Evaluation of resistance to six races of southern root-knot nematode in 87 sweetpotato cultivars in laboratory tests

Abstract

We evaluated resistance of 87 sweetpotato cultivars to 6 races (SP1, SP2, SP3, SP4, SP6-1, and SP6-2) of the southern root-knot nematode (SRKN) using a 5-point scale based on the number of egg masses (NEM) formed in laboratory tests. NEM values for all 6 races showed highly skewed distributions. Significant differences in resistance were observed among the 6 races, with resistance to SP1, SP2, and SP3 generally higher than or equal to that observed for SP4, SP6-1, and SP6-2. Laboratory resistance evaluations for the 6 races were significantly correlated with field test results from Miyakonojo, Yawara, Katori, and Hamaoka. No significant differences in laboratory resistance were observed among cultivars grouped by their 3 major primary usages, nor between resistance evaluations for 5 of the 6 races (SP2 being the exception) and production area ranks. However, cultivars registered after 1970 exhibited higher or equal resistance compared to those registered before 1970 and landrace cultivars, suggesting that although higher SRKN resistance in a cultivar did not correlate with an expansion of its production area, breeding for SRKN resistance across various usages intensified from the 1960s. These results provide valuable insights for breeding SRKN-resistant sweetpotato cultivars.

Introduction

The southern root-knot nematode, Meloidogyne incognita, is one of the most destructive pests of sweetpotato. It infects the fibrous roots of sweetpotato, forming characteristic round to spindle-shaped swellings (galls) on which egg masses of SRKN are often found (Overstreet 2013). SRKN severely damages the quality and commercial yield of sweetpotato by deforming storage roots and reducing overall yield (Overstreet 2013). Consequently, resistance to SRKN is one of the most important traits in sweetpotato breeding programs (Okada et al. 2017).

Sano and Iwahori (2005) isolated 129 SRKN strains from sweetpotato fields in the Kyushu region of southern Japan and identified 9 races, designated SP1 to SP9, based on their differential pathogenicity on 5 reference sweetpotato cultivars: ‘Norin No. 1’, ‘Norin No. 2’, ‘Tanegashimamurasaki 7’, ‘Elegant Summer’, and ‘J-Red’. Of these 9 races, SP1 predominated in Saga, Nagasaki, and Kumamoto Prefectures of Kyushu. SP2 predominated in Miyazaki and Kagoshima Prefectures of Kyushu. In Okinawa Prefecture, also part of the Kyushu region, SP4 and SP6 predominated (Sano and Iwahori 2005). SP4 and SP6 were also identified in Ibaraki and Chiba Prefectures in the Kanto region of central Japan (Momota et al. 2012). Kuranouchi et al. (2018) identified SP1, SP2, and SP3 in Chiba Prefecture. Based on these reports, SP5, SP7, SP8, and SP9 appear to be minor races in these prefectures. In addition, Tabuchi et al. (2017) demonstrated that SP6 comprises 2 races, SP6-1 and SP6-2, distinguished by their differential pathogenicity on the reference sweetpotato cultivar ‘Murasakimasari’.

SRKN resistance has been evaluated in infested fields during sweetpotato breeding programs in Japan. Field evaluations of SRKN resistance are categorized into 5 degrees: strong (S), moderately strong (S′), medium (M), moderately weak (W′), and weak (W), representing a spectrum from highest to lowest resistance. Cultivars classified as S or S′ are considered desirable resistance, whereas M, W′, and W classifications indicate undesirable susceptibility. Of the 8 resistance test fields examined in this study, the predominant SRKN races identified in the Miyakonojo field (Miyakonojo, Miyazaki Prefecture), Koshi field (Koshi, Kumamoto Prefecture), Yawara field (Tsukubamirai, Ibaraki Prefecture), Katori field (Katori, Chiba Prefecture), and Hamaoka field (Omaezaki, Shizuoka Prefecture) were SP1, SP1, SP4, SP6, and SP1, respectively (Kai et al. 2017, Momota et al. 2012, Sano et al. 2002, Takada et al. 2016). It remains unknown whether the predominant race in Katori field is SP6-1, SP6-2, or a mixture of both. The predominant races in Kumage field (Nishinoomote, Kagoshima Prefecture), Yotsukaido field (Yotsukaido, Chiba Prefecture), and Unakami field (Asahi, Chiba Prefecture) have not been determined. In contrast, Sano et al. (2002) conducted laboratory tests to assess the resistance of 24 sweetpotato cultivars to races SP1, SP2, and SP4, quantifying resistance using number of egg masses (NEM) as an index. However, few studies have reported the resistance of sweetpotato cultivars to races SP3, SP6-1, and SP6-2 in laboratory settings. Furthermore, a correlation between field tests and laboratory tests has not been reported. Therefore, this study reports the resistance of 87 sweetpotato cultivars to 6 SRKN races (SP1, SP2, SP3, SP4, SP6-1, and SP6-2) in laboratory tests. We also analyzed the correlation between field and laboratory resistance evaluations.

Materials and Methods

SRKN and plant materials

The laboratory resistance tests used 6 SRKN races (SP1, SP2, SP3, SP4, SP6-1, and SP6-2), originally isolated by Sano and Iwahori (2005).

For laboratory resistance tests, 87 sweetpotato cultivars were used, including major cultivars accounting for more than 85.5% of the cultivation area in Japan (Table 1) (Ministry of Agriculture, Forestry and Fisheries 2025). All 87 cultivars except ‘HE 306’ were maintained in the Kyushu Okinawa Agricultural Research Center, NARO (Miyakonojo, Miyazaki Prefecture, Japan). Commercially available ‘HE 306’ plants, bred by Kaneko Seeds Co., LTD (Maebashi, Gunma Prefecture, Japan), were purchased and used in this study. These 87 cultivars were grown in a greenhouse. ‘Norin No. 1’, ‘Norin No. 2’, ‘Tanegashimamurasaki 7’, ‘Elegant Summer’, ‘J-Red’, and ‘Murasakimasari’ served as reference cultivars for confirming SRKN races (Sano and Iwahori 2005, Tabuchi et al. 2017). ‘Choshu’ served as a reference cultivar instead of ‘Norin No. 1’ because both ‘Choshu’ and ‘Norin No. 1’ were susceptible to all 6 SRKN races, and ‘Choshu’ was easier to handle.

Table 1.Breeding information and resistance evaluations of the 87 cultivars

Cultivar	Typea	Primary usageb	Breeding location	Registration yearc	Production area rank (%)d	Resistance in laboratory testse							Resistance in field tests (predominant race)e
						SRKN race							Miyako-nojo (SP1)f	Koshi (SP1)g	Yawara (SP4)h	Katori (SP6)h	Hamaoka (SP1)i	Other field (NA)
						SP1	SP2	SP3	SP4	SP6-1	SP6-2
Aikomachi	Bred	Table use	Tsukuba	2014		S	S	S	S	S	S				S1	S1	S1
Amahazuki	Bred	Table use	Tsukuba	2024		S	S	S	S	M	S′				S2	S2
Beniazuma	Bred	Table use	Yotsukaido	1985	3 (9.2)	Mj	W′ j	S′	Wj	W′	W				M1	M1	S′ or M1	Mk3
Beniharuka	Bred	Table use	Miyakonojo	2010	1 (21.9)	M4	M4	S4	M4	S′4	W4		S5		S′6	S′6	S′5
Benihinata	Bred	Table use	Miyakonojo	2023l		S4	S4	S4	S′4	S4	M4		S4
Benimasari	Bred	Table use	Miyakonojo	2005	8 (4.4)	M	M	M	W′	W	W		M7				S′7
Beniotome	Bred	Table use	Koshi	1991	23 (<0.1)	Sj	Sj	W′	Wj	W′	W		S′7	S8
Fukumurasaki	Bred	Table use	Miyakonojo	2021		W	W′	W′	W	W	W		M9
HE306m	Bred	Table use	Isesaki	2018	5 (7.4)	W′	W	M	W	W′	W
Kanto No. 14	Bred	Table use	Chiba	1944n		W′	W	W′	W	W′	W				W	W	W1	Wo10
Kanto No. 143	Bred	Table use	Tsukuba	2014n		S	S	S	S	S	S				S	S
Kanto No. 145	Bred	Table use	Tsukuba	2015n		S	S	S	S	S	M				S	S
Kanto No. 147	Bred	Table use	Tsukuba	2016n		S	M	S	S	S′	W′				S	S
Kanto No. 148	Bred	Table use	Tsukuba	2016n		S	S′	S	S′	S	M				S	S
Karayutaka	Bred	Table use	Tsukuba	2016		S	S	M	M	M	M				M11	S	S
Kokei No. 14	Bred	Table use	Nankoku	1945n	4 (8.2)	W′ j4	Mj4	W′4	Wj4	W4	W′4		W′5		W12
Miyaakari	Bred	Table use	Miyakonojo	2023l		S′	M	S	M	S′	M		S13
Norin No. 1	Bred	Table use	Chiba	1942	27 (<0.1)	Wj	W′ j	W	Wj	W	W′		W7		M	W		Wo10
Norin No. 5	Bred	Table use	Chiba	1945		S′	W′	M	W′	S′	W′						S1	So10
Purple Sweet Lord	Bred	Table use	Tsukuba	2004	19 (0.1)	S′	S	W′	W	W	W		S′9		M14	S′14	S or S′14
Quick Sweet	Bred	Table use	Tsukuba	2005	25 (<0.1)	S	S	S	S′	S	S		S15		S′16	S16	S or S′16
Suzuhokkurip	Bred	Table use	Miyakonojo	2019		M	M	S	W	M	W′		S′17				S
Daichinoyume	Bred	Raw material	Miyakonojo	2006	26 (<0.1)	S	S	S	S′	S′	M		S				S or S′
Hi-Starch	Bred	Raw material	Yotsukaido	1989		Sj	Sj	W	Wj	W	W							Sk18
Koganemasari	Bred	Raw material	Miyakonojo	2014		S	S	S	S′	S	S′		S19				S
Koganesengan	Bred	Raw material	Koshi	1966	2 (20.5)	W′ j20	W′ j20	M20	Wj20	W′20	W′20		W′21		W′22
Konahomare	Bred	Raw material	Miyakonojo	2003	22 (<0.1)	S	M	S	S′	S	M		S′21				S′21
Konaishin	Bred	Raw material	Miyakonojo	2022	6 (5.1)	S20	S20	S20	S20	M20	S′20		S		S′
Konamizuki	Bred	Raw material	Miyakonojo	2012		S	S	S	S	S	S′		S15				S15
Konasenri	Bred	Raw material	Miyakonojo	2005		S	S	S′	S′	S′	M		S				S
Kyukei 117	Bred	Raw material	Koshi	1988n		S	S	S	S	S	M			S
Kyukei 185	Bred	Raw material	Miyakonojo	1996n		S	W	S′	W	S	S′		S
Kyukei 236	Bred	Raw material	Miyakonojo	2001n		S	S	M	W	M	M		S
Kyukei 373	Bred	Raw material	Miyakonojo	2020n		W′	W′	W	W′	W	W		W
Kyukei 708-13	Bred	Raw material	Koshi	1972n		M	M	S	M	M	M			S
Kyukei 82124-1	Bred	Raw material	Koshi	1984n		S	S	S	S	S	S′
Kyushu No. 102	Bred	Raw material	Koshi	1987n		M	W′	M	W′	M	W′			S′
Michishizuku	Bred	Raw material	Miyakonojo	2024		S	S	S′	S′	M	M		S23
Norin No. 2	Bred	Raw material	Kagoshima	1942	17 (0.1)	Sj	Wj	S	W′ j	S	S			S24	M	S′
NR617-10	Bred	Raw material	Koshi	1967n		S′	M	M	W′	M	S′
Satsuma Starch	Bred	Raw material	Tsukuba	1998		Mj	W′ j	W′	Wj	W	W		M21		W′22		M or W′22
Satsumamasari	Bred	Raw material	Miyakonojo	2012		S	S	S	S	S	S′		S25				S25
Shirosatsuma	Bred	Raw material	Yotsukaido	1988	10 (0.8)	S′	M	S′	S′	S′	M						M1	Sk26
Shiroyutaka	Bred	Raw material	Koshi	1986	7 (4.9)	Sj20	W′ j20	S′20	S′ j20	S′20	M20			S24
Starch Queen	Bred	Raw material	Miyakonojo	2005		S′	S′	S	S′	S′	W		S				S′
Suzukogane	Bred	Raw material	Miyakonojo	2019		S′	S′	S′	S′	S′	W′		S27		S
Akaneminori	Bred	Processed food	Tsukuba	2022		M	W′	S′	M	W′	W′				S′28
Akemurasaki	Bred	Processed food	Miyakonojo	2010		S	S	S′	M	M	M		S29				S29
Ayamurasaki	Bred	Processed food	Miyakonojo	1999	13 (0.2)	W′ j	W′ j	W′	W′ j	W	W		S′30				W′30
Benihayato	Bred	Processed food	Koshi	1986		Mj	Sj	S	Sj	M	S			S31
Churakanasa	Bred	Processed food	Itoman	2022		S	S	W′	W	W	W		M32		W′
Churakoibeni	Bred	Processed food	Itoman	2012	12 (0.5)	S	S′	S	M	S	S
Hamakomachi	Bred	Processed food	Miyakonojo	2006		S	S	S	S	S	M		S33				S
Hitachi Redq	Bred	Processed food	Tsukuba	1996		W′ j	W′ j	W	Wj	W	W				M or W′34	M34	W34
Hoshikogane	Bred	Processed food	Tsukuba	2014		S	S	S	S′	S	S				S35	S′35	S or S′35
J-Red	Bred	Processed food	Miyakonojo	1999		Sj	Sj	S	Sj	S	S′		S36		S	S	S36
Kanto No. 140	Bred	Processed food	Tsukuba	2012n		M	S	S′	S	W′	S′				M	S′
Kyushu No. 176	Bred	Processed food	Miyakonojo	2012n		S	S	S	S	S	W		S
Kyushu No. 198	Bred	Processed food	Miyakonojo	2018n		S	M	S′	M	S′	M		S
Kyushu No. 199	Bred	Processed food	Miyakonojo	2019n		S	S	S	S	S	M		S′
Murasakihomarer	Bred	Processed food	Miyakonojo	2021		S	S	S	S	S	M		S37		S′		M
Murasakimasari	Bred	Processed food	Miyakonojo	2004	11 (0.6)	S	M	S′	S	S	W′		S38				S or S′
Okikogane	Bred	Processed food	Miyakonojo	2007		W	W′	W′	W	W	W		M39				M
Saemurasaki	Bred	Processed food	Miyakonojo	2021		S	S	S	S	S	S′		S		S′
Satsumaakane	Bred	Processed food	Miyakonojo	2021		S′	M	M	S′	S′	W		S40				M
Tamaakane	Bred	Processed food	Miyakonojo	2011	31 (<0.1)	S	S	S′	M	S′	M		S41				S′
Tamayutaka	Bred	Processed food	Chiba	1960	14 (0.2)	Mj	Mj	W′	Mj	M	M							Mo10
Tokimasari	Bred	Processed food	Miyakonojo	2010		S	S	S′	S′	S	W′		S′42				S′42
Chienoha	Bred	Top use	Miyakonojo	2020		S	S′	S	S	S	W'		S
Elegant Summer	Bred	Top use	Tsukuba	1998		Sj	Sj	S′	W′ j	W′	M		S′		S′12	S′	S or S′12
Suio	Bred	Top use	Miyakonojo	2004		S	S	S	S	S	S′		S
Hanaranman	Bred	Ornamental use	Miyakonojo	2002		M	W′	W	W′	W′	W		S
Annou-benis	Landrace	Table use	Kumage	1998s	9 (1.3)	W	W′	W′	W	W	W							Mt43
Annou-koganes	Landrace	Table use	Kumage	1998s	16 (0.1)	W′	W′	W′	W	W	W′							Mt43
Genki	Landrace	Table use				W′	M	M	W′	W	M
Shichifuku	Landrace	Table use				M	S′	S′	S′	M	S′
Taihaku	Landrace	Table use				S	S	W	W	W	W′
Tsurunashigenji	Landrace	Table use				M	M	W′	W′	W′	W′
Yoshida	Landrace	Table use				M	M	M	M	M	M
Bise	Landrace	Processed food			21 (<0.1)	S	S	M	W′	M	W		S
Choshu	Landrace	Unknown				W	W	W	W	W	W
Matayoshi	Landrace	Unknown				M	W	M	M	M	W′
Tanegashima- murasaki 7	Landrace	Unknown				Sj	Sj	W′	Wj	W	W		S′		M	M
Xushu 18u	GR	Table use				W′	W′	W′	W′	W′	W
90IDN-47	GR	Unknown				M	M	M	W′	M	M		S′
L-4-5	GR	Unknown				M	W′	S′	W′	S′	S′
T11	GR	Unknown				S	S′	S′	S′	S′	S′

^a Cultivars are classified into bred cultivars (bred), landrace cultivars (landrace), and genetic resource cultivars (GR).

^b Primary usages of ‘Genki’, ‘Shichifuku’, ‘Taihaku’, ‘Tsurunashigenji’, and ‘Yoshida’ were described in Kumagai 2010; those of other cultivars were determined based on breeding objectives and market evaluation.

^c Registration years are not available for landrace or genetic resource cultivars, with the exception of ‘Annou-beni’ and ‘Annou-kogane’, which were selected from landraces and registered in 1998.

^d The production area rank and share of each cultivar are based on data from the Ministry of Agriculture, Forestry and Fisheries (2025). Blank indicates that no data were available for the production area shares of these cultivars.

^e Subscripts attached to resistance evaluations indicate the corresponding reference number listed. No subscript indicates that resistance was reported in this study. NA indicates no data on a predominant SRKN race, and a blank space indicates no data on resistance evaluation in field tests. ₁ Takada et al. 2016. ₂ Taguchi 2022. ₃ Shiga et al. 1985. ₄ Kawata et al. 2025. ₅ Kai et al. 2017. ₆ Nishinaka et al. 2022. ₇ Ishiguro et al. 2004. ₈ Kukimura et al. 1992. ₉ Kai 2019. ₁₀ Onoda et al. 1970. ₁₁ Takada 2015. ₁₂ Komaki et al. 1997. ₁₃ Suematsu 2024. ₁₄ Tamiya et al. 2003. ₁₅ Katayama et al. 2012. ₁₆ Katayama et al. 2003. ₁₇ National Agriculture and Food Research Organization (c). ‘Suzuhokkuri’ is described as ‘Kyushu 161 go’ in this manuscript. ₁₈ Tarumoto et al. 1989. ₁₉ Sakai 2012. ₂₀ Kobayashi et al. 2024. ₂₁ Kumagai et al. 2002. ₂₂ Tarumoto et al. 1996. ₂₃ Kobayashi 2023. ₂₄ Sakamoto et al. 1987b. ₂₅ Katayama et al. 2013. ₂₆ Sakamoto et al. 1989. ₂₇ National Agriculture and Food Research Organization (b). ₂₈ Kuranouchi 2021. ₂₉ Sakai et al. 2010. ₃₀ Yamakawa et al. 1997. ₃₁ Sakamoto et al. 1987a. ₃₂ Okada 2021. ₃₃ National Agriculture and Food Research Organization (d). ₃₄ Tarumoto et al. 1995. ‘Hitachi Red’ is described as ‘Healthy Red’ in this manuscript. ₃₅ Kuranouchi et al. 2015. ₃₆ Yamakawa et al. 1998. ₃₇ National Agriculture and Food Research Organization (a). ‘Murasakihomare’ is described as ‘Kyushu 166 go’ in this manuscript. ₃₈ Kashimura et al. 2005. ₃₉ National Agriculture and Food Research Organization (e). ₄₀ National Agriculture and Food Research Organization (f). ₄₁ Sakai 2009. ₄₂ Katayama et al. 2009. ₄₃ Kouzuma et al. 2003.

^f A predominant race in the resistance test field at Miyakonojo was described in Kai et al. 2017.

^g A predominant race in the resistance test field at Koshi was described in Sano et al. 2002.

^h Predominant races in the resistance test fields at Yawara and Katori were described in Momota et al. 2012.

ⁱ A predominant race in the resistance test field at Hamaoka was described in Takada et al. 2016.

^j Sano et al. (2002) reported resistance evaluations for SP1, SP2, and SP4 races in 16 of the 87 cultivars investigated in the present study. In Sano et al. (2002), resistance ratings of HR and R corresponded to S and S′, respectively, in this study. Their ‘S’ rating encompassed M, W′, and W in this study. All resistance evaluations for SP1, SP2, and SP4 in the 16 cultivars were consistent with those reported in Sano et al. (2002), with the exception of SP1 and SP2 in ‘Tamayutaka’. For ‘Tamayutaka’, in Sano et al. (2002) reported R for both SP1 and SP2, whereas our study found M for both.

^k Resistance was tested in the Yotsukaido field, where no predominant SRKN race had been identified.

^l Application year for variety registration.

^m The commercial name of ‘HE306’ is ‘Silk Sweet’.

ⁿ Year of naming.

^o Resistance was tested in the Unakami field, where no predominant SRKN race had been identified.

^p ‘Suzuhokkuri’ is referred to as ‘Kyushu 161 go’ by the National Agriculture and Food Research Organization (c).

^q ‘Hitachi Red’ was also referred to as ‘Healthy Red’ in Tarumoto et al. 1995.

^r ‘Murasakihomare’ is referred to as ‘Kyushu 166 go’ by the National Agriculture and Food Research Organization (a).

^s ‘Annou-beni’ and ‘Annou-kogane’ were selected from landrace cultivars in Kumage. Data for these cultivars were not used for the correlation analysis between resistance evaluations and registration years.

^t Resistance was tested in the Kumage field, where no predominant SRKN race had been identified.

^u ‘Xushu 18’ was bred in China but is treated as a genetic resource cultivar in this study.

Table 1 details the 87 cultivars, specifying their types (72 bred, 11 landrace, and 4 genetic resource), primary usages (30 table use, 24 raw material, 23 processed food, 3 top use, 1 ornamental use, and 6 unknown), and breeding locations of the bred cultivars (36, 9, 15, 4, 3, 2, 1, 1, and 1 cultivars were bred in Miyakonojo, Koshi, Tsukuba, Chiba, Yotsukaido, Itoman, Nankoku, Kagoshima, and Isesaki, respectively). Registration years are provided for 54 cultivars, application years for 2, naming years for 18, and no data for 13. This information was compiled from published literature, unpublished internal breeding records investigated by sweetpotato breeding laboratories, and market evaluations.

Resistance evaluation in laboratory tests

We investigated the resistance of the 87 sweetpotato cultivars using laboratory tests, following the method described by Tabuchi et al. (2017), and originally developed by Sano and Iwahori (2005). In this study, we modified the number of plants and test replications to more accurately evaluate resistance. For each laboratory test, resistance was assessed using 2 to 8 plants (mode: 4) from each of up to 21 test cultivars. To confirm the SRKN race used for inoculation, 1 to 6 plants (mode: 2) from each of 6 reference cultivars were included. Each plant was inoculated with 500 J2 juveniles of a single SRKN race. The NEM formed on the roots of each inoculated plant was counted after 32–35 days of cultivation in a greenhouse. We calculated the mean NEM for each cultivar within each replication. Resistance tests for each SRKN race were replicated at least 3 times per cultivar. Because withered plants were rare among the experimental plants during cultivation, if the total number of plants across 3 replications was less than 10, we performed additional replications. The final NEM for each cultivar was determined as the mean of all replications. A lower NEM indicates higher resistance to SRKN. Supplemental Table 1 presents the NEM, the number of test replications, and the total number of tested plants. To analyze the correlation between resistance evaluations from laboratory and field tests, we categorized laboratory NEM values into 5 resistance classes based on the method by Sano et al. (2002), similar to field test evaluations: 0 ≤ NEM < 2 (S), 2 ≤ NEM < 10 (S′), 10 ≤ NEM < 50 (M), 50 ≤ NEM < 100 (W′), and 100 ≤ NEM (W) (Table 1). Of the 87 cultivars in the table, 5 resistance classes have previously been reported for 6 cultivars: ‘Beniharuka’, ‘Benihinata’, ‘Kokei No. 14’, ‘Koganesengan’, ‘Konaishin’, and ‘Shiroyutaka’ (Kawata et al. 2025, Kobayashi et al. 2024). However, NEM, the number of test replications, and the total number of tested plants are reported in this study for the first time (Supplemental Table 1).

Resistance evaluation in field tests

Sweetpotato resistance to SRKN was investigated in field tests as part of a breeding program, following the methodology described by the Ministry of Agriculture, Forestry and Fisheries (2016). For each cultivar, 5 plants were transplanted per plot into 2 field plots. These plots were either naturally infested with SRKN or artificially infested the previous year by cultivating a susceptible cultivar (e.g., ‘Kanto No. 14’ or ‘Norin No. 1’) known to be highly susceptible to SRKN. Plants were grown for at least 60 days after transplanting. Cultivar resistance was evaluated by comparing the number of SRKN egg masses or galls formed on the roots of the test cultivar with those of 3 reference cultivars: a weak resistant type (e.g., ‘Kanto No. 14’ or ‘Norin No. 1’), a medium resistant type (e.g., ‘Benimasari’), and a strong resistant type (e.g., ‘J-Red’). SRKN resistance was ultimately evaluated through repeated tests over a period of more than 2 years. The relationship between breeding locations and test fields is summarized in Table 2. Cultivars bred in Miyakonojo were evaluated primarily at the Miyakonojo test field and additionally at Hamaoka. Those bred in Koshi were evaluated at the Koshi test field. Cultivars from Tsukuba were tested primarily at the Yawara and Katori test fields and additionally at Hamaoka. Cultivars bred in Chiba were evaluated at Unakami. Those bred in Yotsukaido were evaluated at the Yotsukaido test field. No SRKN resistance data from field tests were available for cultivars bred in Itoman, Nankoku, Kagoshima, and Isesaki.

Table 2.Resistance evaluation values for each cultivar type and resistance test field

SRKN race	Resistance evaluation (No. of cultivars)
	All cultivars (87)	GR cultivarsa (4)	Landrace cultivars (11)	Bred cultivars (72)	Breeding locations of 72 bred cultivars
					Miyakonojo (36)	Koshi (9)	Tsukuba (15)	Chibaa (4)	Yotsukaidoa (3)	Itomana (2)	Nankokua (1)	Kagoshimaa (1)	Isesakia (1)
SP1	3.92 a	3.25	3.00 a	4.10 a	4.28 a	3.89 a	4.33 a	2.50	4.00	5.00	2	5	2
SP2	3.61 ab	2.75	3.09 a	3.74 ab	3.94 a	3.56 a	4.20 a	2.00	3.33	4.50	3	1	1
SP3	3.63 ab	3.25	2.36 a	3.85 ab	4.11 a	3.89 a	4.00 a	2.00	3.00	3.50	2	5	3
SP4	3.00 cd	2.50	1.91 a	3.19 bc	3.56 a	3.11 a	3.53 a	1.75	2.00	2.00	1	2	1
SP6-1	3.23 bc	3.25	1.82 a	3.44 ab	3.83 a	3.33 a	3.27 a	2.50	2.33	3.00	1	5	2
SP6-2	2.60 d	3.00	2.00 a	2.67 c	2.53 b	3.00 a	3.13 a	2.00	1.67	3.00	2	5	1
Resistance test fieldb	Main field				Miyakonojo (SP1)	Koshi (SP1)	Yawara (SP4) Katori (SP6)	Unakami (SP1)	Yotsukaido (unknown)
	Additional field				Hamaoka (SP1)		Hamaoka (SP1)

Resistance values were calculated by assigning numerical scores: S, S′, M, W′, and W = 5, 4, 3, 2, and 1, respectively.

Results sharing the same letter (a, b, c, or d) were not significantly different among the 6 races, based on Tukey’s HSD test (p < 0.05).

^a Statistical analysis was not performed because fewer than 5 samples were available.

^b Predominant SRKN races are indicated below the names of test fields. No data were available on SRKN resistance in field tests for cultivars bred in Itoman, Nankoku, Kagoshima, and Isesaki.

We conducted field resistance tests at Miyakonojo, Yawara, and Katori. The test field at Miyakonojo had been artificially infested by cultivating a weak resistant cultivar, ‘Norin No. 1’, the year before. Resistance was evaluated by comparing the degree of gall formation on the roots of the test cultivar, primarily with that of a reference weak resistant cultivar ‘Norin No. 1’ and secondarily with that of a reference moderately weak resistant cultivar, ‘Koganesengan’ (Kai et al. 2017). At Yawara, the test field had been artificially infested by cultivating a susceptible cultivar, ‘Kanto No. 14’, the year before. Resistance of a cultivar was evaluated by comparing NEM formed by SRKN on the roots of the test cultivar with those of a reference weak resistant cultivar, ‘Kanto No. 14’, a reference medium resistant cultivar, ‘Beniazuma’, and a reference strong resistant cultivar, ‘J-Red’ (Kuranouchi et al. 2016). At Katori, the test field had been naturally infested with SRKN. Resistance of a cultivar was evaluated by comparing the number of galls formed by SRKN on the roots of the test cultivar with those of a reference weak resistant cultivar, ‘Kanto No. 14’, a reference medium resistant cultivar, ‘Beniazuma’, and a reference strong resistant cultivar, ‘J-Red’ (Kuranouchi et al. 2016).

The test field at Koshi had been artificially infested by cultivating a weak resistant cultivar, ‘Norin No. 1’, the year before. Resistance was evaluated by comparing the degree of damage on the roots of the test cultivar, primarily with that of a reference weak resistant cultivar ‘Norin No. 1’, secondarily with that of a reference moderately weak resistant cultivar, ‘Koganesengan’, and a reference strong resistant cultivar, ‘Shiroyutaka’ (Kukimura et al. 1992). The test field at Hamaoka had been artificially infested by cultivating susceptible balsam, the year before. Resistance was evaluated using the following methods. Roots and storage roots were immersed in a 0.05% aqueous solution of Phloxine B for 1 h to stain egg masses. Staining intensity was scored on a 1–5 scale (strong to weak), and judgments were made based on the mean score according to the following criteria: strong, 1.0–1.4; moderately strong, 1.5–2.4; medium, 2.5–3.4; moderately weak, 3.5–4.4; and weak, 4.5–5.0 (Kai et al. 2017).

Statistical analysis

Statistical analyses were performed using JMP (SAS Institute, Cary, NC). Given that the NEM for the 6 SRKN races in laboratory tests showed a strongly biased distribution, rendering it unsuitable for direct statistical analysis (Supplemental Fig. 1), correlations between SRKN races were analyzed using both the raw NEM values and values transformed by adding 1 and applying a natural logarithm (hereafter, ‘logarithm-transformed NEM’). For other statistical analyses, a 5-point resistance evaluation scale was used instead of NEM. The resistance evaluations from laboratory and field tests (S, S or S′, S′, M, M or W′, W′, and W) were converted to numerical forces for statistical analysis: 5, 4.5, 4, 3, 2.5, 2 and 1, respectively. Statistical analysis of differences in resistance evaluations observed in laboratory tests across 6 SRKN races was not performed for genetic resource cultivars or cultivars bred in Chiba, Yotsukaido, Itoman, Nankoku, Kagoshima, and Isesaki because fewer than 5 samples were available for each. Resistance evaluations from field tests at Kumage (n = 2), Yotsukaido (n = 3) and Unakami (n = 4) were excluded from correlation analysis due to insufficient data points. For the same reason, resistance evaluations of top-use cultivars (n = 3) and an ornamental cultivar (n = 1) were also excluded.

Results

Resistance evaluation to SRKN in laboratory tests

In laboratory tests, NEM values for the 6 SRKN races (SP1, SP2, SP3, SP4, SP6-1, and SP6-2) ranged from 0.0 to 189.1, 0.0 to 177.1, 0.0 to 188.7, 0.0 to 250.1, 0.0 to 229.3, and 0.1 to 237.2, respectively (Supplemental Table 1). Specifically, NEM values for SP1, SP2, SP3, SP4, SP6-1, and SP6-2 were between 0 and 10 in 62.1, 51.7, 58.6, 43.7, 47.1, and 25.3% of the 87 cultivars, respectively (Fig. 1). The NEM values for the remaining cultivars were widely scattered, leading to a strongly biased distribution for all 6 races (Supplemental Fig. 1). Both the raw NEM and logarithm-transformed NEM values for the 6 races were significantly correlated with each other (Tables 3, 4, Supplemental Figs. 2, 3).

Fig. 1.

Percentage of cultivars for each resistance evaluation in laboratory tests. S, NEM < 2; S′, 2 ≤ NEM < 10; M, 10 ≤ NEM < 50; W′, 50 ≤ NEM < 100; W, 100 ≤ NEM. Vertical axis: percentage. Horizontal axis: SRKN race.

Table 3.Correlation coefficients for NEM from the 6 SRKN races in laboratory tests

SRKN race	SP1	SP2	SP3	SP4	SP6-1	SP6-2
SP1
SP2	0.699
SP3	0.642	0.460
SP4	0.496	0.513	0.718
SP6-1	0.726	0.445	0.859	0.774
SP6-2	0.472	0.427	0.650	0.637	0.685

All correlation coefficients were statistically significant (p < 0.001).

Table 4.Correlation coefficients for logarithm-transformed NEM from the 6 SRKN races in laboratory tests

SRKN race	SP1	SP2	SP3	SP4	SP6-1	SP6-2
SP1
SP2	0.784
SP3	0.617	0.494
SP4	0.602	0.606	0.785
SP6-1	0.712	0.441	0.833	0.770
SP6-2	0.446	0.441	0.672	0.600	0.585

All correlation coefficients were statistically significant (p < 0.001).

The 5-point resistance evaluations for the 6 races in laboratory tests are presented in Table 1. Significant differences were observed in the resistance evaluations of the 87 cultivars among the 6 races (Table 2). The proportions of S and S′ cultivars (0 ≤ NEM < 10) were higher for SP1, SP2, and SP3 compared to the proportions of M, W′, and W cultivars (10 ≤ NEM) for these 3 races (Fig. 1). Conversely, for SP4, SP6-1, and SP6-2, the proportions of S and S′ cultivars were lower than those of M, W′, and W cultivars (Fig. 1).

Resistance evaluation to SRKN in field tests

We surveyed both published reports and unpublished data (e.g., internal breeding records) for SRKN resistance evaluated in field tests of 87 cultivars (Table 1). Resistance tests were conducted at 8 fields: Miyakonojo, Koshi, Yawara, Katori, Hamaoka, Kumage, Yotsukaido, and Unakami. From published data surveys, we identified resistance data for 30, 14, and 8 cultivars investigated at Miyakonojo, Yawara, and Katori fields, respectively (Ishiguro et al. 2004, Kai 2019, Kai et al. 2017, Kashimura et al. 2005, Katayama et al. 2003, 2009, 2012, 2013, Kawata et al. 2025, Kobayashi 2023, Komaki et al. 1997, Kumagai et al. 2002, Kuranouchi 2021, Kuranouchi et al. 2015, National Agriculture and Food Research Organization (a, b, c, d, e, f), Nishinaka et al. 2022, Okada 2021, Sakai 2009, 2012, Sakai et al. 2010, Suematsu 2024, Taguchi 2022, Takada 2015, Takada et al. 2016, Tamiya et al. 2003, Tarumoto et al. 1995, 1996. Yamakawa et al. 1997, 1998). In addition to these data surveys, we conducted field resistance tests for 18, 15, and 12 cultivars at Miyakonojo, Yawara, and Katori, respectively. Also from published sources, we found resistance data for 4 and 20 cultivars tested at Koshi and Hamaoka fields, respectively (Ishiguro et al. 2004, Kai et al. 2017, Katayama et al. 2003, 2009, 2012, 2013, Komaki et al. 1997, Kukimura et al. 1992, Kumagai et al. 2002, Kuranouchi et al. 2015, Sakai et al. 2010, Sakamoto et al. 1987a, 1987b, Takada et al. 2016, Tamiya et al. 2003, Tarumoto et al. 1995, 1996, Yamakawa et al. 1997, 1998). From unpublished internal breeding records, we identified additional resistance data for 3 and 12 cultivars at Koshi and Hamaoka fields, respectively. Finally, resistance data for all 2, 3, and 4 cultivars investigated at Kumage, Yotsukaido, and Unakami fields, respectively, were obtained from published sources (Kouzuma et al. 2003, Onoda et al. 1970, Sakamoto et al. 1989, Shiga et al. 1985, Tarumoto et al. 1989). The results are summarized in Table 1, showing resistance evaluations for a total of 48, 7, 29, 20, 32, 2, 3, and 4 cultivars tested at Miyakonojo, Koshi, Yawara, Katori, Hamaoka, Kumage, Yotsukaido, and Unakami, respectively. No field resistance data were available for 14 cultivars. Resistance of 48 cultivars was evaluated in 2 or more test fields.

Correlation of resistance evaluations between laboratory tests and field tests

Resistance evaluations for 6 races in laboratory tests and those from 4 field tests (Miyakonojo, Yawara, Katori, and Hamaoka) were significantly correlated (Table 5). For Miyakonojo, where SP1 is the predominant SRKN race, the correlation coefficient between field test evaluations and laboratory evaluations for SP1 was higher than for other races (Table 5). The same pattern was observed at Yawara and Hamaoka, where the predominant SRKN race is SP4 and SP1, respectively (Table 5). At Yawara and Hamaoka, the correlation coefficients between field resistance evaluations and laboratory evaluations to SP4 and SP1, respectively, were higher than those for other races (Table 5). However, in Katori, where the predominant SRKN race is SP6, the correlation coefficient between field resistance evaluations and laboratory evaluations to SP1 was higher than those for other races (Table 5).

Table 5.Correlation coefficients between resistance evaluations in laboratory and field tests

SRKN race or field (predominant race)	Laboratory tests							Field tests
	SP1	SP2	SP3	SP4	SP6-1	SP6-2		Miyakonojo	Koshi	Yawara	Katori	Hamaoka
SP1
SP2	0.722***
SP3	0.610***	0.456***
SP4	0.586***	0.566***	0.785***
SP6-1	0.699***	0.399***	0.845***	0.777***
SP6-2	0.485***	0.404***	0.657***	0.578***	0.630***
Miyakonojo (SP1)	0.741***	0.511***	0.704***	0.636***	0.730***	0.473***
Koshi (SP1)	0.471	0.333	0.415	0.320	0.222	0.344		NAa
Yawara (SP4)	0.649***	0.527**	0.786***	0.813***	0.746***	0.565**		0.792***	NAa
Katori (SP6)	0.832***	0.662**	0.763***	0.777***	0.682***	0.574**		0.939**	NAa	0.775***
Hamaoka (SP1)	0.722***	0.708***	0.674***	0.489**	0.554***	0.595***		0.479*	NAa	0.766**	0.875***

*, p < 0.05. **, p < 0.01. ***, p < 0.001.

Field tests in Kumage (n = 2), Yotsukaido (n = 3) and Unakami (n = 4) were excluded from analysis due to small sample sizes.

For correlation calculations, resistance categories (S, S or S′, S′, M, M or W′, W′, W) were assigned numerical values of 5, 4.5, 4, 3, 2.5, 2, and 1, respectively.

^a Correlation coefficient between resistance scores in the Koshi field and other fields could not be calculated because the sample size was less than 2.

Correlation of laboratory resistance evaluations with primary usages, registration year, and production area ranks

We investigated whether resistance evaluations differed among cultivars based on primary usage, registration year, and production area rank. For primary usage, after excluding 3 top-use cultivars and 1 ornamental cultivar (due to small sample sizes), no significant differences were observed in resistance evaluations to 6 SRKN races among the 3 primary usages: table use (n = 22), raw materials (n = 24), and processed foods (n = 22) (Table 6).

Table 6.Resistance scores for 3 primary usages of bred cultivars in laboratory tests

Primary usage	SRKN race
	SP1	SP2	SP3	SP4	SP6-1	SP6-2
Table use (n = 22)	3.73	3.50	3.73	2.68	3.09	2.36
Raw materials (n = 24)	4.33	3.63	3.96	3.21	3.63	2.92
Processed foods (n = 22)	4.14	4.05	3.86	3.64	3.59	2.73

Resistance scores were determined by assigning numerical values to resistance categories: S, S′, M, W′, and W = 5, 4, 3, 2, and 1, respectively.

Resistance scores for top-use cultivars (n = 3) and an ornamental-use cultivar (n = 1) were excluded from the analysis due to small sample sizes.

No significant differences in resistance scores to any of the SRKN races were observed among the 3 usages.

For registration year, we compared 3 groups of cultivars: those bred between 1972 and 2024 (n = 64), those bred between 1942 and 1967 (n = 8), and landrace cultivars (n = 11), because the resistance tests for SRKN begun in earnest around 1970 (Kyushu nogyo shikenjo sakumotsu dainibu 1971). Significant differences in laboratory resistance evaluations were observed for races SP1, SP2, SP3, SP4, and SP6-1 when comparing 3 groups of cultivars (Table 7).

Table 7.Resistance scores for bred and landrace cultivars in laboratory tests

Cultivar type	SRKN race
	SP1	SP2	SP3	SP4	SP6-1	SP6-2
Registered in 1972–2024 (n = 64)	4.25 a	3.94 a	4.00 a	3.39 a	3.55 a	2.67 a
Registered in 1942–1967 (n = 8)	2.88 b	2.13 b	2.63 b	1.63 b	2.63 ab	2.63 a
Landrace (n = 11)	3.00 b	3.09 ab	2.36 b	1.91 b	1.82 b	2.00 a

Resistance scores were determined by assigning numerical values to resistance categories: S, S′, M, W′, and W = 5, 4, 3, 2, and 1, respectively.

Values followed by the same letter (a or b) are not significantly different among cultivar types based on Tukey’s test (p < 0.05).

No significant differences were observed between resistance evaluations for 5 of the 6 races (SP2 being the exception) and production area ranks for 24 cultivars (Table 8).

Table 8.Correlation coefficients between resistance scores in laboratory tests and production area ranks of 24 cultivars

SRKN race
SP1	SP2	SP3	SP4	SP6-1	SP6-2
0.376	0.497*	–0.039	0.101	0.207	0.257

For correlation calculations, resistance categories were assigned numerical values as follows: S, S′, M, W′, and W = 5, 4, 3, 2, and 1, respectively.

*, p < 0.05.

Discussion

Threshold values for a 5-degree resistance scale using NEM

To analyze the correlation between laboratory and field resistance evaluations, we converted SRKN NEM values from laboratory tests into a 5-degree resistance scale, mirroring the field test scale. When thresholds for S/S′, S′/M, M/W′, and W′/W were set to 2, 10, 50, and 100 NEM, respectively, laboratory and field resistance evaluations were significantly correlated. Notably, field evaluations at Miyakonojo, Yawara, and Hamaoka (where the predominant SRKN races are SP1, SP4, and SP1, respectively) showed the highest correlation with laboratory evaluations for their corresponding predominant races (SP1, SP4, and SP1) (Table 5). These results suggest that the chosen threshold values are appropriate. Incidentally, the lack of a significant correlation between the field tests at Koshi and the laboratory tests for 6 SRKN races seems to be due to the skewed results, as 6 of 7 cultivars tested at the Koshi field were S and 1 was S′.

Stability of the resistance scale using NEM

Sano et al. (2002) reported resistance evaluations for SP1, SP2, and SP4 races in 16 of the 87 cultivars investigated in the present study (Table 1). The resistance categories HR and R in Sano et al. (2002) correspond to S and S′ in this study, respectively. The S category in Sano et al. (2002) included M, W′, and W in this study. For 16 cultivars, resistance evaluations for SP1, SP2, and SP4 were consistent with those reported in Sano et al. (2002), except for SP1 and SP2 in ‘Tamayutaka’. In that study, ‘Tamayutaka’ was classified as R for both SP1 and SP2, whereas in the present study it was classified as M for both races. While there was a difference in the average temperature used for culturing SRKN-inoculated plants—25.5°C in this study versus 27.0°C in that study by Sano et al.—, Sano et al. and we conducted the laboratory resistance tests under identical conditions at different locations: Miyakonojo, Miyazaki Prefecture and Koshi, Kumamoto Prefecture, respectively. These results suggest that consistency was maintained for at least 3 resistance evaluation categories (S, NEM < 2; S′, 2 ≤ NEM < 10; M, W′, and W, 10 ≤ NEM) when the laboratory resistance tests were conducted under the identical conditions in different laboratories.

QTLs associated with resistance to SRKN races SP1, SP2, SP3, SP4, and SP6-1

We investigated the resistance of 87 cultivars to 6 SRKN races in laboratory tests by counting NEM. The NEM values ranged from 0 to 10 in 25.3–62.1% of the 87 cultivars, and the distributions were strongly skewed toward lower values (Supplemental Fig. 1). In addition, NEM and logarithm-transformed NEM values for the 6 SRKN races were significantly correlated (Tables 3, 4, Supplemental Figs. 2, 3). This skewed distribution of NEM and the significant correlation among SRKN races have also been reported in experiments using F₁ progeny plants (Kurihara et al. 2022, 2024, Nakayama et al. 2012, Obata et al. 2022, Sasai et al. 2019). Nakayama et al. (2012) reported that resistance to SP1 and SP2 in ‘Hi-starch’ was controlled primarily by single dominant simplex genes, and that the genes conferring resistance against each race were closely linked. Kurihara et al. (2022) revealed that the resistance of ‘J-red’ to 5 SRKN races (SP1, SP2, SP3, SP4, and SP6-1) was controlled primarily by a combination of 3 common QTLs. Although it is unknown whether the QTLs associated with SRKN resistance in ‘Hi-starch’, ‘J-red’, and the 85 other cultivars investigated in this study are identical, resistance to 5 SRKN races in these 87 cultivars appears to be controlled mostly by common single and/or a few simplex QTLs, leading to correlations among NEM values for these 5 SRKN races. This suggests that resistance of a cultivar to any one of the 5 SRKN races is likely to be accompanied by resistance to the remaining 4 races. In practice, for the cultivars bred in Miyakonojo and Koshi—where SP1 was the predominant SRKN race in the resistance test fields (Miyakonojo, Koshi, and Hamaoka)—no significant differences were observed in resistance evaluations among 5 SRKN races (Table 2).

Difference between QTLs associated with resistance to SP6-2 of SRKN and QTLs associated with resistance to other races of SRKN

‘Konasenri’, derived from a cross between ‘Hi-Starch’ and ‘Kyukei 82124-1’, exhibited S, S, S′, S′, and S′ resistance to SRKN races SP1, SP2, SP3, SP4, and SP6-1, respectively (Table 1, Fig. 2). Resistance profiles of SP1 and SP2 likely originated from ‘Kyukei 82124-1’ and/or ‘Hi-Starch’ both of which were strong resistant (S) to SP1 and SP2. Resistance profiles of SP3, SP4, and SP6-1 likely originated from ‘Kyukei 82124-1’, as ‘Hi-Starch’ was weak resistant (W) to these races, while ‘Kyukei 82124-1’ was strong resistant (S). In contrast, ‘Konahomare’, also derived from a cross between these 2 parents, exhibited medium (M) resistance to SP2. As resistance to SP2 in ‘Hi-Starch’ is controlled primarily by a single dominant simplex gene (Nakayama et al. 2012), if resistance to SP2 in ‘Kyukei 82124-1’ were also predominantly governed by a single dominant gene (simplex, duplex, or triplex), this could account for the M resistance observed in ‘Konahomare’.

Fig. 2.

Pedigrees of ‘Hi-Starch’ and other cultivars. The red arrow and blue dotted arrow to the left of the cultivar name indicate the maternal and paternal parents, respectively. Above each cultivar name (except ‘99L04-3’), resistance scores for SRKN races SP1, SP2, SP3, SP4, SP6-1, and SP6-2 are indicated from left to right. No resistance data for SRKN was available for ‘99L04-3’ because the cultivar had withered. Resistance scores indicated in blue include those for SP1, SP2, SP3, SP4, and SP6-1 (above ‘Hi-Starch’, ‘Kyukei 82124-1’, and ‘Konasenri’); and SP2 (above ‘Konahomare’). Resistance scores for SP6-2 above ‘Tokimasari’, ‘Satsumamasari’, ‘Kyushu No. 102’, ‘Starch Queen’, ‘Koganemasari’ and ‘Kyukei 236’ are indicated in red.

Conversely, ‘Starch Queen’ and ‘Kyukei 236’ exhibited W and M resistance to SP6-2, respectively (Table 1, Fig. 2). Nevertheless, ‘Koganemasari’, derived from a cross between ‘Starch Queen’ and ‘Kyukei 236’, showed S′ resistance to SP6-2. A similar pattern was observed for ‘Satsumamasari’, which was derived from a cross between ‘Tokimasari’ and ‘Kyushu No. 102’. ‘Satsumamasari’, ‘Tokimasari’, and ‘Kyushu No. 102’ exhibited S′, W′, and W′, respectively. Kurihara et al. (2024) revealed that resistance to SP6-2 in ‘J-Red’ was controlled by multiple QTLs that individually confer low resistance effects but collectively exhibit additive resistance effects. While it is unknown if the resistance genes in ‘Koganemasari’, ‘Satsumamasari’, and other cultivars are identical to those in ‘J-Red’, it is plausible that a combination of multiple genes, each conferring low resistance and present in both parents, collectively confers resistance to SP6-2 in ‘Koganemasari’ and ‘Satsumamasari’. The scarcity of cultivars exhibiting S or S′ resistance to SP6-2 in laboratory tests (Fig. 1) further supports the hypothesis that resistance to SP6-2 is controlled by multiple QTLs, each with a low resistance effect. Furthermore, statistically significant correlations were observed between resistance evaluations (NEM and logarithm-transformed NEM) to SP6-2 and the other 5 races (Tables 3–5), suggesting that one or a few QTLs associated with SP6-2 resistance may be identical to QTLs involved in resistance to the other 5 races.

Differences in the number of resistant cultivars among SRKN races

For SP1, SP2, and SP3, the proportion of resistant cultivars (S and S′; 0 ≤ NEM < 10) was higher than that of susceptible cultivars (M, W′, and W; 10 ≤ NEM) (Fig. 1). Conversely, for SP4, SP6-1, and SP6-2, the proportion of resistant cultivars was lower than that of susceptible cultivars (Fig. 1). Overall, the mean resistance evaluation values for all 87 cultivars were 3.92 (SP1), 3.61 (SP2), 3.63 (SP3), 3.00 (SP4), 3.23 (SP6-1), and 2.60 (SP6-2), respectively, with a significant difference observed among the 6 races (Table 2). When the 87 cultivars were categorized into 72 bred cultivars, 11 landrace cultivars, and 4 genetic resource cultivars, a significant difference in resistance evaluations among the 6 races was observed for bred cultivars (Table 2). Further categorization of the 72 bred cultivars by breeding location revealed a significant difference in resistance evaluations among the 6 races for cultivars bred at Miyakonojo (Statistical analysis was not performed for genetic resource cultivars or cultivars bred in Chiba, Yotsukaido, Itoman, Nankoku, Kagoshima, and Isesaki because fewer than 5 samples were available at each). When the 87 cultivars were grouped by origin (landrace or breeding location), evaluations for SP1, SP2, and SP3 were generally equal to or higher than those for SP4, SP6-1, and SP6-2. Collectively, these 87 cultivars exhibited a greater tendency toward resistance to SP1, SP2, and SP3 compared to SP4, SP6-1, and SP6-2; however, the underlying reason for this observation remains unclear.

Correlation of resistance evaluations with primary usages, registration year, and production area ranks in laboratory tests

Resistance to 5 SRKN races (SP1, SP2, SP3, SP4, and SP6-1) in 64 cultivars registered between 1972 and 2024 was significantly higher than in 8 cultivars registered between 1942 and 1967 or 11 landrace cultivars (Table 7). Among the 8 cultivars registered between 1942 and 1967, SRKN resistance was investigated during the breeding process only for ‘Tamayutaka’ (Hamada 1976, Onoda et al. 1970) (Table 1). In contrast, SRKN resistance was investigated during the breeding process for all cultivars registered between 1972 and 2024, with the exception of 3 cultivars: ‘Kyukei 82124-1’, ‘Churakoibeni’, and ‘HE306’ (Table 1). On the other hand, no significant differences were observed in resistance levels among cultivars based on their 3 primary usages (Table 6). Furthermore, no significant differences were found between resistance to 5 SRKN races (SP1, SP3, SP4, SP6-1, and SP6-2) and production area ranks (Table 8). Considering the approximately 10-year period from the initiation of breeding to the registration of a new cultivar, these results suggest that although higher SRKN resistance in a cultivar does not necessarily correlate with an increase in its production area, breeders have made continuous efforts since the 1960s to develop cultivars with higher SRKN resistance for various usages.

This study determined the resistance of 87 cultivars to 6 SRKN races (SP1, SP2, SP3, SP4, SP6-1, and SP6-2), a level of detail not feasible in field tests where multiple races may coexist. These findings can significantly aid in breeding SRKN-resistant cultivars by selecting cultivars resistant to multiple SRKN races as breeding parents.

Author Contribution Statement

HT conducted the laboratory tests, analyzed the data, and drafted the manuscript. YK and TK conducted the field tests. All authors contributed to the article and approved the submitted version.

 Acknowledgments

We thank Ai Tokiwa, Masayo Nishidome, Sanae Nagahashi, Mizuki Maniwa, and the members of Kyushu Operation Unit 3 of the Technical Support Office of Department of Research Promotion (Miyakonojo Research Station, KARC/NARO) for their assistance. We also thank M.S. M. Kurihara (CARC/NARO, Tsukuba, Japan) and Dr. Y. Monden (Okayama University, Okayama, Japan) for their review of the manuscript. The authors would like to express their gratitude to the sweetpotato breeders who were previously involved in field resistance tests to SRKN for their valuable contributions. This work could not have been accomplished without the SRKN races maintained by the late Dr. Hiroki Nakayama (KARC/NARO), who passed away in 2012. Funding for this work was provided by the Ministry of Agriculture, Forestry and Fisheries of Japan [Smart breeding technologies to Accelerate the development of new varieties toward achieving “Strategy for Sustainable Food Systems, MIDORI” (Grant Number JPJ012037).

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