In October 2020, a postharvest fruit brown rot symptom was observed on navel orange (Citrus sinensis Osbeck cv. Newhall) fruits in a local fruit market in Ganzhou, Jiangxi Province, China. The disease incidence increased up to 15% in 40 fruits with a 7-day-long storage at room temperature. The disease symptoms on the infected fruit were circular, light brown to brown, slightly sunken lesions, covered with whitish mycelium mass, and brown rot in the center. To isolate the causal organism, infected fruits were surface sterilized with 1% NaClO solution for 30 sec, and rinsed thrice with sterilized water. Symptomatic tissues at the margins were cut into 5-mm2 pieces, placed on potato dextrose agar (PDA) medium and incubated at 25℃ for 5 days. Thirteen morphologically similar single-spore fungal isolates were obtained from the isolation experiment. Fungal colonies were white, fluffy, cottony texture, reverse buff to light yellow, with black stromata at maturity. Alpha conidia were hyaline, aseptate, ellipsoid to clavate, tapering towards the ends, often biguttulate, and ranged in size from 6.8 to 9.8 µm × 2.7 to 4.5 µm (n=50). Beta conidia were hyaline, aseptate, smooth, straight to sinuous, and with size ranging from 12.1 to 21.3 µm × 0.9 to 2.2 µm (n=50). Morphological features were consistent with those of Diaporthe sojae (Dissanayake et al. 2015). For molecular identification, DNA was extracted from the representative isolate JFRL 03-13, the internal transcribed spacer (ITS) region, beta-tubulin (TUB), calmodulin (CAL), partial translation elongation factor 1-alpha (TEF1-α), and histone H3 (HIS) genes were amplified by using primers ITS1/ITS4, Bt2a/Bt2b, CAL228F/CAL737R, EF1-728F/EF1-986R, and CYLH3F/H3-1b (Udayanga et al. 2015), respectively. The resulting sequences were deposited in GenBank (Accession Nos. OM281710 for ITS, OM289961 for TUB, OM289964 for CAL, OM289963 for TEF1-α, and OM289962 for HIS). BLAST analysis revealed that these sequences were 100% similar to the sequences of ITS (MN816426), TUB (MK941336), CAL (MN894375), TEF1-α (MN894447), HIS (MN894409) published for D. sojae. Phylogenetic analysis was conducted based on the concatenated sequences (ITS, TUB, CAL, TEF1-α, and HIS) by Maximum likelihood analysis (ML) and Bayesian inference (BI) using IQtree v.1.6.11 and MrBayes v.3.2.7 (Guo et al. 2020). The phylogenetic tree showed that the isolate clustered with D. sojae. To confirm pathogenicity, mature and healthy harvested fruits of navel orange (Citrus sinensis Osbeck cv. Newhall) were surface sterilized. Ten fruits were wounded by a sterile scalpel and put a 7-mm-diamter agar plug with 7-day-old mycelium of the isolate JFRL03-13 cultured on PDA at 25°C, noncolonized PDA plugs were used as the control. Inoculated fruits were incubated at 25℃ with 80% relative humidity. After 10 days, the similar symptoms were observed on the inoculated sites and spread on the surface of fruits, whereas the control remained symptomless. The pathogen was re-isolated from the lesions of inoculated fruits and confirmed as D. sojae via morphological and molecular analysis. The assays were repeated twice, fulfilling the Koch's postulates. Although D. sojae is known as the major causative agent of pod and stem blight, and has been reported as an endophyte in the twigs and leaves of citrus (Huang et al. 2015; Santos et al. 2011), but to our knowledge, this is the first report of postharvest fruits brown rot disease on navel orange caused by D. sojae in China. However, further investigation of the specific causes of this disease is necessary to help the local fruit farmers develop effective disease management strategies.
Keywords: Diaporthe sojae; brown rot; navel orange.