First Record of Aphelenchoides oryzae on Foxtail Millet (Setaria italica (L.) P. Beauv.) in Jilin Province of China.

Abstract

Aphelenchoides oryzae Yokoo caused a large reduction in rice yields in Japan (1948). It was later synonymised with A. besseyi by Allen (1952), but Subbotin et al. (2021) considered it a valid species. In the main foxtail millet (Setaria italica (L.) P. Beauv.) production area of western Jilin Province, China, many plants were stunted, with thin spikes and open, smooth, shiny glumes. Severely affected spikes were noticeably shorter, fluffy at the top, and erect. In August 2023, 10 foxtail millet samples were collected and nematodes were isolated from 9 of them. A population from Songyuan City, Jilin Province (E:123.64, N:44.86) was studied. The average number of nematodes isolated per gram of ear was 510.7 ± 15.17. Female body slender, lip region rounded, lateral fields with 4 or 6 incisures, vulva located at 71.6% of the body length, post vulvar uterine sac (PUS) 3.7 times anal body width long but less than quarter distance from vulva to anus (VA), tail conical with 3 or 4 terminal spikes. The body length (L), maximum body width (W) and tail length of the female (mean, n=25) were 648 μm, 15.9 μm and 36.7 μm, respectively. PUS length / (VA)% = 23.5, L/W = 41.1, L/ tail length = 17.8. Male body tail curves like a sickle, lacks bursa and shows three pairs of copulatory papillae. Spicules typical of the genus except that the proximal end lacks a dorsal process and has only a moderately developed rostrum. Male measured (mean, n = 25): L = 525.8 μm, W = 14.8 μm, tail length = 34.0 ± 0.7 μm, spicule length = 16.4 μm; L/W = 35.6; L/tail length = 15.6 μm. Amplification of the D2-D3 expansion segments of the 28S ribosomal RNA and the cytochrome oxidase subunit I (COI) gene of mitochondrial DNA (mtDNA) with primers the forward D2A (5'-ACAAGTACCGTGAGGGAAAGTTG-3') and the reverse D3B (5'-TCGGAAGGAACCAGCTACTA-3') (Subbotin et al. 2006), and forward COI-F1 (5'-CCTACTATGATTGGTGGTTTTGGTAATTG-3') and the reverse COI-R2 (5'-GTAGCAGCAGTAAAATAAGCACG-3') (Kanzaki and Futai 2002). PCR conditions were as described by Ye et al. (2007). The sequences of 28S D2-D3 region (726 bp, PP573753- PP573761) of rDNA were 100% identical to A. oryzae (KY123700, KY123694) and COI region (698-700 bp, PP733171-PP733179), were 98.88% identical to A. oryzae (GU367867). Bayesian inference was used to construct phylogenetic tree of 28S D2-D3 region and COI gene, which showed that the Jilin populations clustered together with A. oryzae, which was a sister branch of A. besseyi. Pathogenicity was established via the infection of foxtail millet (cv. Jiyou 2). The germinated foxtail millet seeds were planted in pots containing 350 ml of sterile soil mixture. On the 15th day, every 10 seedlings were inoculated with 100 A. oryzae at the leaf sheet wounds and 3 plants were noninoculated as control. Three independent replicates were performed on different dates. Forty days post-inoculation, an average of 88.3 ± 2.26 A. oryzae were extracted from each nematode-inoculated plant, and the plants were yellowed and necrotic, uninoculated plants grew normally. The molecular and morphological identification of the nematodes obtained by re-isolation from the plants was identical to that of A. oryzae. Our findings clarify the pathogen species, which can be targeted to develop prevention and control strategies, which are important for ensuring safe grain production and helping promote sustainable local agricultural production. To our knowledge, this is the first record of A. oryzae on foxtail millet in Jilin, China.