Occurrence of Leaf Spot Caused by Neodeightonia phoenicum on Pygmy Date Palm (Phoenix roebelenii) in China.

Abstract

The pygmy date palm (Phoenix roebelenii) is a popular ornamental plant widely cultivated in tropical regions as well as in China. In June 2018, a new leaf spot symptoms were observed on P. roebelenii in several different parks in Zhanjiang City of China. The early symptoms of infected leaves were presented with small, round, pale brown spots. As the size of these spots increased, they coalesced to form larger irregular necrotic lesions surrounded by dark brown edges, which eventually led to leaf wilted and defoliation. A filamentous fungus was consistently isolated from infected leaf samples. Colonies on PDA at 25°C (12 h light/dark) were initially white with abundant aerial mycelium, which turned fluffy and dark olivaceous after one-week culture. Pycnidial conidiomata were black and globose and formed on pine needles in water agar at 25°C (12 h light/dark) after 21 days. Conidiogenous cells were hyaline, cylindrical, holoblastic. The conidia was ovoid to ellipsoid, thick-walled, which was initially hyaline and aseptate, later turned into dark brown and 1-septate with a striate appearance to conidia, 11.6~25.0 μm×9.6~12.0 μm (av. 20.4 μm×10.1 μm). For molecular identification, the partial sequences of internal transcribed spacer (ITS) regions, translation elongation factor (EF-1α) and β-tubulin (TUB) genes of two representative isolates RYCK-1, RYCK-2 were amplified and sequenced using primer pairs ITS/ITS4 (White et al. 1990), EF-688F/EF-986R (Carbone and Kohn 1999), and Bt2a/Bt2b (Glass and Donaldson 1995), respectively. The sequences of the above three loci of the two isolates (accession nos. ITS, OK329968 and OK329969; EF-1α, OK338067 and OK338068; TUB, OK338069 and OK338070) showed 98.4-100.0 % identity with the existing sequences of ex-type culture CBS 122528 of N. phoenicum. A multilocus phylogenetic analysis of the three loci concatenated sequences using the maximum likelihood method showed the isolates that belongs to N. phoenicum. Based on the morphological characteristics and molecular analysis of the isolates, the fungus was identified as N. phoenicum (Phillips et al. 2008). To confirm pathogenicity, five one-year-old potted plants were used for each isolate (RYCK-1 and RYCK-2) and the plants were inoculated by pricking the epidermis of the leaf with a needle. Five leaves of each plant were sprayed with 100 µl of a conidial suspension (1 × 106 conidia/ml) to the wounded surface for each plant. Sterilized distilled water was used as the control and the experiment was repeated. All the plants were incubated at 26 ± 2°C (12 h light/dark) and covered with plastic bags to maintain constant high humidity. After 14 days, all the inoculated leaves showed the same symptoms as those observed in the original diseased plants, but the control plants remained health. The reisolated fungus was identified as N. phoenicum by morphological and molecular characteristics. N. phoenicum is an important pathogen of Phoenix species plants worldwide, which have been reported to cause shoot blights and stalk rots on P. dactylifera and P. canariensis in Greece (Ligoxigakis et al. 2013) and root rot on P. dactylifera in Qatar (Nishad and Ahmed 2020). To our knowledge, this is first report N. phoenicum causing leaf spot on P. roebelenii in China.