Southern root-knot nematode race SP6 is divided into two races

The southern root-knot nematode (SRKN) includes at least nine races, SP1 to SP9, which were identified based on their different responses to five differential sweetpotato cultivars, ' Norin-1', ' Norin-2', 'Tanegashimamurasaki-7', 'Elegant Summer', and 'J-Red'. In this study, we propose a modified method for SRKN race identification in which the threshold of response is 10 egg masses instead of the two-egg-mass threshold used in the original method, when a sweetpotato plant was inoculated with 500 second-stage juveniles. According to this method, two isolates of SP6, Okiishi-12 and Ishigaki-2, showed the same response to each of these five cultivars. We found, however, that three sweetpotato cultivars, 'Murasakimasari', 'Suzukogane', and 'Chienoha', were resistant to the Okiishi-12 isolate and susceptible to the Ishigaki-2 isolate. These results indicated that the Okiishi-12 and Ishigaki-2 isolates were distinct and that SP6 included at least two races. We suggest that the Okiishi-12 and Ishigaki-2 isolates should be distinguished by sub-numbers and designated SP6-1 and SP6-2, respectively. Five isolates of SP6 collected from a sweetpotato field in Chiba prefecture were examined, and all of them were identified as SP6-1.