Banana Shrub (Michelia figo (Lour.) Spreng.) is widely cultivated in most of southern China (Wu et al, 2008). It can be used to make essential oil and flower tea（Ma et al, 2012; Li et al, 2010）.The first symptoms were observed in Sept. 2020 at a grower's field in Banana shrub seedlings (0.6 ha), Ya'an city (29°30'N, 102°38'E), Hanyuan county. The symptoms re-occurred in May-June of 2021 and became prevalent from August to September. the incidence rate and the disease index were 40% and 22%, respectively. Initially, purplish-brown necrotic lesions appeared at the leaf tip with dark-brown edges. Progressively, necrosis spread, to the middle of the leaves, and the older area turned gray-white. Dark sunken lesions appeared in the necrotic areas and orange conidial masses were visible under humid conditions. Ten isolates were obtained on potato dextrose agar (PDA) from 10 leaf samples using previously described tissue isolation method (Fang et al. 1998). All the 10 isolates exhibited similar morphological characteristics. Grey to white aerial mycelium at the center and in dispersed tufts, with numerous dark conidiomata scattered over the surface, reverse was pale orange with numerous dark flecks corresponding to the ascomata, orange conidial masses were formed from mature conidiomata. Conidia were hyaline, smooth-walled, aseptate, straight, cylindrical, apex round, the contents appearing granular 14.8 to 17.2 × 4.2 to 6.4 μm (average: 16.26 × 4.84 μm, n=30) as Colletotrichum spp. (Damm et al. 2012). For molecular identification, DNA was extracted from a representative isolate HXcjA using a plant genomic DNA extraction kit (Solarbio, Beijing). and the partial sequences of internal transcribed spacer region (ITS, OQ641677), glyceraldehyde-3-phosphate dehydrogenase (GAPDH, OL614009), actin (ACT, OL614007), beta-tubulin (TUB2, OL614011), histone3 (HIS3, OL614010), and calmodulin (CAL, OL614008) were amplified and sequenced using the primer pairs ITS1/ITS4 (White et al. 1990), GDF/GDR (Templeton et al. 1992), ACT-512F/ACT-783R, CAL 228F/CAL 737R (Carbone et al. 1999), TUB1F/Bt2bR, CYLH3F/CYLH3R (Crous et al. 2004), respectively. BLASTn analysis for ITS, GAPDH, CAL, ACT, TUB2 and HIS3 sequences showed ≥99.7% identity to C. Karstii, namely, NR_144790 (532/532 bp), MK963048 (252/252 bp), MK390726 (431/431 bp), MG602039 (761/763 bp), (KJ954424, 294/294 bp), (KJ813519, 389/389 bp), respectively. The fungus was identified as C. karstii based on morphology and a multigene phylogeny. The conidial suspension (1 × 107 conidia/mL) with 0.05% Tween 80 buffer was used for pathogenicity test, by spraying 2-year-old Banana Shrub plants. Ten plants were inoculated with spore suspensions (approximately 2ml per plant). An equal number of plants were sprayed with 0.05% Tween 80 buffer to serve as a control. Fifteen days later, the inoculated plants showed similar symptoms as the original diseased plants but the controls remained asymptomatic. C. karstii was re-isolated from the infected leaves and identified by morphology and a multigene phylogeny. The pathogenicity test was repeated three times with similar results, confirming Koch's postulates. To our knowledge, this is the first report of Banana Shrub leaf blight caused by C. karstii in China. This disease reduces the ornamental and economic value of Banana Shrub, and this work will provide a basis for the prevention and treatment of the disease in the future.
Keywords: Banana Shrub; Colletotrichum karstii; Leaf blight; Michelia figo.