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.
A greenhouse experiment was conducted to examine the effect of initial population density (Pi) of Helicotylenchus dihystera on six commercial tomato cultivars. Two-week-old tomato seedlings of six commercial cultivars were transplanted in d-10-cm clay pot and was inoculated with to give 0, 0.02, 0.2, and 2 nematodes/g soil. Plants were grown in a greenhouse for 60 days. Root and plant weights were unaffected but plant height declined only at inoculum level of 2 nematodes/g soil. At the highest initial population density (2 nematodes/g soil), plant height of tomato cv. Poseidon was reduced by 24%. Tomato cv. Hoyong produced the most nematodes with 7.0 nematodes/g soil and the least was tomato cv. Miniheuksu with 2.2 nematodes/g soil.
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