A new soybean cyst nematode Heterodera sojae n. sp. was found from the roots of soybean plants in Korea. Cysts of H. sojae n. sp. appeared more round, shining, and darker than that of H. glycines. Morphologically, H. sojae n. sp. differed from H. glycines by fenestra length (23.5-54.2 mm vs. 30-70 mm), vulval silt length (9.0-24.4 mm vs. 43-60 mm), tail length of J2 (54.3-74.8 mm vs. 40-61 mm), and hyaline part of J2 (32.6-46.3 mm vs. 20-30 mm). It is distinguished from H. elachista by larger cyst (513.4-778.3 mm 3 343.4-567.1 mm vs. 350-560 mm 3 250-450 mm) and longer stylet length of J2 (23.8-25.3 mm vs. 17-19 mm). Molecular analysis of rRNA large subunit (LSU) D2-D3 segments and ITS gene sequence shows that H. sojae n. sp. is more close to rice cyst nematode H. elachista than H. glycines. Heterodera sojae n. sp. was widely distributed in Korea. It was found from soybean fields of all three provinces sampled.
Meloidogyne mali was originally described in Japan on roots of an apple rootstock (Malus prunifolia) (Itoh et al. 1969) and found on elm trees in Italy, Netherlands, Belgium, France and United Kingdom, and euonymus in the USA (EPPO 2018; Prior et al. 2019). In Italy, the nematode was initially described as a new species, Meloidogyne ulmi, but was later synonymized with M. mali (Ahmad et al., 2013). During the study of Meloidogyne species in Republic of Korea, galled roots were found on Acer palmatum collected in Naejangsan National Park, Republic of Korea located at 35°29'29.1"N, 126°55'42.7"E, altitude 147.8 m. Morphologically, the perineal patterns of the females was very similar to M. mali due to rounded dorsal arch and smooth, finely-spaced, indistinct striae. Lateral field shallow, narrow, and faint. Phasmids large, very distinct. Head region of second–stage juveniles flattened anteriorly to hemispherical, slightly set-off from body, without annulations, low head cap. Stylet slender, sharply pointed cone, cylindrical shaft with rounded knob sloping posteriorly. Tail conoid with irregular, and rounded end. Rectum undilated. Several micrographs were made from 25 J2s and females for mean, standard deviation and range. J2s were measured with a body length: 408.2 ± 25.1 (366-449) µm, maximum body width: 15.9 ± 1.0 (14.1-17.9) µm, stylet length: 14.1 ± 0.5 (13.1-15.3) µm, hyaline tail terminus: 10.0 ± 0.9 (8.3-11.0) µm and tail length: 31.7 ± 3.0 (26.0-36.1) µm. Females (n=25) were characterized by a body length: 656.7 ± 102.7 (516-947) µm, a stylet length: 16.4 ± 2.2 (13.9-19.0) µm, a vulval slit length: 22.2 ± 1.8 (19.8-25.7) µm, and a vulva-anal distance: 20.2 ± 2.4 (17.1-25.4) µm. Morphological measurements and configuration of perineal patterns (Fig. 1S) were comparable to M. mali (Itoh et al. 1969; Ahmed et al. 2013; Gu et al. 2020). To confirm pathogenicity, a modified version of Koch’s postulates was conducted in the greenhouse by inoculating 300 eggs from a single egg mass onto each of three, two-year-old A. palmatum plants, grown in sterilized sandy soil. After about one year, symptoms developed on the maple tree roots, with numerous galls containing females and egg masses by visual inspection. In addition, PCR was performed for the 28S rDNA D2-D3 segment and ITS region using the primers D2A, D3B, TW81 and AB28. The resulting sequences (MW522548, MW522549, MW523004 and MW523005) were at least 99% identical to other 28S rDNA D2-D3 segment and ITS region sequences on Genbank (MT406757 and JX978229). The molecular phylogenetic relationships of this species strongly supports M. mali (Fig. 2S). To the best of our knowledge, this is the first report of M. mali in Republic of Korea. The host range of M. mali includes many species which are of economic importance in fruit trees (e.g. apple, chestnut, fig, mulberry), forestry trees (e.g. elm, maple, oak, Yew), and vegetable crops (e.g. cabbage, carrot, cucumber, eggplant, soybean, watermelon). The potential danger to these economically important plants caused M. mali to be added the EPPO Alert List and also the Quarantine List of the Korean Animal and Plant Quarantine Agency. Additionally, in our survey around the Naejangsan National Park, M. mali was not found on other economically important host crops, such as grapes. Although this nematode was not detected other crops, it requires regular monitoring because it poses a serious threat to the future production of these crops.
Soybean cyst nematode (Heterodera glycines, SCN) is the most harmful pathogen of soybean (Glycine max (L.) Merr.) worldwide. In 2016, a new soybean-parasitic cyst nematode, Heterodera sojae (the white soybean cyst nematode) was found parasitizing the roots of soybean plants in Korea. To investigate the distribution and population density of H. sojae, 943 soil samples were collected from soybean fields in all 9 provinces in Korea in 2017-2018. Cyst nematodes were detected in 343 samples (36.4%) from 8 of the 9 provinces, except the island of Jeju province. Among the 343 samples, H. glycines was found in 227 samples (66.2%), H. sojae in 95 samples (27.7%) and 21 samples (6.1%) were infested with both H. sojae and H. glycines. Wide distribution of H. sojae in soybean fields indicates that H. sojae is an important cyst nematode species parasitizing soybean together with H. glycines.
Morphological (cyst shape, color, and sizes [length (L), maximum width (W), volume and "a" (L/W)]), structural (vulvar cone slope angle [VCSA], surface wrinkle [VCSW], cyst wall thickness, composition, and texture) and biological characteristics (fecundity, hatching, and emergence [number of second-stage juveniles (J2) from a cyst]) in preceding Heterodera glycines (Hg), currently-recorded H. sojae (Hs) and H. trifolii (Ht) were examined by microscopy. Cysts were lemon-shaped, indicating the genus is Heterodera except for Hs that formed frequently globular cysts with significantly flatter VCSA (102.2°) with smooth VCSW than Hg (50.6°) and Ht (82.0°), but not genus Globodera because of the presence of vulvar cone in Hs. Ht was significantly larger in all morphological characteristics than Hg and Hs, suggesting Ht may be diagnosed differentially by cyst sizes and also host plant preferences. Hs showed smaller "a" value with more globular shape and stronger structures with more thickened and strengthened collagen-like texture of cyst wall than Hg and Ht. This suggests Hs may be diagnosed differently by structural characteristics from the others, especially Hg with similar cyst sizes. There were no significant differences in emergence (inoculum potential) among cyst nematodes due to the offset of fecundity and hatching rate; however, the inoculum potential of Hs may be not so persistent as Hg and Ht in fields because of its lower fecundity and higher hatching rate (causing rapid inoculum loss) than the others. These characteristics of cysts provide information useful for simple and differential diagnoses and reliable management of cyst nematodes.
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