First Report of Diaporthe hongkongensis Causing Leaf Blight on Macadamia in China.

ABSTRACT

Macadamia (Macadamia ternifolia Maiden and Betche) belongs to the Proteaceae family (Li et al. 2022). In the hilly areas of Guangxi (southern China), macadamia trees are an important source of revenue. The planting area in Guangxi has increased in recent years, exceeding 53,333 hectares by the end of 2022, but this increase is also associated with emergency of, macadamia diseases. Leaf blight symptoms were observed in 37/241 macadamia trees (15% incidence) in a plantation in Nanning, Guangxi province in China, during June, 2022. Disease severity on infected trees ranged from 5% to 60%. The disease developed from the tips or margins of leaves, causing the leaves to turn brown, and later gradually withered (Fig. 1 A). Ten leaves with lesions were collected from five macadamia trees (two leaves per tree. Thereafter, small segments (3 to 4 mm²) excised from the margins of ten lesions were surface sterilized in 75% ethanol for 30 s and 1% hypochlorite for 90 s and Page 1 of 6 2 rinsed in sterile water, before plating onto potato dextrose agar (PDA) medium. Plates were incubated under lighting during the daytime, and darkness at night-time for 5 days at 25℃. Twenty-two purified colonies were generated by subculturing hyphal tips, of which eight exhibited similar morphology and were further characterized. The colonies on PDA were gray with a white outer ring and flat lawn on the surface (Fig. 1 B). The pycnidia were superficial to semi-immersed on PDA, solitary to aggregated, globose to sub-globose, brown to black and oozed yellow mucilaginous masses (Fig.1 C). The α-conidia were unicellular, hyaline elliptical or fusiform, and measuring 4-8 × 1.9-4 µm (n=30) ， whereas the ß-conidia were hyaline, long, straight or curved, measuring 20-23 × 0.9-2 µm (n=30) (Fig. 1 D-E). The morphological features were similar to Diaporthe hongkongensis (Dissanayake et al. 2015). The eight morphologically similar isolates were identified as D. hongkongensis using the internal transcribed spacer (ITS) region, but only one isolate, JG11, was selected for further molecular identification. Five target genes, including the ITS region, translation elongation factor 1 alpha (EF1-α), beta-tubulin genes (TUB2), calmodulin (CAL), and histone H3 (HIS) were amplified and sequenced using primers ITS1/ITS4, EF1-728F/EF1-986R, Bt2a/Bt2b, CAL-228F/CAL-737R, and CYLH3F/H3-1b, respectively (Carbone and Kohn 1999). The sequences were deposited in GenBank under accession numbers OQ932790 (ITS) and OR147955-58 for EF1-α, TUB, CAL and HIS genes, respectively. BLAST search of GenBank showed that ITS, EF1-α, TUB, CAL, and HIS sequences of JG11 were similar to Page 2 of 6 3 those of D. hongkongensis NR111848 (99.22% identity), KY433566 (99.72%), MW208603 (99.42%), MW221740 (99.80%), and MW221661 (99.79%), respectively. Phylogenetic analysis of concatenated sequences was performed with IQ-TREE software. JG11 was grouped in the same clade as other Diaporthe hongkongensis isolates (Fig. 2). Pathogenicity experiments were carried out on healthy macadamia trees in a greenhouse. Three macadamia trees were used as negative controls where five uninjured leaves per tree were sprayed with sterile distilled water. Uninjured five leaves per tree of three other macadamia trees were sprayed with conidia suspension of the isolate JG11 at a concentration of 1×106. Each treatment was repeated 3 times independently, with 5 leaves per tree (Liu et al. 2023; Havill et al. 2023; Zhang et al. 2022). Plastic bags were placed over all inoculated leaves. The average daily temperature and relative humidity in the greenhouse were 32°C and 65%, respectively. Two days later, browning appeared on the leaves inoculated with the spore suspension and expanded outward. After 5 days, all macadamia leaves inoculated with the fungal spores began to wither, while controls remained asymptomatic (Fig. 1 H-I). D. hongkongensis was consistently re-isolated and purified from inoculated leaves and the identity was confirmed by morphological identification and molecular analysis, completed Koch's postulates. D. hongkongensis has been reported on peach (Zhang et al. 2021), grapevine trunk (Dissanayake et al. 2015) and Cunninghamia lanceolata (Liao et al. 2022). To our knowledge, this is the first report of D. hongkongensis causing leaf blight on macadamia in China. These findings provide a foundation for future research on the epidemiology and control of this newly emerging disease of macadamia.