant nutritional benefits and economic value (Kuzmanović et al. 2019). In October 2021, leaf spots were detected on 'Rabbiteye' blueberry at the Agricultural Science and Technology Park of Jiangxi Agricultural University in Nanchang, China (28°45'51"N, 115°50'52"E), which caused severe defoliation of the crop and fruit yield losses of 25% (Figure 1A). Disease surveys were conducted at that time; the results showed that disease incidence was 75.5%, observed in 151 of the 200 accessions sampled, and this disease had not been found at other cultivation fields in Nanchang. Lesions with taupe to dark brown margins were irregularly shaped and associated with leaf margins. Spots coalesced to form larger lesions, with black pycnidia present in more mature lesions. To identify the causal agent, 10 small pieces (5 mm2) of leaf tissue excised from the lesion margins were surface sterilized in 75% ethanol solution for 30 s and 0.1% mercuric chloride solution for 2 min, rinsed three times with sterile distilled water, then placed on potato dextrose agar (PDA) at 25°C for 5 to 7 days in darkness. Five fungal isolates showing similar morphological characteristics were obtained as pure cultures by single-spore isolation. All fungal colonies on PDA were floccose, dense, and white (Figure 1B.C). Black pycnidia developed on PDA at 25°C under a 12/12 h light/dark cycle for 30 days. Alpha conidia were 6.17 to 8.53 × 1.64 to 3.20 µm (average 6.94 × 2.52 µm, n = 100), aseptate, hyaline, fusiform to ellipsoidal, often biguttulate. Beta conidia were 15.26 to 25.41 × 0.92 to 1.40 µm (average 20.14 × 1.27 µm, n = 30), aseptate, hyaline, linear to hamate (Figure 1D). Based on morphological characteristics, the fungal isolates were suspected to be Diaporthe spp. (Gomes et al. 2013). To further confirm the identity of this putative pathogen, two representative isolates (LGM1 and LGM2) were selected for molecular identification. The internal transcribed spacer region (ITS), translation elongation factor 1α (EF1-α), histone H3 (HIS), calmodulin (CAL), and β-tubulin (TUB2) genes were amplified from gDNA and sequenced using primers ITS1/ITS4 (Peever et al. 2004), EF1-728F/EF1-986R and CAL228F/CAL737R (Carbone et al. 1999), CYLH3F/H3-1b (Crous et al. 2004), Bt2a/Bt2b (Glass and Donaldson 1995), respectively. GenBank accession numbers of isolate LGM1 and LGM2 were OM778771 to 72 for the ITS region, OM868228 to 29 for EF1-α, OM837771 to 72 for TUB2, ON206971 to 72 for CAL, ON206973 to 74 for HIS. BLAST results showed that the ITS, EF1-α, TUB2, HIS, and CAL sequences showed 99% (538/545 bp), 100% (322/322 bp), 99% (480/484 bp), 99% (459/460 bp), 99% (430/433 bp) identity, respectively, with those of Diaporthe phoenicicola (GenBank accession no. MW504735, MW514099, MW514142, MW514067, MT409304). Two maximum likelihood phylogenetic trees were built based on the sequences of ITS, EF1-α, HIS, CAL, and TUB2 by using MEGA 5. The two isolates LGM1 and LGM2 clustered with D. phoenicicola (Figure 2 and 3). The fungus was identified as D. phoenicicola by combining morphological and molecular characteristics. To evaluate the pathogenicity, three healthy young potted V. virgatum plants were spray inoculated with a conidial suspension of 106 conidia/ml. Another set of three plants that were sprayed with sterilized distilled water served as the controls. The experiment was repeated three times, and all plants were maintained in a climate box (12 h light/dark) at 25°C with 80% relative humidity. Five days after inoculation, no symptoms were observed on control plants (Figure 1F), and all inoculated plants showed symptoms (brown flecks) similar to those observed in the field (Figure 1E). The fungus was reisolated from the infected tissues and confirmed as D. phoenicicola by morphological and molecular identification, and could not be isolated from the controls, fulfilling Koch's postulates. To our knowledge, this is the first report of D. phoenicicola causing leaf spot on blueberry in China. The discovery of this new disease and the identification of the pathogen will provide useful information for developing specific control measures and potential sources for resistance to this disease caused by D. phoenicicola.
Keywords: Diaporthe phoenicicola; Vaccinium virgatum; leaf spot.