Host-status of 20 sweet potato lines to Meloidogyne species in South Africa

ABSTRACT

Experimental system: Due to serious economic challenges posed by root-knot (Meloidogyne species) nematodes in sweet potato (Ipomoea batatas) production, the Sweet Potato Programme (SPP) of the Agricultural Research Council (ARC) in South Africa has since included screening for nematode host-status in its breeding-selection activities. Procedures: 20 sweet potato lines were screened against M. javanica, M. incognita race 2 and M. incognita race 4 in parallel trials inoculated with 3000 eggs and second-stage juveniles (J2) per established cutting. Results: At 56 days after inoculation, the reproductive potential (RP) of all test Meloidogyne species on sweet potato line 1990-10-2 was zero, whereas RP values on other lines were 19.48–342.7, 31.9–995.1 and 10.3–380.44 ranges for M. javanica, M. incognita race 2 and M. incognita race 4, respectively. Conclusion: Among the test sweet potato lines, line 1990-10-2 was non-host to populations of tropical Meloidogyne species in South Africa and could, therefore, be subjected to nematode resistance tests.

KEYWORDS:

• Biofortification
• cv. ‘Beauregard’
• cv. ‘Bophelo’
• cv. ‘Bosbok’
• cv. ‘Mvuvhelo’
• nematode resistance

Introduction

Environmental changes caused by global warming require that adjustments have to be made in agricultural activities (FAO 2018). Increases in soil temperature, for example, have since led to the shortening of the life cycle of certain nematode species (Ghini et al. 2008), in the process increasing their aggressiveness towards crop damage (Ghini et al. 2008). The advent of global warming coincided with the withdrawal of fumigant synthetic nematicides from the agrochemical markets (Mashela et al. 2017a), along with intensive biofortification efforts, particularly in sweet potato (Ipomoea batatas L.) (Bouis et al. 2011; Burri 2011; Low 2011; Laurie, Faber, et al. 2015). Due to multi-pronged challenges being induced by global warming, inclination towards climate-smart research has since been imposed on multidisciplinary-focused studies (Laurie et al. 2009; Mashela et al. 2017a). In terms of climate-smart research, characterised by multidisciplinary approaches, sweet potato plant breeders incorporated resistance to root-knot (Meloidogyne species) nematode in the breeding programme at the Agricultural Research Council (ARC) of South Africa (Pofu et al. 2016).

In addition to the life cycle of Meloidogyne species being temperature-dependent (Ghini et al. 2008), some of the identified nematode resistance genes were shown to be lost at soil temperatures above 30°C (Mashela et al. 2017b). Furthermore, honeydew-producing insects like the greenhouse whitefly (Trialeurodes vaporariorum Westwood, 1856) on wild watermelon (Cucumis africanus L.) and red sugarcane aphid (Sipha flava Forbes) on sweet stem sorghum cv. ‘Ndendane’, which are increasing under global warming (Ghini et al. 2008), were shown to break resistance to Meloidogyne species (Pofu et al. 2011). In another review study (Mashela et al. 2017b), it was shown that genes that confer resistance to insects do not necessarily confer resistance to Meloidogyne species. Consequently, a multi-pronged approach is necessary in plant breeding for climate-smart agriculture (Laurie et al. 2009).

The Sweet Potato Programme (SPP) of the ARC has been in the forefront of biofortifying sweet potato cultivars (Laurie, Faber et al. 2015; Laurie, Tjale et al. 2015). In its multi-pronged approaches to climate-smart agriculture, the programme has since included the selection of nematode resistance as one of the desirable attributes. After selecting for attributes like high β-carotene content, drought tolerance, high yield, storability, sweetness and/or dry taste (Omotobora et al. 2014; Laurie, Faber et al. 2015; Laurie, Tjale et al. 2015), lines with desirable attributes are screened for host-status to tropical Meloidogyne species, namely, M. javanica, M. incognita race 2 and M. incognita race 4. Lines with non-host status are subjected to nematode resistance in pot trials and those that are resistant to nematodes are further tested for the related nematode resistance mechanisms to establish whether the related resistant genes could be used in plant breeding through introgression (Thurau et al. 2010). The objective of this study was to screen 20 sweet potato lines from the ARC SPP against M. javanica, M. incognita race 2 and M. incognita race 4 to illustrate how the multi-pronged approach works in the context of climate-smart agriculture.

Materials and methods

Description of the study locations

Three parallel trials for tropical Meloidogyne species (Trial 1: M. javanica, Trial 2: M. incognita race 2 and Trial 3: M. incognita race 4) were conducted under greenhouse conditions in Gauteng Province (ARC-Vegetable and Ornamental Plants: 25°59″S, 28°35″E) and Limpopo Province (University of Limpopo (UL): 23°53′10″S, 29°44′15″E). At each location, the greenhouse conditions were not homogeneous due to fan-induced wind streams for extracting heat from the facilities and therefore necessitating blocking for this factor during the design of the experiments. The experiments were initiated in autumn (February–April) 2017 and validated in 2018.

Procedures, experimental design and cultural practices

At each location, 25-cm-diameter plastic pots were filled to the mark with pasteurised (300°C) loam soil (37% clay, 23% silt, 40% sand) containing 2.5% organic carbon, pH 6.8 and electrical conductivity (EC) 0.371 dS/m. The growing mixture/pot was irrigated to field capacity using 1.5 L chlorine-free tapwater. Cuttings of each sweet potato line were set in different pots and irrigated with 500 ml chlorine-free tap water. When required, eggs and second-stage juveniles (J2) of each species/race were extracted from roots of nematode-susceptible tomato (Solanum lycopersicum) cv. ‘Floradade’ in 1% NaOCl solution for 60 s (Hussey and Barker 1973). At 10 days after transplanting, uniform lines were arranged at 0.25 m × 0.25 m spacing on greenhouse benches. Lines in each trial were inoculated with 3000 eggs and J2 of their respective nematode species/race using a 20 ml plastic syringe. Inocula were placed into 5-cm-deep holes on cardinal points at 10 cm further from the vine-crown, with the hole slightly sealed with the growing mixture.

At the ARC location, treatments in each trial comprised 2005-5-5, 2008-3-1, 199062.1x.Ndou, Monate x 1999-5-1, 1988-7-7, 2000-12-16, 1987-19-5, 2008-5-5, 2012-8-4, 1990-10-2 and 1987-2-1 lines, with nematode-susceptible ‘Bophelo’ and ‘Beauregard’ serving as standards (n = 78) for substantiating the infectivity of the inocula. In contrast, at the UL location treatments comprised 2013-26-3, 2012-15-5, 2012-40-1, C5-1, 2013-28-2, 2008-12-5, 2008-8-5, IIAM-3 and 2010-03-1 lines, with the standard being ‘Beauregard’ (n = 60). In each trial, treatments were arranged in a randomised complete block design, with six replications. After the initial irrigation to field capacity, three Hadeco moisture metres (Hadeco Magic^R, RSA) were randomly installed and plants irrigated with 300 ml tap water when readings averaged below 2 units. A week after transplanting, lines were fertilised with 3 g 2:1:2 (43) fertiliser mixture to provide 0.70 mg N, 0.64 mg K, 0.64 mg P, 1.8 mg Mg, 1.5 mg Fe, 0.15 mg Cu, 0.7 mg Zn, 2 mg B, 6 mg Mn and 0.14 mg Mo/ml tapwater. Greenhouse whiteflies were monitored daily and populations suppressed using (Deltamethrin 5 ml/10 L) when more than 20 insects were observed.

Data collection and analysis

At 56 days after inoculation, vines were severed and discarded, roots were removed from pots, slightly rinsed in water to remove soil particles, pressed between tissue paper to remove excess water and fresh root mass weighed. Eggs and J2 were extracted from total roots/line using maceration and blending in 1% NaOCl solution (Hussey and Barker 1973), followed by the sugar-flotation centrifugation method (Coolen and D’Herde 1972). The reproductive potential (RP = eggs + J2/g roots) was computed in each trial and data were subjected to the analysis of variance using SAS software (SAS Institute 2008). Means were separated using Waller–Duncan multiple range test. Unless otherwise stated, significant treatments were discussed at the probability level of 5%.

Results

Seasonal interaction within each location for RP was not significant and, therefore, data were pooled (ARC: n = 156; UL: n = 120) and re-analysed as above. At the ARC location, lines had significant effects on RP of M. javanica, M. incognita race 2 and M. incognita race 4, contributing 65%, 80% and 57% in total treatment variation (TTV) of the respective variables (data not shown). The RP values had 0.0–3726.7, 0.0–254.7 and 0.0–65.8 ranges for M. javanica, M. incognita race 2 and M. incognita race 4, respectively. All test Meloidogyne species did not reproduce on line 1990-10-2, where RP values were zero (Table 1). Meloidogyne species had high RP values on the standard cultivars ‘Bophelo’ and ‘Beauregard’ in all trials.

Table 1. Fresh root mass (FRM), eggs, second-stage juveniles (J2) and reproductive potential (RP) of Meloidogyne javanica and Meloidogyne incognita race 2 and Meloidogyne incognita race 4 on sweet potato lines and nematode-susceptible standards at 56 days after inoculation.

	M. javanica				M. incognita race 2				M. incognita race 4
Line	FRM (g)	Eggs	J2	RP^y	FRM (g)	Eggs	J2	RP	FRM (g)	Eggs	J2	RP
ARC-Vegetable and Ornamental Plants (25°59″S, 28°35″E)
2005-5-5	34.1	14180	1810	464.8^bcd	20.1	3840	110	196.5^b	42.2	340	90	10.2^bc
2008-3-1	23.0	8900	1140	436.5^bcd	32.0	4530	80	144.1^b	47.0	770	60	17.7^bc
199062.1x.Ndou	37.4	64400	4950	1854.3^b	40.9	3830	200	98.5^bc	33.0	1240	100	40.6^b
Monatex1999-5-1	42.7	37330	5150	994.8^bcd	50.2	4330	210	90.4^bc	45.0	940	110	23.3^bc
1988-7-7	23.3	69160	7800	3303.0^a	37.7	3450	140	95.2^bc	43.1	2770	70	65.9^a
2000-12-16	31.6	24960	1030	822.5^bcd	76.3	2350	80	31.8^c	36.5	540	120	18.1^bc
1987-2-1	40.9	7980	1550	233.0^cd	25.2	6250	170	250.8^a	51.5	1990	230	43.1^b
1987-19-5	29.6	34930	3960	1313.9^bc	27.5	5750	110	213.1^b	51.0	990	140	22.2^bc
2008-5-5	30.5	22440	5460	914.8^bcd	42.4	6240	230	152.6^b	47.6	540	150	14.5^bc
2012-8-4	20.6	18580	3170	1055.8^bcd	19.5	1670	130	92.3^bc	42.2	910	90	23.7^bc
1990-10-2	15.2	0	0	0^d	23.7	0	0	0^d	42.0	0	0	0^d
Bophelo^z	32.1	8960	1140	314.6^cd	29.2	2370	40	82.5^bc	53.0	700	120	15.5^bc
Beauregard^z	35.1	9270	1630	310.5^cd	23.1	3840	100	170.6^b	44.0	650	150	18.2^bc
University of Limpopo (23°53′10″S, 29°44′15″E)
2013-26-3	16.2	1890	90	122.2^bc	8.4	1810	240	244.0^cde	11.2	1200	100	116.1^bc
2012-15-5	9.0	1790	70	206.7^b	5.7	3660	190	675.4^bcd	6.3	1150	80	195.2^b
2012-40-1	12.2	1270	50	108.2^bc	4.8	600	100	145.8^e	7.8	1000	70	137.2^bc
C5-1	14.0	3070	110	227.1^b	8.1	2170	130	284.0^cde	7.7	1470	90	202.6^b
2013-28-2	14.9	2220	80	154.4^bc	11.4	1790	160	171.1^de	9.6	920	80	104.2^c
2008-12-5	13.8	810	40	61.6^c	6.5	1580	70	253.8^cde	8.7	1610	110	197.7^b
2008-8-5	14.3	1170	60	86.0^bc	7.6	970	160	148.7^de	9.5	1430	100	161.1^bc
IIAM-3	11.1	1280	40	118.9^bc	11.2	4410	370	426.8^bcde	7.3	950	110	145.2^bc
2010-3-1	6.8	2210	110	341.2^a	5.5	4710	720	987.3^a	6.7	2400	130	377.6^a
Beauregard^z	12.0	1490	80	130.3^bc	8.2	4520	730	640.2^bcd	7.8	1250	120	175.6^bc

^yRP = $Pf/FRM$, column means followed by the same letter were not different (P ≤ 0.05) according to Waller–Duncan multiple range test.

^zNematode-susceptible sweet potato standard cultivars.

Similarly, at the UL location lines had significant effects on RP of M. javanica, M. incognita race 2 and M. incognita race 4, contributing 69%, 60% and 79% in TTV of the respective variables (data not shown). In contrast, Meloidogyne species reproduced on all lines, with RP ranges being 61.2–342.7, 147.4–995.1 and 104.16–380.44 for M. javanica, M. incognita race 2 and M. incognita race 4, respectively (Table 1). Also, Meloidogyne species had high RP values on the standard cultivar ‘Beauregard’ in all trials.

Discussion

At both locations, the sweet potato lines had significant effects on RP values of the test tropical Meloidogyne species, with high percentage contributions in TTV of the respective variables. This agreed with previous observations where lines had significant effects on RP values of M. javanica, M. incognita race 2 and M. incognita race 4, with high percentage contributions in TTV (Pofu et al. 2016). In the current study, all test nematodes did not reproduce on line 1990-10-2, which should join cultivars ‘Bosbok’ and ‘Mvuvhelo’, the cream-fleshed cultivars shown to have been non-host to tropical Meloidogyne species in South Africa (Pofu et al. 2016). Additionally, the USA orange-fleshed cv. ‘W-119’ was non-host to M. incognita race 4 (Pofu et al. 2016), which is predominant in the cotton-producing regions of South Africa (Kleynhans et al. 1996).

In the previous screening trials (Pofu et al. 2016), M. javanica, M. incognita race 2 and M. incognita race 4 had RP values of 1.35, 1.68 and 2.18, respectively, on cv. ‘Bophelo’. Due to its desirable orange-fleshed attributes and yielding capacity as a local-released commercial cultivar (Laurie, Booysen et al. 2015), a decision was taken to include it in the current study as a standard. In the current study, RP values for M. javanica, M. incognita race 2 and M. incognita race 4 on the cultivar were of 323.05, 82.3 and 15.5, respectively (Table 1), convincingly suggesting that the cultivar was a host to all test nematodes. Also, as it was previously confirmed, cv. ‘Beauregard’ which was shown to be host to all USA Meloidogyne populations (Cervantes-Flores et al. 2002), continued to serve as a good standard for tropical Meloidogyne species in South Africa.

Sweet potato line 1990-10-2, due to its non-host status to all test Meloidogyne species (Table 1), should undergo two further trials for each of the test nematodes, namely, nematode resistance and mechanism trials. In the current study, lines were screened using one inoculation level, which does not provide guarantee that the non-host status is equivalent to host-resistant status. In tests involving the latter, various levels of inoculation, below and above the level used during screening are used, in order to guard against confounding inoculation levels above the equilibrium point as non-host status (Seinhorst 1967). Once nematode resistance is confirmed using the reproductive factor (RF = Pf/Pi), it becomes imperative to assess the mechanisms of nematode resistance. Subjection of cultivars ‘Bosbok’ and ‘Mvuvhelo’ to the suggested trials demonstrated that both were resistant to the test nematodes, with post-infectional mechanism of nematode resistance (Makhwedzana et al. 2018; Maseko 2018). Among the two existing mechanisms of nematode resistance, namely, pre- and post-infectional (Roberts et al. 1998), only post-infectional nematode resistance could be used in plant breeding programmes through introgression (Thurau et al. 2010).

In conclusion, sweet potato line 1990-10-2 is a non-host to M. javanica, M. incognita race 2 and M. incognita race 4. Ten of the 20 lines tested were used in a crossing scheme, availing a set of progenies, which can be tested for resistance to the nematode species to understand the mechanism of inheritance in sweet potato, and opening up possibilities in marker development for marker-assisted breeding.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes on contributors

K. M. Pofu is researcher and associate researcher of nematology at the Agricultural Research Council and University of Limpopo.

P. W. Mashela is senior professor of nematology at the University of Limpopo.

S. Laurie is senior researcher of plant breeding at the Agricultural Research Council.

Additional information

Funding

The authors are grateful to the National Research Foundation of South Africa and the Land Bank Chair of Agriculture – University of Limpopo for the financial support.