In June 2021, a previously unreported leaf blight disease of peanut (Arachis hypogaea) was observed on field-grown peanut (Jinhua19) in Laixi city, Shandong province of China. Approximately 5% of plants showed disease symptoms in the fields we investigated. The symptoms first appeared as yellow round or irregular spots on leaves, and then the spots became brown. As the disease progressed, spots became larger and even converge, which later produced leaf chlorosis and abscission. Symptomatic leaves were cut into small pieces, surface disinfested with 70% ethanol for 30s, 1% NaClO for 60s, rinsed three times in sterile water, dried on sterile filter papers, placed on potato dextrose agar (PDA) media, and incubated at 25°C in darkness. Fungal cultures were initially white, with red pigment, then turned gray, and eventually turned black, and aerial hyphae were dense. Conidia were spherical or slightly ellipsoidal, black, smooth, and 8.6 to 11.5 × 8.7 to 14.5μm (n=50). Morphological characteristics of the isolates matched the description of Nigrospora aurantiaca (Wang et al. 2017). Molecular identification was performed by sequencing beta tubulin gene (TUB) with Bt2a/Bt2b and translation elongation factor 1-alpha (TEF) with EF1-728F/EF1-986R (Wang et al. 2021) of a representative isolate ZHX11. TUB (OK489789) and TEF (OK489790) of ZHX11 obtained 100% (401/401 nucleotides) and 99.64% (279/279 nucleotides) similar to those of N. aurantiaca (MN329935, MN264010), respectively. Alignment was conducted separately for each gene set using Clustal W algorithm implemented in MEGA 7.0 (Kumar et al. 2016), and multi-gene (TUB and TEF) phylogenetic analyses using Neighbor-Joining (NJ) method showed that the isolate was N. aurantiaca. To complete Koch's postulates, nine 2-week-old peanut (Zhonghua 12) seedlings were sprayed with conidia suspensions (106 conidia mL-1 in 0.05% Tween 20 buffer). The same number of seedlings were only treated with 0.05% Tween buffer as controls. The experiment was repeated three times. Plants were incubated in a growth chamber (30°C in the day and 25°C at night, a 12-h photoperiod and 80% RH). Ten days after inoculation, typical symptoms were observed on inoculated leaves but not on the controls. N. aurantiaca was reisolated from the diseased leaves but not from the controls. N. sphaerica was observed on peanut in China (Liu et al. 2020). To our knowledge, this is the first report of N. aurantiaca causing leaf blight on peanut in shandong province, China. These findings will help to develop better preventive measures in accordance with the emergence of the new disease.
Keywords: Leaf Blight; Nigrospora aurantiaca; Peanut.