First Report of Root Rot Caused by Pleiocarpon algeriense on Peony (Paeonia suffruticosa) in China

Peony (Paeonia suffruticosa Andr.) is a perennial plant of Ranunculaceae. Its root bark (Danpi in Chinese) is a traditional Chinese medicine, which has the effects of clearing heat and cooling blood, promoting blood circulation to resolve blood stasis. Peony is mainly planted in the provinces of Anhui, Gansu, Henan and Shandong. Peony is also called Fengdan in the Fenghuang Mountain of Tongling, Anhui Province. In November 2021, a root rot-like disease was observed on the root of peony in several fields located in Tongling county, Anhui Province, China (118°0'51" N, 30°48'11" E). Approximately 20-40% of the peony plants were affected in the fields. The roots of the diseased plants were rotten and blackened, the bark of the roots was detached, and the leaves were withered, causing the whole plants to die. To isolate the pathogen, the symptomatic roots were sampled, and small pieces (5 × 5 mm) of diseased tissues were surface sterilized with 0.5% NaClO solution and 75% ethanol for 5 min, rinsed with sterile distilled water three times, and finally incubated on potato dextrose agar (PDA) at 28°C in the dark for 7 days. A total of 16 isolates were obtained from the infected tissues. Among isolates, six isolates were morphologically similar to B4. Colonies were passaged multiple times on fresh PDA medium, and pure isolate B4 exhibiting cinnamon-to-honey coloration on PDA with pale yellow aerial hyphae, was then selected. Microscopic observations revealed that microconidia were straight to curved, ellipsoid or subcylindrical, and ranged from 7.14 to 14.29 × 2.85 to 5.00 μm (n = 20). The morphological characteristics were similar to the description of Pleiocarpon algeriense by Aigoun-Mouhous et al. (2019). To further identify the taxonomic status of B4 strain, three genes of the internal transcribed spacer (ITS) region of rDNA, beta-tubulin (TUB2), and the RNA polymerase II second subunit (RPB2) were respectively amplified and sequenced using primers ITS1/ITS4 (White et al. 1990), T1/Bt-2b (O'Donnell and Cigelnik 1997), and 5F2/7cR (O'Donnell et al. 2007). Sequences for the isolate B4 were deposited in GenBenk (OP810684, ITS; OP882301, TUB2; OP863337, RPB2). BLAST analysis showed the ITS, TUB2, RPB2 sequences of B4 were 99.80% (505/506), 99.51% (609/612) and 100.00% (854/854) homology with those of P. algeriense Di3A-AP52 (MT613337, ITS; MT597145, TUB2; MT635004, RPB2). A phylogenetic tree was built using MEGA11 based on sequences of three genes showing that B4 strain was closely clustered with reference strain of P. algeriense, which has not been reported in peony in China. The pathogenicity test of the isolates was performed by inoculating 50 mL of conidial suspension (1 × 108 conidia/mL) on the roots of ten healthy peonies, ten peonies inoculated with 50 mL of sterile water were used as a control group. After one-month, typical symptoms of root rot appeared on the inoculated plants and the control plants were asymptomatic. The fungus (P. algeriense) was reisolated from the diseased roots and identified by sequencing of ITS gene, conforming to Koch's postulates. Pleiocarpon algeriense has been reported to cause stem and crown rot in avocado (Aiello et al. 2020). To the best of our knowledge, this is the first report of P. algeriense causing root rot in peony. Control methods of P. algeriense on peony fields will be studied in-depth in the future.