Occurrence of Trichoderma hamatum causing soft rot in Paeonia lactiflora roots in China

Paeonia lactiflora Pall., whose roots yield the traditional Chinese medicine “Radix Paeoniae Rubra”, is an important cash crop in Northeastern China, with Heilongjiang as its main planting area. In September 2024, severe root rot was observed during harvesting in Jiamusi, Heilongjiang (46.24°N, 130.55°E), affecting 85%–95% of 3-year-old P. lactiflora plants in four 5-hectare fields. Diseased roots exhibited white aerial mycelium, soft disintegrated internal tissue, brown empty xylem, and intact epidermis. To identify the causal agent, 20 symptomatic roots (5 per field) were collected. Small pieces (0.5 cm×0.5 cm) from the diseased-healthy junction were surface-sterilized (75% ethanol for 2 s, 1.5% sodium hypochlorite for 5 min), rinsed thrice with sterile water, dried, plated on PDA (50 µg ml⁻¹ streptomycin), and incubated at 26°C in darkness. Twenty-one identical fungal isolates were obtained: dense white PDA colonies turning green in 5–7 days; lageniform to ampulliform phialides (2.9–9.75 × 1.36–4.3 µm, n=50); green, ellipsoidal to oblong conidia (2.1–3.3 × 1.6–2.33 µm, n=50), consistent with Trichoderma sp. (Leslie and Summerell 2006). For molecular identification, genomic DNA from three representative isolates (CS1, CS2, CS3) was extracted. ITS, tef1, and rpb2 genes were amplified with primers ITS1/ITS4 (White et al. 1990), EF1-728F/EF1-986R (Carbone and Kohn 1999), and RPB2-5F2/RPB2-7cR (Sung et al. 2007), respectively. Consensus sequences (accession nos. PQ867578–PQ867580, PQ884217–PQ884219, PQ884220–PQ884222) showed 100% identity to Trichoderma hamatum strains JAHLH2, 2019002, and EEK-D8. Phylogenetic trees constructed via PhyloSuite v1.2.2 and edited in FigTree v1.4.4 (Ronquist et al. 2012; Rambaut et al. 2020) confirmed the isolates as T. hamatum. Pathogenicity was tested by inoculating 3-year-old healthy potted P. lactiflora with sorghum grains colonized by CS1–CS3 isolates (1:5, w/w with sterile soil). Each treatment included 15 seedlings (3 replicates of 5), with 5 non-inoculated seedlings as controls. At 21 days post-inoculation, inoculated plants showed typical root rot symptoms, while controls were asymptomatic. T. hamatum was re-isolated from symptomatic roots, fulfilling Koch’s postulates. Trichoderma species are mainly biocontrol agents, but pathogenicity has been reported (Yang et al. 2021; Shu et al. 2023). To our knowledge, this is the first report of T. hamatum causing root rot on P. lactiflora in China, threatening its yield and quality. Effective disease management is urgently needed.