Interrelations between commercial beetroot (Beta vulgaris) cultivars and Meloidogyne species

ABSTRACT

Most beetroot (Beta vulgaris) cultivars in South Africa are exotic and were specifically bred for root-knot (Meloidogyne species) nematode populations in their countries of origin. Due to the widespread distribution of different Meloidogyne species and races, exotic cultivars should be matched with nematode populations in importing countries. The objective of this study was to determine the interrelations between exotic beetroot cultivars ‘Detroit Red Dark’ and ‘Crimson Globe’ with Meloidogyne species in the predominant beetroot-producing regions in South Africa. Different inoculum series of M. incognita and M. javanica were used on the two beetroot cultivars. At 56 days after initiating the treatments, roots of both cultivars had small undeveloped root galls, with the reproductive factor values of M. incognita on ‘Detroit Dark Red’ being above and below unity at low (≤125 inoculation) and high (≥250 inoculation) nematode levels, respectively. Growth of ‘Detroit Dark Red’ was significantly stimulated and inhibited at low and high nematode infection levels, respectively. In contrast, RF values for M. javanica on ‘Crimson Globe’ were below unity, without any significant effects on plant growth. In conclusion, ‘Detroit Dark Red’ was tolerant to M. incognita, whereas ‘Crimson Globe’ was resistant to M. javanica.

KEYWORDS:

• Beetroot
• Beta vulgaris
• host-status
• host-sensitivity
• nematode resistance

Introduction

Internationally, the major root-knot (Meloidogyne species) nematodes associated with beetroot (Beta vulgaris) include M. arenaria, M. incognita, M. chitwoodi, M. hapla and M. mayanguensis, with limited information on M. javanica (Sikora & Fernández 2005). Meloidogyne incognita is internationally viewed as being more aggressive in damaging crops than M. javanica (Sasser 1980). In South Africa, M. javanica populations are more aggressive than those of M. incognita (Kleynhans et al. 1996). The South African M. incognita populations contain races 2 and 4 (Kleynhans et al. 1996). Meloidogyne incognita race 2 is widely distributed, whereas M. incognita race 4 occurs mainly in cotton-producing regions. Infection by Meloidogyne species in beetroot increases fibrous roots, resulting in unsightly roots due to the formation of root galls (Sikora & Fernández 2005). Also, nematode infection negatively affects the accumulation of potassium in roots and leaves of different crops (Mashela et al. 2016), which has been associated with various health benefits in humans (Lundberg et al. 2011; Hobbs et al. 2012; Siervo et al. 2013; Mente et al. 2014; Wootton-Beard et al. 2014).

Prior to the withdrawal of fumigant chemical nematicides from the agrochemical markets, most beetroot farmers relied on methyl bromide for managing nematodes (Sikora & Fernández 2005). However, following the 2005 cut-off-date for withdrawing methyl bromide from the agrochemical markets, successful crop production has been based on the use of alternative nematode management strategies, including nematode resistance (Stirling 2014). Due to the widespread distribution of Meloidogyne species and races, the first step in successful use of nematode-resistant cultivars is to identify the local nematode species or races, followed by matching them with potential nematode-resistant cultivars (Sasser 1980).

The major commercially produced beetroots in South Africa include the exotic ‘Detroit Red Dark’ and ‘Crimson Globe’ cultivars. Preliminary desktop studies suggested that ‘Detroit Red Dark’ and ‘Crimson Globe’ were being produced in regions with high population densities of M. incognita race 2 and M. javanica, respectively. The objective of this study was to determine the interrelations between exotic ‘Detroit Red Dark’ and ‘Crimson Globe’ cultivars with M. incognita and M. javanica populations, respectively.

Materials and methods

Two parallel greenhouse experiments were conducted at the Green Technologies Research Centre (GTRC), University of Limpopo, South Africa (23°53′10″S, 29°44′15″E) during spring (August–October) 2015. Ambient minimum/maximum greenhouse temperatures averaged 22/28°C, with maximum temperatures controlled using thermostatically activated fans and wet walls. Due to the large size (30 m × 20 m) of the greenhouse structure, the extraction of warm air to the exterior for cooling invariably induces heterogeneous conditions of different temperatures and wind speeds within the facility.

Twenty-five-cm-diameter (2700 ml) plastic pots were filled with a mixture of steam-pasteurised (300˚C for 1 h) river sand (20 L), loam soil (65% sand, 30% clay, 5% silt; 1.6% organic C, EC of 0.148 dS/m, pH of 6.53) and Hygromix-T (Hygrotech, Pretoria) at 2:1:1 (v/v), containing 10 g calcitic lime. Hardened-off uniform four-week-old seedlings of each beetroot cultivar were transplanted, with pots placed on the greenhouse benches at 0.25 m inter-row and 0.25 m intra-row spacing.

Inoculation and experimental design

Nematode eggs and second-stage juveniles (J2) were extracted from roots of greenhouse-raised nematode-susceptible kenaf (Hibiscus cannabinus) plants using the maceration and blending method in 1% NaOCl solution (Hussey & Barker 1973). Two days after transplanting, seedlings were inoculated with a mixture of eggs and J2, using the pre-determined inoculation series of 0, 50, 75, 125, 250, 500, 1125 and 2625 M. incognita race 2 for ‘Detroit Dark Red’ and 0, 100, 200, 400, 600, 800, 1600 and 2200 M. javanica for ‘Crimson Globe’. The inoculum was dispensed into 5-cm-deep furrows around the seedling stems using a 20-ml plastic syringe and then covered with the growing mixture. The eight treatments in each experiment were arranged in a randomised complete block design, with eight replicates (n = 64). Blocking was done to account for different micro-temperatures within the greenhouse. Eight nematode-susceptible tomato (Solanum lycopersicum) cv. ‘Floradade’ seedlings were inoculated with 2625 M. incognita eggs and J2 and the other eight with 2200 M. javanica eggs and J2 for serving as standards for verifying the infectivity of the inoculum.

Cultural practices

Each pot was originally irrigated to field capacity and then three iTuin4-in-1 Moisture Meters installed randomly in three pots, with plants being irrigated with 300 ml tapwater/plant when readings averaged below 2 units. Seedlings were fertilised once at 7 days after transplanting using 5 g 2:3:2 (22) NPK to provide 310 mg N, 210 mg P and 260 mg K/ml water and 3 g at 2:1:2 (43) NPK Multifeed fertiliser 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.

Data collection and analysis

At 56 days after inoculation, plants were removed from the soil, with roots, tubers and leaves separated. Soil particles from roots and tubers were rinsed off in 20 L water and excess water removed by pressing materials between pieces of paper towel. Root galls were assessed using the North Carolina Differential Scale, where 0 = no galls, 1 = 1–2 galls, 2 = 3–10 galls, 3 = 11–30 galls, 4 = 31–100 galls and 5 ≥ 100 galls per root system (Taylor & Sasser 1978). Nematode eggs and J2 were extracted from 10 g roots per plant using the maceration and blending method for 30 s in 1% NaOCl solution (Hussey & Barker 1973), followed by the sugar-flotation centrifugation method (Coolen & D’Herde 1972). Materials were passed through top-down nested 150-, 45- and 25-μm mesh sieves and eggs and J2 collected from the 25-µm mesh sieves. Nematode J2 were extracted from 250 ml soil sample (Jenkins 1964), with nematodes from soil and roots separately counted using a stereomicroscope. The reproductive factor (RF = Pf/Pi) values, which are proportions of the final (Pf) and initial (Pi) nematode population densities, were calculated. Leaves, tubers and the remaining roots were dried at 70°C for 72 h in air-forced ovens for dry mass determination. The RF and plant data were subjected to analysis of variance using SAS software (SAS Institute 2008), with the mean sum of squares partitioned to establish the total treatment variation (TTV) for significant variables (Gomez & Gomez 1984) prior to mean separation using the Waller–Duncan multiple-range test. Significant treatment means were subjected to lines of the best fit. Unless otherwise stated, results were discussed at the 5% level of probability.

Results

Host-status

Treatments had highly significant effects on root galls, contributing 92% and 85% in TTV of undeveloped root galls in ‘Detroit Dark Red’ and ‘Crimson Globe’, respectively (Data not shown). In both cultivars, the scale rating of the undeveloped root galls was proportional to nematode inoculation levels, exhibiting positive quadratic relations, with the models being explained by 97% and 82% in ‘Detroit Dark Red’ and ‘Crimson Globe’, respectively (Table 1). In contrast, tomato roots had fully developed root galls, with the mean rating in both cultivars being at the upper end of the differential scale.

Table 1. Root galls and population densities of Meloidogyne species on beetroot cultivars in pot trials under greenhouse conditions at 56 days after inoculation (n = 64).

	Beetroot cv. ‘Detroit Red Dark’ with Meloidogyne incognita						Beetroot cv. ‘Crimson Globe’ with Meloidogyne javanica
Pi^u	Root galls^w	Eggs	Juveniles	Pf^y	RF^z	Pi	Root galls	Eggs	Juveniles	Pf	RF
50	1.00^e ± 0.00^x	35	90	125	2.50^a ± 0.63	100	0.00^c ± 0.00	20	18	38	0.38^ab ± 0.10
75	1.00^e ± 0.00	30	141	171	2.28^a ± 0.26	200	1.33^b ± 0.21	37	17	53	0.27^b ± 0.10
125	1.17^de ± 0.17	38	121	159	1.27^b ± 0.16	400	1.50^b ± 0.22	133	37	170	0.43^a ± 0.20
250	1.50^cde ± 0.34	90	131	221	0.88^bc ± 0.11	600	1.33^b ± 0.21	63	25	88	0.15^c ± 0.0
500	1.67^bcd ± 0.40	63	126	190	0.38^cd ± 0.65	800	1.50^b ± 0.22	103	18	122	0.15^cd ± 0.0
1125	2.00^bc ± 0.26	358	211	569	0.51^cd ± 0.10	1600	2.50^a ± 0.34	387	83	470	0.29^b ± 0.1
2625	2.71^a ± 0.20	105	144	249	0.09^d ± 0.01	2200	2.67^a ± 0.21	297	117	413	0.19^d ± 0.0
Relation	Quadratic				Quadratic		Quadratic				Quadratic
R^2	0.97				0.70		0.82				0.27

^wPi = Initial nematode population density.

^wColumn means with the same letter were not statistically different (P ≤ .05) according to the Waller–Duncan multiple-range test.

^zMean ± SE (Standard error).

^yPf = Final nematode population density.

^zRF = Reproductive factor.

Treatments had highly significant effects on RF values, contributing 87% and 90% in TTV of RF values in ‘Detroit Dark Red’ and ‘Crimson Globe’, respectively (Data not shown). In ‘Detroit Dark Red’, the RF values at the lower (≤125) and higher (≥250) levels of inoculation with M. incognita were above and below unity, respectively (Table 1). In contrast, in ‘Crimson Globe’ the RF values were below unity at all levels of inoculation with M. javanica. In both cultivars, the RF values were inversely proportional to nematode inoculation levels, exhibiting negative quadratic relations, with the models being explained by 70% and 27% in ‘Detroit Dark Red’ and ‘Crimson Globe’, respectively (Table 1). In tomato plants, M. incognita and M. javanica had the mean RF values of 1.55 and 2.65, respectively (Data not shown). In ‘Detroit Dark Red’, the egg/juvenile ratios of M. incognita were mainly below unity, whereas those of M. javanica in ‘Crimson Globe’ were above one.

Host-sensitivity

Masses of dried leaves, roots and tubers in ‘Detroit Dark Red’ were each significantly affected by M. incognita infection (Table 2), contributing 68%, 84% and 95% in TTV of the three variables, respectively (Data not shown). Relative to the control, M. incognita infection at all inoculation levels stimulated leaf, root and tuber mass in ‘Detroit Dark Red’ (Table 2). Although inoculation with M. javanica did not have significant effects on growth of ‘Crimson Globe’, similar relative stimulation patterns due to nematode infection were conspicuous.

Table 2. Responses of beetroot growth to infection by Meloidogyne species in pot trials under greenhouse conditions at 56 days after inoculation (n = 64).

Beetroot cv. ‘Detroit Red Dark’ with M. incognita							Beetroot cv. ‘Crimson Globe’ with M. javanica
	Dry leaf mass (g)		Dry root mass (g)		Dry tuber mass (g)		Pi	Dry leaf mass (g)	Dry root mass (g)	Dry tuber mass (g)
Pi	Variable^y	Impact (%)^z	Variable	Impact (%)	Variable	Impact (%)
0	4.78^b ± 0.75	–	0.22^c ± 0.03	–	3.83^b ± 0.60	–	0	3.49	0.22	4.62
50	6.42^ab ± 0.73	34	0.35^a ± 0.10	59	6.05^a ± 0.30	58	100	4.15	0.28	6.17
75	6.42^ab ± 0.36	34	0.32^ab ± 0.09	45	6.02^a ± 0.7	57	200	3.73	0.27	5.50
125	6.50^a ± 0.60	36	0.35^a ± 0.02	59	6.42^a ± 0.4	68	400	5.20	0.25	5.03
250	5.35^ab ± 0.26	12	0.28^abc ± 0.10	27	5.15^ab ± 1.06	34	600	4.40	0.32	6.67
500	6.13^ab ± 0.70	28	0.25^bc0.08	14	5.18^ab ± 0.20	35	800	3.95	0.28	5.90
1125	5.90^ab ± 0.65	23	0.32^ab0.12	45	5.88^a ± 0.60	54	1600	4.02	0.23	5.07
2625	6.20^ab ± 0.35	30	0.33^ab ± 0.00	50	6.30^a ± 0.40	64	2200	4.67	0.27	5.38

Discussion

Host-status

In both beetroot cultivars, the root galls were undeveloped at the lower end of the North Carolina Differential Scale. Although successful host-status in plant–Meloidogyne species interrelations is usually accompanied by the formation of root galls, this is not an absolute indicator of host-status. The observed undeveloped root galls as shown in Table 1 were similar to those that were occasionally observed in highly nematode-resistant Cucumis myriocarpus and Cucumis africanus indigenous to South Africa (Pofu et al. 2010, 2012), certain moderately nematode-resistant exotic Cucumis species (Fassuliotis 1970) and nematode-resistant maize (Zea mays) cultivars (Ngobeni et al. 2012). The observations led others (Fassuliotis 1970; Ngobeni et al. 2012) to believe that host-status was feasible without the development of root galls. This view, however, was challenged by another view which suggested that the presence of juveniles does not necessarily imply the existence of host-status (Pofu et al. 2012).

In nematode–plant relations, another available indicator for non-host status is the RF values, which when above and below one suggest that the plant is a host and non-host, respectively (Seinhorst 1967a). The RF values above unity at low inoculation levels in ‘Detroit Dark Red’ (Table 1) suggested that the cultivar partially allowed M. incognita to feed, develop and reproduce as observed by Seinhorst (1967a). However, since at higher nematode levels on this cultivar the RF values were less than one, some degree of nematode resistance existed, thereby resulting in the equilibrium (E) point, where Pf = Pi, being achieved at low inoculation levels (Seinhorst 1967b). Generally, beyond E point, the RF values are invariably less than one due to interspecific competition for resources that include feeding sites (Seinhorst 1967b) as shown in Table 1. In plants with non-host status attributes as observed on ‘Crimson Globe’ and M. javanica interrelations, the RF values at all inoculation levels are usually below unity (Seinhorst 1967b). The latter agreed with non-host status observed in C. myriocarpus and C. africanus for various Meloidogyne species in South Africa (Pofu et al. 2010, 2012).

In the current study as shown in Table 1 and other studies (Ngobeni et al. 2012), fewer nematode eggs were reported. In plant-parasitic nematodes, eggs can only be produced if the feeding site was allowed, which would then be followed by feeding, nematode development and then reproduction (Wallace 1963). Currently, there was no plausible explanation of the lower and higher than unity egg/juvenile ratios observed in ‘Detroit Red Dark’ and ‘Crimson Globe’, respectively.

Host-sensitivity

Stimulated plant growth due to nematode infection in ‘Detroit Dark Red’ (Table 2) could be viewed on the basis of density-dependent growth (DDG) patterns, which have three phases, namely, stimulation, neutral and inhibition phases (Mashela et al. 2015). The first report on stimulated plant growth in response to nematode infection was in the early 1960s (Wallace 1963). Stimulation of growth in ‘Detroit Dark Red’ infested by M. incognita infection (Table 2) agreed with observations in tomato–Meloidogyne (Mashela 2002; Mashela & Nthangeni 2002), citrus–Tylenchulus (Mathabatha et al. 2016), cowpea–Meloidogyne (Mashela 2014) and sorghum–Meloidogyne (Mashela & Pofu 2016) interrelations. In their review, Mashela et al. (2015) argued that stimulation of plant growth by nematodes was a common phenomenon whenever nematode population densities were below the damage threshold level.

The observed neutral effects of M. javanica infection on ‘Crimson Globe’ as shown in Table 2 is the second stage within the DDG patterns (Mashela et al. 2015). This is a common phenomenon in nematode-resistant plants (Seinhorst 1967b). The Curve-fitting Allelochemical Response Dosage computer-based model (Liu et al. 2003) had been used to enhance clarity of the neutral effect concept (Mashela et al. 2015), which had been previously observed in nematode–phytonematicide (Mashela et al. 2015), nematode–phytonematicides (Dube & Mashela 2016), nematode–rhizobium (Huang 1987) and nematode–mycorrhiza (Smith 1987) interrelations. Inhibitive effects, which were not observed in the current study, occur as the third phase of the DDG patterns and is widely reported in nematode-susceptible plant species (Mashela et al. 2015).

Degree of nematode resistance

Seinhorst (1967a) described the degree of nematode resistance in nematode–plant interrelations using three concepts. Susceptible hosts referred to interrelations where RF values were greater than unity, with nematode infection reducing plant growth; whereas in tolerant hosts the RF values were greater than one, with nematode infection resulting in either stimulated plant growth or no effects at all. In contrast, in resistant hosts the RF values are less than one and plant growth not affected by nematode infection. Using this model, ‘Detroit Dark Red’ was tolerant to M. incognita, whereas ‘Crimson Globe’ was resistant to M. javanica. In conclusion, ‘Crimson Globe’ as shown in Tables 1 and 2 could be used in areas with M. javanica populations, whereas ‘Detroit Dark Red’ is not suitable for crop rotation in areas with M. incognita race 2 since it would increase nematode population build-up for the successor crops.

Disclosure statement

No potential conflict of interest was reported by the author.

Notes on contributor

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

Additional information

Funding

This work was supported by the Agricultural Research Council-Universities Collaboration Centre [grant number R538].