First Report of Southern Root-Knot Nematode, Meloidogyne incognita, on Large Crabgrass, Digitaria sanguinalis, in China

Large crabgrass, Digitaria sanguinalis (L.) Scop., is a weed seriously damaging agricultural production throughout tropical and temperate regions of the world (King and Oliver 1994). However, in China, D. sanguinalis is used as a traditional Chinese medicine that may have beneficial health effects on both lung and eyesight (Hu et al. 1999). In July 2017, numerous plants of D. sanguinalis with small to large galls on the root systems, typical of Meloidogyne infection, were observed in Qingdao, Shandong Province, China. The affected plants did not exhibit obvious above-ground symptoms. Infected roots were collected from a greenhouse, and nematodes were extracted from root samples. The morphological identification of the nematode was performed based on the characteristics of females, males, second-stage juveniles (J2s), and eggs (Eisenback et al. 1981). The measurement data are shown in the supplementary materials. The perineal patterns of females had a high dorsal arch, which was square or trapezoid, with smooth or wavy striae and lacking obvious lateral lines. The stylet knob of males extended horizontally and had a distinct boundary with the stylet shaft. All data and descriptions conformed to the morphological characteristics of Meloidogyne incognita (Kofoid & White, 1919) Chitwood, 1949. DNA was extracted from mature females. Species molecular identification was confirmed by sequence characterized amplified region (SCAR) markers specific for M. incognita. Two pairs of the species-specific SCAR primers, Finc/Rinc and IncK-14F/IncK-14R, were used to diagnose whether these nematodes from large crabgrass were M. incognita (Zijlstra et al. 2000; Randig et al. 2002). The fragments of 1,200 and 400 bp were observed under a UV light, respectively, confirming this population to be M. incognita. In addition, primers specific for M. arenaria (Far/Rar) and M. javanica (Fjav/Rjav) were also used but failed to amplify any fragments from the nematode DNA (Zijlstra et al. 2000; Randig et al. 2002). Ten D. sanguinalis plants about 15 cm long were transplanted into 20-cm-diameter pots in a growth chamber, and each seedling was inoculated with 4,000 J2s accompanied by uninoculated plants as the control. After 16 weeks, all inoculated plant roots showed galling symptoms, which were similar to those produced in the greenhouse. There were different life stages of the nematode in dissected galled tissues, with population densities ranging from 728 to 3,563 eggs and J2s per 5 g of fresh roots. The control seedlings had no galls. On the basis of these results, the population of Meloidogyne sp. on D. sanguinalis was confirmed to be M. incognita. M. incognita is one of the most devastating nematodes of economic crops and vegetables worldwide. D. sanguinalis has also been reported as a major host of M. incognita in Florida (Noling and Gilreath 2002), but in another study, D. sanguinalis was a nonhost of M. incognita in Florida (Kaur et al. 2007). The contradiction between the above two studies might be because of the different M. incognita races used. The discovery that D. sanguinalis could be infected by M. incognita is significant for integrated nematode management, particularly for crop rotation. To our knowledge, this is the first report of D. sanguinalis as a host of M. incognita in China.