First report of cobweb disease of Auricularia heimuer caused by Hypomyces mycophilus in Fujian Province, China.

Abstract

Auricularia heimuer is a gelatinous fungus with great edible and medicinal values. In September 2021, a suspected cobweb disease was found in some A. heimuer farms in Fujian Province, China. The disease caused white cottony mycelium to grow on the basal surface of the A. heimuer at the beginning of infection and gradually spread along the outer edge of the fruiting body, and eventually the white pathogen mycelium covered the entire fruiting body, which eventually led to the wilting and death of about 35% of A. heimuer . Two heavily infected tissues of A. heimuer were collected and two isolates were obtained by single spore isolation and purification technique. The pathogen colonies grew 10 to 12 mm per day on potato dextrose agar (PDA), and the colonies were initially white in color and gradually changed to yellowish brown with neat margins. Well-developed mycelium with septum, Conidiophores are bottle-stemmed and whorl-shaped branches, Conidia solitary, as ovoid, colorless singletons or doublets. The chlamydospores are yellowish, smooth surface, with 2-3 septa, size 9-22 μm × 30-38 μm. These morphological features are consistent with the Hypomyces mycophilus (Gea et al. 2019;Wang et al. 2021). For molecular identification, genomic DNA of the two isolates was obtained using an extraction kit (Biocolor, Shanghai, China), internal transcribed spacers (ITS) regions and 5SrRNA were amplified using ITS1 and ITS4 primers (White et al. 1990). A 590 bp DNA fragment was obtained and the sequences were deposited in GenBank (Accession Nos. OP715875 and OP782039),A BLAST search in GenBank revealed the highest similarity (≥99%) toH. mycophilus (GenBank Accession Nos.MH857567 and KU937111) .To fulfill Koch's postulates, the isolates cultured on PDA plates for 10 days were made into a spore suspension with sterile water at a concentration of 5 × 106conidia/ml and sprayed onto twenty healthy fruiting bodies grown to about 2 cm in diameter, and another ten healthy fruiting bodies sprayed with sterile water as control, and incubated in an artificial climate chamber at 25℃ and relative humidity of 90%-95% (Back et al. 2012). After 4 days of inoculation, the pathogen started to germinate and slowly grew white mycelium, then the white mycelium multiplied at the base of the fruiting bodies and spread from the base to the periphery, and finally the fruiting bodies were completely covered by the pathogenic mycelium and gradually wilted. The symptoms were consistent with the natural disease symptoms under cultivation conditions, while the control group had normal growth of the seeds and no disease symptoms. H. mycophilus was reisolated and purified from symptomatic cotyledons and identified by the above method, and the results of the two experiments were consistent. To our knowledge, this is the first report of H. mycophilus causing cobweb disease in A. heimuer.