First report of alfalfa root rot caused by Trichothecium roseum in China.

Alfalfa (Medicago sativa L.) is perennial leguminous forage, which is cultivated throughout the world due to its high yield, high quality, satisfactory palatability, and wide adaptability. With the increase of planting area in China, root diseases caused by Fusarium spp., Sclerotium rolfsii, Phytophthora spp. (Yang et al. 2022), and new pathogens have been found that reduce the yield and quality of alfalfa and cause economic losses (Li at al. 2019). In 2021, an alfalfa disease occurred under conditions of high temperature and high humidity at the Jiaozhou Experimental Base of Qingdao Agricultural University (Jiaozhou Modern Agricultural Science and Technology Demonstration Park, 36.33°N 120.40°E, Qingdao, Shandong, China), and about 2 ha of alfalfa were infected. The disease affected up to 35% of the plants and caused grass spots. Infected plants developed black-brown lesions with irregular shapes on roots with yellowing of the foliage; the leaves of the whole plant turned yellow. In the late stage of the disease, defoliation occurred and the plants stopped growing, wilted and died. Ten infected plants with typical symptom were collected for isolation and identification of pathogen. The infected roots were cut into 3-5 mm2 sections and then soaked in 75% ethanol for 30 s, followed by a 3-minute immersion in 2% sodium hypochlorite for surface sterilization. Next, the tissues were rinsed in sterile water five times and then placed on potato dextrose agar (PDA) medium. After three subcultures and subsequent single spore isolation, one representative strain named as DC1 was isolated from the infected roots. Based on morphological observation, the colony of DC1 was flat, granular, and powdery in appearance. Four days after inoculation on PDA medium, the size of the colony were 2.1-2.6 cm. After 8 to 20 days, the colonies were initially white and gradually change a light pink to peach color. The conidia are two-celled (Hamid et al. 2014), elliptic to pear-shaped, colorless or translucent, smooth to slightly rough with thick walls. The size of conidia ranged from 11.3 to 23.5 μm long × 6.1 to 12.7 μm wide (n =30). For the identification, the rDNA--ITS gene of the fungus was amplified using the primers ITS1/ITS4 (White et al.1990), and the EF1α gene was amplified using primers EF1-983F/EF1-2218R (Rehner and Buckley 2005). Then the PCR amplicons were cloned into the pCE2 TA/Blunt-Zero vector. The results of the rDNA-ITS (OM049197.1, 515 bp) and EF1α (OM069381.1, 926 bp) sequences were deposited in GenBank. DNA analysis showed that the two sequences were 100% similar to the rDNA-ITS sequence (MN882763.1) and EF1α sequence (DQ676610.1) of Trichothecium roseum, respectively. A pathogenicity test was done by placing one piece (0.5 cm in diameter) of fungal culture (PDA plug) 1cm below the crown of 40-day-old healthy alfalfa (cv. Zhongmu No.3) plants, 3 replicates and 20 plants in each replicate. PDA plug without the pathogen were used for control. All plants were cultivated in a growth chamber at 25±1°C with a light cycle of 15 h (90% relative humidity). After 18 days, the roots of inoculated plants had dark brown lesions and the leaves of their plants turn yellow, while those control plants had no symptoms. To fulfilling Koch's postulates, the same pathogen was re-isolated from necrotic root tissue of inoculated plants and confirmed by morphology and the rDNA-ITS and EF1-α sequences. Based on disease symptoms, morphological characteristics DNA sequences and pathogenicity, the pathogen of alfalfa disease in Jiaozhou Experimental Base of Qingdao Agricultural University was identified as T. roseum. To our knowledge, this is first report of T. roseum causing alfalfa root rot. The newly emerging disease may pose threat to alfalfa production of central and southern China in future.