Peanut (Arachis hypogaea L.) is a widely grown oilseed crop of great agricultural importance worldwide. In July 2022, disease symptoms were observed on peanut roots in Laixi (36º85' N, 120º54' E), Shandong Province, China. About 25% of the plants showed various symptoms, including stem and root rot and blackening, microsclerotia on the stem, yellowing and wilting of leaves, and even death. Twenty diseased plants were collected to confirm the pathogen. Symptomatic roots were cut into small pieces, disinfested with 75% ethanol for 1 min and 0.5% NaClO for 2 min, rinsed three times with sterile water, dried on sterile filter paper, and then spread on potato dextrose agar (PDA) supplemented with 100 μg/mL chloramphenicol and incubated at 25°C in the dark. At the beginning of growth, the fungus formed sparse, white mycelia, which white, then darkened with age and microsclerotia were formed in the medium after 5 days. The mycelium aggregated into black, round to oblong or irregularly shaped microsclerotia 84 to 163 μm long and 54 to 125 μm wide (n=40). These morphological characteristics were consistent with the description of Macrophomina phaseolina (Holliday and Punithalingam, 1970). Molecular identification was performed by sequencing the internal transcribed spacer (ITS) region with ITS1 and ITS4 and translation elongation factor 1-alpha (TEF) with EF1-728F/EF1-986R (Glass and Donaldson 1995) of a representative isolate SXY183. ITS (OR056369) and TEF (OR098356) of SXY183 showed 100% and 97.74% similarity with M. phaseolina (KF951622, KF951997), respectively. Phylogenetic analysis was performed using Neighbor-Joining (NJ) analysis based on the gene sequences of ITS and TEF. The fungus was identified as M. phaseolina based on molecular analysis and morphological characteristics. The pathogenicity of a representative isolate (SXY183) was tested on peanuts under greenhouse conditions. Two-week-old peanut (Huayu No. 9115) seedlings were inoculated with a mycelial plug (8 mm diameter) at the root base of each plant and cultured in a greenhouse (30°C during the day and 25°C at night, a 12-h photoperiod, and 80% RH). Ten plants were inoculated with a plug of non-colonized PDA as a control. Brown lesions were observed on the stem and root of all inoculated seedlings 7 days after inoculation, but not on the control plants. The experiment was repeated three times. M. phaseolina was re-isolated from the symptomatic root and confirmed based on morphological characteristics and DNA sequence analysis of ITS and TEF. M. phaseolina is a soil-borne fungus that is distributed worldwide and has a broad host range. Disease agent has previously been reported on several host plants such as adzuki bean, faba bean, watermelon, Plukenetia volubilis, Atractylodes lancea and Curcuma longa in China (Cai et al., 2020; Sun et al. 2016; Sun et al., 2019; Sun et al., 2020; Wang et al., 2020; Wu et al., 2022). However, this is the first report in which M. phaseolina was found to cause peanut root rot in Shandong Province, China. Our report will provide important information for studying the epidemiology and management of this disease.
Keywords: Causal Agent; Fungi; Macrophomina phaseolina; Microsclerotia; Peanut.