First Report of Pink Snow Mold Caused by Microdochium majus on Wheat in Xinjiang Uygur Autonomous Region, China.

Abstract

Pink snow mold (PSM) is a destructive disease that primarily occurs in cold, humid regions with persistent snow cover. The main pathogens, Microdochium nivale and M. majus, can infect a wide range of graminaceous crops under snow cover (Ponomareva et al., 2021). In Xinjiang, PSM has caused significant damage to wheat in years with heavy snowfall. In March 2023, a detailed survey was conducted in Nilek County, Yili Prefecture, Xinjiang Uygur Autonomous Region, to investigate prevalent diseases impacting winter wheat (Triticum aestivum L.) following snow melt. Widespread infection was observed on seedlings in wheat fields, with dead leaves near the soil surface covered in a typical dusky-pink mold and the upper leaves displaying watery spots. Over 50% of the wheat exhibited these symptoms in approximately 10 hectares of the field. To identify the pathogen, 12 symptomatic leaf segments approximately 0.5 cm in length were collected, surface disinfected, placed on potato dextrose agar (PDA), and incubated at 20°C for 5 days. All symptomatic tissues yielded fungal colonies that exhibited vigorous growth, and pink to orange fluffy mycelium. After 1 week, orange-red conidial mounds appeared at the edges of the PDA. The conidia measured 2.52-6.41 × 9.23-21.27 μm (av = 4.05 × 14.79 μm; n = 60), with 1-6 septa. These morphological characteristics were consistent with previous reports and all 12 isolates were identified as M. majus (Glynn et al., 2005; Hernández-Restrepo et al., 2015). Molecular confirmation was performed using four primer pairs targeting the 28S ribosomal RNA (LSU) gene (Hernández-Restrepo et al., 2015), internal transcribed spacer (ITS) (White et al., 1990), RNA polymerase II second largest subunit (RPB2) gene, and β-tubulin gene (Jewell et al., 2013). BLAST analysis showed high similarity (≥ 99.58%) with the corresponding sequences of LSU (KP858937, 832/832 bp), ITS (OM949048, 509/509 bp), RPB2 (MZ734201, 727/727 bp) and β-tubulin (JX280568, 717~719/720) of M. majus in the database. The new sequences were deposited in GenBank as follows: LSU: PQ319764 to PQ319775; ITS: PQ252935 to PQ252946; RPB2: PQ261093 to PQ261104; β-tubulin: PQ261081 to PQ261092. To assess the pathogenicity of the isolates, germinated seeds of the susceptible wheat cultivar Mingxian169 were grown in test tubes containing Murashige and Skoog medium, each inoculated with a 0.8 mm diameter plug of M. majus. Of the 12 isolates, seven were selected for pathogenicity test. In the control group, seeds were cultivated without the fungal inoculum. The test tubes were incubated at 20°C under a 16-hour photoperiod for 20 days, with 10 replicates per isolate (Gorshkov et al., 2020). The control showed no symptoms, whereas all inoculated treatments resulted in wheat seedlings with brown discoloration at the stem base, watery leaf spots, and dusky-pink mold in severe cases, leading to plant death. No pathogens were isolated from the control, whereas M. majus was reisolated from the symptomatic leaves, confirmed morphologically and molecularly, fulfilling Koch's postulates. In conclusion, this study provides the first confirmation of M. majus as the causal agent of PSM in Xinjiang. Future efforts should prioritize monitoring and managing the occurrence and spread of M. majus-induced PSM in wheat to mitigate its potential threat to wheat production and prevent significant economic losses.