First Report of Clonostachys rosea Causing Rot of Morchella sextelata in Anhui Province, China

Morel mushroom (Morchella spp.) is a valuable mushroom, which has extremely high nutritional and economic value. In the early March of 2022, a serious rot disease was observed on approximately 30% fruiting bodies at an M. esculenta farm of Suzhou City, Anhui province, China. A white mold-like hyphae was initially present on the pileus, which then gradually spread to the whole fruiting body, eventually resulting in softening of the fruiting body and death. This disease developed rapidly at relatively high temperature (>20°C) and humidity (>85%), resulting in approximately 80% loss of yield. Twenty infected tissues were cut into small pieces (5 × 5 mm) and placed on potato dextrose agar (PDA) and incubated at 25°C for 5 days. Fifteen morphologically similar isolates were obtained and purified using the single spore isolation technique. Colonies of these isolates were yellowish-white, and tomentose with thick aerial hyphae after 7 days at 25°C on PDA plates. Conidiophores were dimorphic: primary conidiophores were Verticillium-like, secondary conidiophores were penicillate. Primary conidiophore stipe length ranged from 68.6 to 180.5 µm, and the base width was 3.2-5.6 µm. Phialides were solitary, straight, generally slightly tapering towards the tip, each producing a small, hyaline drop of conidia. Secondary conidiophores stipe length ranged from 68.4 to 120.5 µm, the base measured 3.3-6.1µm. Phialides were straight to slightly curved, slightly flask-shaped, with widest point below the middle, slightly tapering in the upper part, without visible collarette. Conidia were colorless, smooth, slightly curved, and distally broadly rounded with an average size of 6.3 to 8.2 × 2.4 to 3.7 μm (n=30). These isolates were initially identified as Clonostachys rosea based on morphological features (Schroers et al. 1999). To confirm the identity of C. rosea, primers ITS1/ITS4 (White et al. 1990) and EF1-728F/ EF1-986R (Carbone and Kohn 1999), were used to amplify the internal transcribed spacer (ITS), and translation elongation factor 1-alpha (EF-1α) genes of the representative isolate 5-3-2. These sequences were deposited in GenBank (GenBank accession nos. ON614093 and ON630916) and had 100% and 99.45% nucleotide identity with Clonostachys rosea E5R(17) and Clonostachys rosea KGSJ26 (GenBank accession nos. MK752437 and MT462122), respectively. Single conidium were isolated and multiplied on PDA for pathogenicity testing. To fulfill Koch's postulates, pathogenicity tests were performed using the fruiting bodies. Spores of C. rosea isolates 5-3-2 were collected and diluted with sterile distilled water at a concentration of 1 × 106 conidia/ml. Five healthy fruiting bodies were inoculated with 1 ml of the spore suspension, which were maintained in an artificial climate chamber at 22°C and 85% humidity. Sterile water inoculated on other 10 healthy fruiting bodies served as control. Mycelia grew rapidly and overgrew more than half of the fruiting bodies within 3 days. The fruiting bodies died five days after inoculation. Clonostachys rosea isolates were re-isolated from symptomatic fruiting bodies and identified by the methods described above. The control group showed no symptoms. The experiment was conducted twice. To our knowledge, this is the first identification of Clonostachys rosea as the causal agent of the Morchella sextelata rot.