First Report of Trichoderma Hamatum Causing Postharvest Bulb Rot on Lilium Davidii Var. Willmottiae in China.

Lilium davidii var. willmottiae, known as Lanzhou lily, is a famous edible crop that is mostly distributed in the middle area of Gansu Province in China. In the winter of 2019, symptoms of bulb rot were observed on Lanzhou lilies harvested from Lanzhou, Gansu Province, during storage at the Institute of Grassland, Flowers and Ecology (39°57'55.984" N, 116°20'8.124" E), Beijing Academy of Agriculture and Forestry Sciences, at an incidence of nearly 50%. The decayed bulb (Fig.1a)was washed under tap water and surface disinfested with 75% ethanol for 1 min, followed by 2.5% sodium hypochlorite for 5 min, and washed with sterile distilled water three times. The 5 mm×5 mm tissue pieces from the junction of the diseased part and the healthy part were clipped, placed on potato dextrose agar (PDA) medium and subsequently incubated at 25 °C. Thirteen dominant pure fungal isolates with the same morphological characteristics were obtained by the hyphal-tip method. Three representative isolates LZ-8, LZ-9-2 and LZ-10 were chosen for phylogenetic analyses. The internal transcribed spacer (ITS), translation elongation factor 1-alpha (TEF-1a), and RNA polymerase II second largest subunit (RPB2) sequences were PCR amplified using the primer pairs ITS1/ITS4 (White et al. 1990), EF1-728F/EF1-986R (Carbone and Kohn 1999), and RPB2-5F2/RPB2-7cR (O'Donnell et al. 2022), respectively. BLAST analysis showed that the ITS,TEF-1a, and RPB2 sequences of the isolates LZ-8 (GenBank accession nos. PP422096, PP447248, and PP447251), LZ-9-2 (GenBank accession nos. PP422098, PP447249, and PP447252) and LZ-10 (GenBank accession nos. PP422099, PP447250, and PP447253) had 99.27 to 99.71% identity with multiple GenBank sequences of Trichoderma hamatum, and the three DNA fragments of the three isolates showed 100% sequence identity. A phylogenetic tree based on concatenated sequences of the three genes using maximum -likelihood analyses revealed that the three isolates LZ-8, LZ-9-2 and LZ-10 were in the same clade with T. hamatum strains (Fig.2). One representative isolate, LZ-10, was chosen for morphological studies and test of the pathogenicity. The colony of LZ-10 on PDA appeared white with cotton-shaped aerial hyphae early, which later turned light green to green and formed concentric rings (Fig.1d-1f). At the end of conidiophores, three to six pear-shaped branches were irregularly gathered(Fig.1h). Conidia were ellipsoid with the size of 3.1 to 4.4 × 2.2 to 3.1 µm (n =20) (Fig.1g). These morphological characteristics were consistent with the description of Trichoderma hamatum. (Kamala et al. 2015, Han et al. 2017).To test pathogenicity, healthy bulbs were punctured with disposable sterilized needles and soaked in equal amounts of sterile water and conidial suspension (1×107 conidia/mL) for 30 min respectively. The pathogenicity experiment was repeated three times. After 6 days of inoculation at 25 °C and 80% relative humidity, the surface of the inoculated bulbs produced water-stained spots and mycelium layers(Fig.1b-1c) consistent with the symptoms exhibited by Lilium davidii var. willmottiae bulbs during storage, meanwhile the uninoculated lily bulbs remained symptomless. Trichoderma hamatum was reisolated from the infected bulbs and identified based on morphological and molecular characteristics, fulfilling Koch's postulates. To our knowledge, this is the first report of bulb rot on Lilium davidii var. willmottiae caused by Trichoderma hamatum in China. This study will contribute to a better understanding and controlling of this postharvest disease in Lilium davidii var. willmottiae.